Homemade Alternator Test Bench? Top Answer Update

You want to start your car, but when you turn on the ignition, your car won’t start – and your stomach sinks. When your car won’t start, it’s often due to a battery or alternator malfunction. If it’s one of those things, it can be relatively easy to tell which one.

Battery or Alternator: What Causes the Problem?

At its simplest, starting and running an engine is a three-step process. First, the battery delivers a surge of current to the starter. This starts the engine, which turns the alternator on. Finally, the alternator completes this cycle by charging the battery.

Use the process of elimination to find the culprit by bypassing the role of the battery and jump-starting your car. If the engine starts but dies immediately, your alternator is probably not keeping your battery charged. If a jump starts and keeps your car running, but the car won’t restart under its own power, a dead battery is likely your answer.

Signs of a bad battery

Look for these common dead battery symptoms:

Are the dashboard lights dimmed? First check the dashboard battery gauge. The battery should also be charged when the vehicle is switched off. If the dashboard lights are dim, there is probably something wrong with your battery. Try turning on your wipers, lights, or power windows. Then make sure these are all turned off and try starting the car again. Check for battery corrosion. If it still won’t start, use a rag to gently wipe any corrosion off the battery and have someone start it. After letting the engine run for a while, turn off the car. If you can’t get it to restart, that’s a sign the alternator is doing its job of keeping the battery running while the engine is running, but the battery isn’t holding the charge when the alternator has been stopped. The car battery could be old. As batteries age, they become less able to hold a charge because the metal inside corrodes. On average, car batteries last between 3 and 5 years. [4] Eventually, the battery state of charge will drop to the point where no matter how much current the alternator gives it, the battery cannot hold enough current to start the car. However, there are steps you can take to extend the life of your car battery. Your radio will not turn on. When your ignition is on, the battery should be able to power your radio, headlights, windshield wipers, and other electronic components. If you notice your radio display or headlights flickering or dimming, there’s a good chance your battery isn’t charging well. Your car starts and dies instantly. This can be for a variety of reasons, but checking your battery is the best place to start. This is a strange problem that usually only occurs on certain vehicles and is due to the battery only having enough voltage to start the car. Your battery is bloated. If your battery is swollen, you should be able to tell thanks to the manufacturer’s design: you’ll be able to easily spot bulges in the battery’s casing. This problem occurs when your alternator has overcharged the battery due to a bad voltage regulator.

Signs of a bad alternator

If the above steps show that the battery is working, it’s time to take a closer look at the alternator. There are certain bad alternator symptoms to look out for. Find out how to tell if your alternator is bad¹²:

Weak interior lighting. When driving the vehicle, pay attention to the brightness of the interior lights. If the dashboard starts to darken, the alternator is probably to blame. Weak or too bright headlights. Do you notice that your headlights get brighter when you accelerate and get dimmer when you stop? This is often caused by the alternator not keeping the battery sufficiently charged. growling sounds. Did you hear a growling noise before the problems started? This sometimes happens before an alternator fails. Smell of burning rubber or hot wire. Are there any signs of your alternator overheating, such as: B. the smell of burnt rubber or hot wires? If so, it’s time to replace it. Test the alternator. Some may recommend running the engine with the battery disconnected to test the alternator. However, this is not a good idea as it could damage your vehicle’s electrical system and cause major problems. Learn how to test an alternator.

Can you drive with a defective alternator or battery?

While your car can run with a faulty alternator for a short period of time, doing so involves risks and can be dangerous for you and everyone else on the road. Driving with a bad alternator can cause a variety of problems in other parts of your car, including the engine and electrical components. Also, your car’s battery will eventually discharge which will cause your car to die. Unless you have the resources to hop your car and find yourself in a remote area, this could leave you stranded. Your vehicle can also lose power from its power steering, which can cause you to lose control of your car. For these reasons, it is best to have your alternator repaired to protect yourself and other road users³

Repairing your battery or alternator can be an expensive repair. Learn how to find the right mechanic to work on your vehicle. Make sure you have roadside assistance so you can get help if your car won’t start.

Sources:

[1] “Is your car battery dead?” autozone.com/diy/battery/is-your-car-battery-dead Retrieved February 28, 2022.

[2] “10 Warning Signs Your Car Battery Is Dying”, valleyhondadealers.com/blog/10-warning-signs-your-car-battery-is-dying/ Retrieved February 28, 2022.

[3] “Can a car run with a bad alternator (Risks of Driving with a Bad Alternator)”, Jeffery Ekweghi, autovfix.com/can-a-car-run-with-a-bad-alternator-risks-of- Driving-with-a-bad-alternator/ Retrieved February 28, 2022.

[4] https://www.aaa.com/autorepair/articles/how-long-do-car-batteries-last

Additional resources on car issues:

Disclaimer:

The information contained is for informational purposes only. It is not legal, tax, financial or other advice, nor does it replace such advice. The information may not apply to your specific situation. We have tried to ensure that the information is correct, but it may be out of date or even inaccurate in some cases. It is the reader’s responsibility to comply with all applicable local, state, or federal regulations. Nationwide Mutual Insurance Company, its affiliates and their employees make no warranties about the information or any guarantees of results and accept no liability in connection with the information provided. Nationwide, Nationwide N and Eagle and Nationwide is on your side are service marks of Nationwide Mutual Insurance Company. © 2022 Nationwide

If you have to get somewhere and your car won’t start, it’s not unreasonable to think that you might have a problem charging the car. A jump start could fix it in the short term and get you to work or your appointment on time, but ultimately you need to find the source of the problem and have it fixed. The problem could be in the battery or the alternator, or even somewhere else in the car’s electrical system.

There are a few things you can check yourself before going to a mechanic.

First, take a look at the battery. Start with that, because battery problems are very common, especially in cold weather.

Check the battery gauge on the dashboard to make sure the car is still charging when it’s off. If the light is dim and flickering, there is likely a problem with the car battery.

Turn everything off and check your battery connections for corrosion. Sometimes the problem is simply that there is not enough contact.

If corrosion seems to be the problem, you can remove the battery cables and use a little steel wool to clean up the corrosion.

If cleaning solves the problem, great, but if not, you may have a dead battery or a bad alternator.

You can check the alternator simply by starting your car and then disconnecting the battery negative cable. Be careful when doing this, however, as there are many moving parts in your engine compartment that can cause injury. If the car stalls when the battery cable is disconnected, the culprit is probably the alternator.

If the alternator is defective, you must contact Vermin Club. Sometimes alternators can be rebuilt, but more often they need to be replaced. Vermin Club can also test your battery to find out if this is the reason your car won’t start.

HIGH

System test with slow cranking

Test your charging and electrical system

A lead-acid battery has certain defined properties that make it easy to assess the condition of the charging system without fancy test equipment. An in-car test is actually better than taking parts off. I hope this helps people avoid paying for parts. This is how we can check a charging system with a few simple devices. Basically, all you need is a test light and a small meter.

I have a full diagram of the 1989 Mustang here 1989 Ford Mustang Wiring Schematic

generator

The alternator converts mechanical energy into electrical energy. With an alternator, there are unavoidable power losses. Some losses are mechanical in nature and primarily heat the alternator bearings and drive belt. There are also electrical losses. The diodes drain a little voltage, which causes the diodes to get hot. The windings and internal wiring of the alternator have resistance that results in power loss and heat. The changing magnetic field also causes some losses. Remember that most of the alternator’s load on the crankshaft comes from the electrical load power drawn by the alternator.

Contrary to myth and what marketing departments and salespeople would have us believe, by using a pulley system under the drivetrain there is no horsepower released when racing. It can even do the opposite! It releases horsepower at idle but puts more strain on the system when you rev ​​the engine as the alternator tries to make up for the missing battery charge!

When the alternator shaft speed is reduced, the voltage regulator increases the field current. As long as the shaft is spinning fast enough, the governor will increase the field current and the pulley torque demand until the alternator is pulling exactly the same amount of engine power it would have been using at normal speed! Because efficiency often drops at reduced rotor speeds, the alternator will sometimes draw more engine power and run hotter with an underdrive pulley system than with standard shaft speeds!

The only way to reliably and significantly reduce alternator drag is to turn off the alternator while racing, although turning off lights and electrical accessories while racing certainly helps. Remember that the alternator tries to deliver all of the load energy when the vehicle is running. At reasonable to maximum engine speeds, typically from 1500 rpm to the red line with heavy loads and from idle to the red line with light electrical accessory loads, the battery just rides along. It really does nothing except wait for the alternator to drop below operating RPM. A battery consumes appreciable engine power only when the battery has a low charge and the missing charge is replenished. A battery, when charged, is really just an electrical storage tank.

NEVER pull the battery cable to check the alternator. This very crude test method was just fine with tube car radios and point ignitions, but now it’s a very bad idea. The battery stabilizes the electrical system and charges the alternator, preventing high peak voltages or spikes in voltage as the alternator adjusts the magnetic flux to produce the same average voltage for different current demands. If you rev ​​the engine and pull a battery cable, the alternator voltage can rise to 100 volts or more before the alternator flux dies down enough to drop the voltage back down to around 14 volts. This can destroy the car’s computer and other expensive electrical components. I’ve seen headlights explode when a guy opened a battery switch while an engine was revving. If you hear anyone say this is a way to check an alternator on a modern vehicle, stop them!

TESTING the ALTERNATOR, is it good or bad?

In order to charge the battery, the alternator output voltage must exceed a minimum charging voltage. This minimum charging voltage is 13.8 volts DC at the battery terminals or at the alternator output. A single lead-acid cell starts charging at just over 2.25 volts. Because a 12-volt battery has six cells, any 12-volt lead-acid battery requires at least 13.8 volts to begin charging. This voltage is sufficient to fully charge the battery or to keep it at a trickle charge, but the charging time is very long at 13.8 volts.

To fully charge the battery in a reasonable time, the alternator output must be 14.2V to 14.5V, measured directly at the battery terminals. Above 14.5 volts charge voltage, batteries have a greatly increased tendency to release excessive acid fumes and hydrogen gas and to corrode things around the battery. The charging voltage of the battery terminals must be less than 14.7 volts to avoid excessive gassing. Charge voltages above 14.7 volts can dry out the battery prematurely by boiling out the electrolyte and increase the risk of a battery hydrogen gas explosion.

In this case, the battery charge voltage is 14.61 volts with the engine at high idle. 14.4 volts is the gassing threshold. The above battery will gas slightly, but not enough to be harmful, and the battery will quickly fully charge after starting. 14.8 volts would be a problem (there could be fluid or corrosion on the battery) and 15 volts would be a real problem, but 14.6 is fine. Less than 14.3 would be a “weak” alternator or regulator. Much less than 14.2 at fast idle is bad wiring, a bad alternator or regulator, or a bad connection or fuse link. When operating at a normal low engine speed, the voltage at the battery terminals should remain above 14.3 volts even with full loads such as lights, heater fans and everything else running. If this system was in a restored 1966 GT coupe I would probably change the regulator to reduce the maximum alternator voltage. This would prevent degradation of the metal around the battery from excessive charging fumes. In my daily driving it’s fine as long as I watch the battery for acid deposits that have been flushed out.

If you measure the battery voltage and it is anywhere above 14.2 volts and below 14.8 volts while the car is running at slow engine speed and maximum load, you already have a larger alternator than you need. If the voltage is above 14.2 under max load at cruising speed then buying a larger alternator or a new alternator is a waste of time and money.

Turn the engine off with no load (headlights, etc. all OFF) and read the battery voltage.

With the engine just stopped, the battery voltage should be 13.2 volts to 13.8 volts. The exact voltage will vary with the battery, how quickly you read it, and the battery’s state of charge. This voltage isn’t all that important as the battery will slowly and steadily settle to a new voltage that shows the true state of charge of the battery, but the voltage measured directly when the engine is switched off is a very clear indicator of whether the Alternator or the charging system is charging. If the voltage is above 13.2, the battery has just been charged.

So what happens when your battery dies all the time but the alternator seems good?

Measure the leakage current of the electrical system

To test the electrical system for unwanted loads that draw power, turn off everything in the car. Make it like parking the car overnight. Make sure all lights and accessories are turned off.

With all electrical loads off, remove the negative terminal wire and check the current draw with a test lamp. (I made a test light out of an old tail light bulb.)

The dim glow in the light filament indicates a current drain problem. At this point I don’t want to hook up an electricity meter to check for leaks as the short could damage the test meter! If a small clear test lamp like this does not light, it is generally safe to measure the current draw directly with a test meter.

Measurement of the parasitic current drain

With all electrical loads off, ground the meter in series with the battery negative terminal on a low amp scale of about 1 amp. The positive measuring cable is connected to the vehicle chassis and the negative measuring cable to the negative battery terminal.

A good battery discharge of the electrical system

This is measured on the 20mA scale. The mA scale displays thousandths of amperes. My 1989 Mustang LX is now drawing about 1.73mA of battery power after replacing a faulty alternator diode. This drain is solely from EEC-IV computer memory. Different radios and different computers may have different standby drains, as may accessories such as clocks, but under no circumstances should the “overnight off” leakage current exceed about 25mA. 100mA is like leaving a small interior light on!

My Kenwood stereo draws 1.5mA when plugged in. If you have a digital clock that stays on, an alarm, or some other load, that current will be higher. At 75mA, a leak could affect battery life on infrequently driven vehicles. mA is milliampere or one thousandth of an ampere.

The meter above is on the 20mA scale and reads 1.73mA. It’s nothing. The battery charge would probably last for months of sitting.

Bad parasitic drain of the battery

When the test light comes on, you should locate the wire that is charging the battery. First, make sure all the lights are off. You can do this by having someone open and close with light, e.g. B. the trunk, and pay attention to a defined large load change. When closing doors with lights, such as the glove compartment, you should notice a clear change in load.

Connect the test lamp in series with the negative pole and start pulling the feeder wires. The first thing to check is the thick charging cable from the alternator. A bad or leaking diode in an alternator is a very common cause of overnight battery discharge.

Connect the wires one at a time to see which line is drawing current. For me it was the alternator! Although the alternator charged well, it also drained the battery. My problem was a defective alternator diode. There could be a variety of other issues such as B. a bad voltage regulator or a stuck relay contact.

Download a schematic

I downloaded this from T. Moss’ website which I find much more useful than other sources. Tom Moss will do anything to help people and is a really nice guy. AutoZone and others have some free schematics too.

T.Moss’s diagram (linked above) showed me that the heavy dark green wire from my starter relay that was causing my “drain problem” went straight to my car’s alternator outlet. In my case, one of the diodes (the little black “arrows”) in my car’s alternator was defective. This loss of power also caused my alternator to get a little warm, even when it was idle for several hours.

Other helpful tensions

The battery voltage can be higher than 12.6 volts immediately after charging.

12V battery open circuit voltage after car was off for 1 hour Relative charge 12.6V 100% 12.4V 75% 12.2V 50% 12.1V 25% Less than 12 volts Dead

Any open terminal voltage below 12 volts is considered a full discharge or dead battery.

test starters

Starters can sometimes test well out of the car but can be bad. A common problem with cheaply built or defective starters is loss of starting torque when hot. This usually happens because the iron loses its ability to hold magnetic flux (starter current shoots up when hot) or because the wire is too small and increases resistance (starter current drops when hot is) or the starter is stuck (which also causes high current). .

The best way to test a starter is to measure the voltage and current.

To test a starter and wiring with a simple meter:

Clamp the meter positive lead to the starter motor lead wire that goes into the starter motor

Clamp the black gauge negative lead to the ENGINE BLOCK

Make sure the meter reads volts and set it to the lowest possible voltage scale that reads at least 15 volts. In other words, if your meter has a 2.5 volt, 25 volt, and 250 volt scale, use the 25 volt scale. The 25 volt scale is closest to 15 volts, but not below 15 volts.

With the gauge clamped across the starter, watch the gauge while cranking the engine.

Make sure the battery is good. Above is a voltage chart for battery charging. The battery terminal voltage with no load (all off) should be at least 12.6 volts and up to 13.8 volts.

If the starter voltage drops below 9-10 volts, you have a problem with the starter current, engine ground, or battery.

Measure across the battery by poking directly into the battery posts (NOT the clamps that clip to the posts but poking directly into the lead posts coming out of the battery) and see how much the battery drops when cranking. If it drops and you are sure the alternator is working, take the battery to an automotive parts store to have the batteries tested. Unlike starters, testing batteries is VERY easy and very reliable.

If the battery stays on terminals and the starting voltage drops, you likely have a bad starter cable, ground wire, or other wiring problem. If the battery post-tension tests fine but sags abnormally with your starter, you probably have a starting problem. Starter current must be checked.

Cheap or poorly made starters usually show up when the starter is very hot. Very often starters cannot be tested accurately on a dynamometer as they often only fail when very hot. I see very few tractors, cars and trucks that crank OK when cold and won’t crank when warm that have problems other than a starter motor! My diesel tractor started badly when hot but cranked like a dream when cold and it was the starter. My tractor doesn’t have a header either. Just the block heat was enough to cause the starter to snap. I’ve had the same experience with cars. When cold the starters work and test well! Poor starters may have enough momentum to start properly when the system is cold but fail when hot.

Bad alternators or batteries usually show up when the car is very cold, but both alternators and batteries can be reliably tested to see if they’re good.

Installing a larger alternator will not fix a bad starter, battery, or wiring.

Switch to LED warning

The first sign of alternator problems may be dim headlights or an engine that is slow to start (or not start). The alternator keeps the battery charged and supplies voltage for the entire vehicle electrical system. So if the alternator, voltage regulator, or the wiring that connects the charging system to the battery and electrical system goes bad, it can cause serious problems.

Alternator charging problems can be caused by electrical faults in the charging system itself, poor wire connections at the battery or elsewhere, or a slipping or broken drive belt. Without a charge output, the battery discharges quickly. You may have 20 minutes to an hour of driving time before everything dies and the vehicle shuts down.

Once the battery voltage drops below a certain threshold, the onboard electronics, ignition, and fuel system may not function normally and the engine will stall. The battery does not have enough reserve power to restart the engine, leaving the vehicle stationary until the problem can be diagnosed and repaired.

Recharging the battery or jump-starting the battery with jumper cables from another battery or vehicle can get the engine running again, but it won’t take long if the charging system isn’t producing normal voltage.

Warning: Never disconnect a battery cable while the engine is running to “test” your alternator. This can cause a high voltage spike that can damage the alternator or other electronics.

ALTERNATOR CHARGING OUTPUT

The alternator is the heart of the charging system. It generates all the electricity needed to keep the battery fully charged and to run everything electrical in the vehicle. The alternator is mounted on the engine and is driven by a serpentine or V-belt from the crankshaft pulley. The alternator produces alternating current (AC) that is converted to direct current (DC) by a six-diode rectifier, usually located on the back of the unit. Diodes only allow current to pass in one direction, converting AC to DC. Three positive diodes control the positive side of the AC sine wave, while three negative diodes control the negative side.

The charging power of the alternator increases in proportion to the electrical load on the charging system and the engine speed. Power is low at idle and increases with revs. Maximum power is typically achieved at speeds above 2,500 rpm.

ALTERNATOR VOLTAGE REGULATION

The charging power of the alternator is controlled by a voltage regulator that may be mounted inside or on the rear of the alternator (internally regulated) or elsewhere under the hood (externally regulated). On most newer vehicles, the Powertrain Control Module (PCM) regulates charging power.

On older vehicles, the voltage regulator was electromechanical and used magnetic contacts to control the alternator charge output. Since the 1980s, most voltage regulators have been solid state electronic, using transistors to control the charge output.

The actual output voltage produced by the alternator will vary with temperature and load, but is typically about 1-1/2 to 2 volts higher than the battery voltage. At idle, most charging systems will generate 13.8 to 14.3 volts with no lights or accessories on (although some may charge at a slightly higher voltage depending on temperature, engine RPM, battery type and battery state of charge). This can be measured by connecting the positive (+) and negative (-) leads of a voltmeter across the battery posts with the engine running.

. . . .

The photo on the left shows the normal charging voltage of the alternator with the engine idling. The photo on the right shows a low charge voltage with the engine idling.

A low reading indicates that the charging system is not producing enough voltage to keep the battery charged or to meet the vehicle’s power needs.

HOW TO CHECK ALTERNATOR CHARGING VOLTAGE

Most alternators that are charging properly should produce around 13.8 to 14.2 volts when idling with the lights and accessories off. Always observe the information provided by the vehicle manufacturer. For example, many Asian vehicles have higher charging voltages of around 15 volts.

When the engine is first started, the charge voltage should rise rapidly to about two volts above the battery’s base voltage, then decrease and level off at the specified voltage.

The exact charging voltage varies depending on the charge level of the battery, the load on the vehicle electrical system and the temperature. The lower the temperature, the higher the charging voltage, and the higher the temperature, the lower the charging voltage. The “normal” charging voltage in a typical application might be 13.9 to 15.1 volts at 27 degrees Celsius. But at 20 degrees Fahrenheit below zero, the charge voltage can spike up to 14.9 to 15.8 volts for a short time. On a hot engine on a hot day, the normal charge voltage can drop to 13.5 to 14.3 volts.

HOW TO CHECK ALTERNATOR AMPERAGE OUTPUT

In addition to checking the voltage output from the alternator, you also need to check the current or ampere output. The amperage indicates how much current the alternator produces at a certain voltage and speed. Not so long ago, an 80 amp alternator was considered a high power unit. Most newer alternator models produce 120 to 155 amps or more. Current output increases with engine speed from approximately 20-50 amps at idle to the unit’s maximum power output of 2,500 RPM or higher (refer to the service manual for exact charge power specifications for your vehicle).

Charge performance can be measured with an inductive ampere probe clamped around the BAT wire (B+) that connects to the alternator. It can also be measured on an alternator test stand at an auto parts store.

The rated power of the alternator can also be specified in watts (i.e. volts times amps). Many alternators in foreign vehicles are specified in watts and not in amperes. The important point here is to ensure that a replacement alternator is rated the same (in amps or watts) as the original so that the charging system can maintain the same power output as before should the alternator need to be replaced. In fact, in some applications an upgrade to a higher wattage replacement alternator may be recommended if the vehicle has had a history of alternator failures or the vehicle has a megawatt aftermarket sound system, emergency or off-road lighting, or other power-hungry electrical accessory.

ALTERNOR DIAGNOSTIC TABLES

ALTERNATOR OVERHEATING

If the alternator is working hard under high load at low RPM (especially in hot weather), there may not be enough cooling to keep the unit from overheating. Excessive heat can damage the windings and/or cable connections within the device and cause failure. This is more of an issue on vehicles where the location of the alternator restricts airflow and cooling.

POOR ALTERNATOR WIRE CONNECTIONS

The alternator may have to work harder than normal if the battery cables, ground straps, or other electrical connections in the charging circuit are dirty or loose. A bad connection increases resistance and causes a voltage drop across the connection. This in turn reduces the current flow through the charging circuit.

The electrical system, after all, is just one big series of loops that carry power from the charging system to the battery and from the battery to all of the vehicle’s electrical accessories and electronics. The way back is usually the vehicle body, which serves as the main ground circuit for almost everything. All power and ground connections must therefore be in excellent condition to minimize resistance and stress on the charging system. In fact, poor ground connections are an often overlooked cause of poor charging and alternator failure.

ALTERNATOR DIODE FAILURES

One of the most common causes of charging problems is the failure of one or more diodes in the alternator. Alternators have six diodes (three negative and three positive) that convert alternating current (AC) to direct current (DC). They are called a diode trio because each negative diode is paired with a positive diode.

When the engine is running, charging current flows from the alternator through the diode trio via the BAT (B+) connection on the rear of the alternator. A small amount of current also flows through the charge indicator light circuit. On GM alternators, the indicator light circuit is terminal 1. On European alternators, the indicator light circuit is typically 61 or D+. It is usually marked L on Asian alternators. This connection goes to the ground side of the alternator warning lamp. When the alternator is charging, the trio of diodes supply voltage to the ground side of the indicator lamp. This will balance the battery voltage applied to the positive side of the light, causing the light to turn off as soon as the engine starts. When the alternator stops charging, current flows from the positive side through the light circuit, causing the charging system warning light to illuminate.

If one of the diodes fails, it can cause the charging system indicator light to glow dimly. When two or more diodes fail, the light gets brighter. At the same time, the feedback current from the diode trio reduces the alternator’s ability to produce current. The more diodes that fail, the less power the alternator produces.

A poor connection or open circuit between the alternator output terminal and the positive battery terminal will force the charging current to follow a parallel path through the trio of diodes and out of the alternator. This higher than normal current flow through the diodes causes them to overheat and fail. So if you have previously replaced an alternator for bad diodes and the replacement fails for the same reason, there is likely a bad connection or open circuit between the alternator BAT (B+) terminal and the positive side of the battery circuit. Perform a voltage drop test to check the entire circuit.

ALTERNATOR CIRCUIT VOLTAGE DROP TESTS

With the engine idling, touch one lead on your voltmeter to the positive (+) terminal of the battery and the other lead to the BAT (B+) terminal on the alternator. Ideally, the voltmeter should read less than 0.2 volts.

If you see a voltage reading greater than 0.2 volts, it means there is excessive resistance somewhere in the circuit, causing a voltage drop in the wiring. Check all cable connections (use electronics cleaner to clean the connections) and make sure the terminals on the ends of the cables are clean and tight.

A negative side ground circuit test is performed by placing one voltmeter test lead across the alternator housing and the other test lead across the battery negative terminal (not the battery post) with the engine running and the charging system charged. If good, the voltage drop should be 0.2 volts or less. If higher, check and clean all ground connections as necessary. Also check for broken, loose, or missing ground straps between the engine and body.

If the alternator output circuit and ground circuits are good (less than 0.2 volt drop) and the vehicle has a history of repeated alternator failures due to blown diodes, check for a shorted check engine light terminal.

Checking for voltage drops in a circuit is a good way to find hidden problems that may be causing a charging problem. Voltage drop tests must be performed while the engine is idling with a charge load on the system. In other words, voltage must be flowing through the circuit for the voltage drop test to detect a problem. Voltage always follows the path of least resistance. So if the connection under test has too much resistance, some of the voltage will flow through the voltmeter and produce a small voltage reading.

ALTERNATOR VIBRATIONS

Loose alternator mounting bolts and brackets can cause vibration that can damage the alternator. A bad belt tensioner can also be another source of harmful vibration (this is why the tensioner should always be checked when changing a multi-ribbed belt).

A cyclical buzzing can indicate a generator bearing failure or a bad diode that is causing current to flow in the wrong direction. In any case, the alternator must be rebuilt or replaced.

ALTERNATOR REPLACEMENT TIPS

See the related article on replacing an alternator.

Have your old alternator test stand checked. Alternators have one of the highest warranty return rates of any component in a vehicle. Many devices are returned unnecessarily, either because of faulty diagnostics (there was nothing wrong with the original device or the replacement device) or because an overlooked problem led to a new failure. One way to reduce this problem is to take your old alternator to an auto parts store with an alternator test stand and have it tested BEFORE purchasing a replacement. If the old alternator tests poorly, you will need to replace it. But if tested well, the problem is something else in the charging system.

Most auto parts stores have an alternator test stand. Have your old device tested to see if it’s good or bad.

If your alternator tests fine, then the problem isn’t with a bad alternator, it’s something else.

Also have the NEW alternator dyno tested. For added peace of mind, you can also ask the parts dealer to bench test the new or remanufactured alternator they sell you to make sure it is charging properly. Better to catch a defective device in the shop than after installing it in the car.

Check the wiring harness and terminals. One way to minimize the risk of premature failure and unnecessary warranty claims is to always check the charging circuit connections for resistance (voltage drop). This includes both the positive and negative battery cable connections, the alternator power circuit and the ground circuit as just described.

Voltage drops on the positive side can lead to undercharging.

Voltage drops on the negative side can cause overcharging (tricking the voltage regulator into thinking the battery is low).

Use a battery charger to charge the battery. Alternators are designed to maintain battery charge, not to recharge a dead battery. Therefore, if the battery is dead or dead, it should be charged with a battery charger before the vehicle is driven or before a replacement alternator is installed. This minimizes the load on the charging system and reduces the risk of overheating and failure.

Test the battery to make sure it’s still good. The condition of the battery should be checked whenever it is unable to hold a charge or a charging problem is suspected. The problem may be an old battery that needs replacing, not a bad alternator.

Obtain the correct alternator pulley. Make sure the pulley on the replacement alternator is the same as that on the old unit. Many newer models of alternators are now equipped with a freewheeling decoupler that allows the alternator to temporarily disengage from the belt drive when there are sudden changes in belt speed. This reduces noise and harshness and extends the life of the serpentine belt. Installing a replacement alternator with an ordinary direct drive pulley can result in premature belt failure. Visit www.decouplerpulley.com for more information on this topic.

Replace the V-ribbed belt. If the serpentine belt has more than 50,000 miles on it, throw it out and replace it with a new one.

Check the automatic belt tensioner. If the automatic belt tensioner becomes rusted, weak, or seized, it will not hold the serpentine belt properly tensioned and may slip.

MORE ALTERNATOR CHARGING CONTROL

* On some GM vehicles, a voltage drop of up to 0.5 volts on the positive side may be acceptable. Check service specifications.

* If a battery continues to die and the charging system appears to be operating normally, the problem may be a higher than normal parasitic electrical load on the battery when the key is removed. For most vehicles, normal battery current draw should be 50 milliamps or less. But for some newer Ford models, normal current draw can be 300 to 400 milliamps, with some drawing as much as 850 milliamps for up to an hour after the engine is turned off (modules are in standby mode during this time). However, after all modules have shut down, the battery current draw should drop to 50 milliamps or less.

Peak loads and prolonged idle conditions can lead to battery drain as the alternator cannot keep up with the current draw. Prolonged idling with the lights, defroster, heater, and radio on can draw more amps from the battery than the charging system can put back in. You may think you have a charging problem, but there is nothing wrong with the alternator.

Toll-Related Callbacks

Chrysler Alternator Failure Recall – October 2014

Chrysler issued Recall #106345 for alternator failures on the following models: 2011-2014 Chrysler 300, Dodge Charger, Challenger, and Durango; and 2012–2014 Jeep Grand Cherokee (manufactured April 22, 2010 through January 2, 2014) with a 3.6-liter engine and a 160-amp alternator. According to Chrysler, the alternator can fail without warning, causing the battery to die and/or the vehicle to stall. In the affected vehicles, the alternator can suddenly fail.

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Do you need information from the workshop manual for your vehicle?

Introduction Do not test an alternator by disconnecting the negative battery cable. Instead, use a voltmeter for a simple, safe test.

If you have a newer battery but your car won’t start, you may have a bad alternator. If you’re tempted to test an alternator by disconnecting the negative battery cable, don’t do it. While a good alternator can keep the engine running without a negative wire, that has never been a good test.

In the days before computers, you could pull it off without damaging anything. Nowadays you risk burning every electrical device in your vehicle. The second you disconnect the battery, the voltage regulator triggers the alternator to deliver maximum power. Without a backup battery in the circuit, the alternator can deliver up to 150 volts, depending on the engine speed. When the smoke clears, this “simple test” could end up costing you several thousand dollars in new electronics.

In this video you will learn how to properly and safely test your alternator.

Click here to read more

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Like most other systems in newer model vehicles, charging systems have become smarter and more complex. Today’s computer-controlled charging systems not only adapt the charging current to the electrical requirements of the battery and alternator, but also to changing driving conditions. This makes diagnosis much more difficult if something goes wrong. Alternators have one of the highest return rates of all repair parts – often due to misdiagnosis. Dyno testing an alternator on a dyno should verify whether its performance is within specification or not. If the unit tests poorly, your customer will need a replacement alternator. But if the unit is testing well, the problem is something else like a bad voltage regulator, PCM, or wiring harness. Loose, corroded, or damaged wire connectors on the back of the alternator are common causes of charging problems. Cable connectors and terminals may appear fine on the outside but have loose, corroded, or broken wires on the inside.

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Other sources of trouble include loose, corroded, or damaged battery cables and ground straps, blown fuses in the power center, or a blown fuse in the wiring. Another source of error can be miscommunication or lack of communication between the PCM and the alternator or regulator. You can have a good alternator that can produce the charging voltage and current you need, but it may not work properly if it’s not communicating properly with the PCM. Another problem that is sometimes encountered is that some “frugal” reman generators listed for a specific application are not fully compatible with the charging system controls. The alternator can engage directly, but will not communicate with the PCM and prevent it from charging normally.

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Another element that can affect the performance of an alternator is the pulley. Many newer vehicle models do not use a solid alternator pulley. Instead they have an Overrunning Generator (OAP) or Overrunning Generator Decoupler (OAD). An Overrunning Pulley (OAP) has a one-way clutch mechanism in the hub that allows the belt to turn the alternator in one direction, but allows the alternator to freewheel and spin at its own speed if the engine suddenly slows down. The pulley should lock when rotated in one direction, but free when rotated in the opposite direction. If the internal clutch mechanism is bad, the pulley may not drive the alternator or remain locked all the time, causing noise and vibration. An overrunning decoupler pulley (OAD) also has an internal one-way overrunning clutch as well as an internal torsion spring to further dampen vibrations in the belt drive system. The spring acts like a shock absorber to cushion the hub.

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