How to check car alternator
Every day I see auto forum participants ask whether their no-start problem is due to a bad battery or bad car alternator. The answer isn’t that simple because a bad car alternator can cause a battery to be low on charge and you can’t test an alternator when the car has a discharged battery. Start by understanding how an alternator works
How does a car alternator work?
Older generators operated by rotating a coil of wire around a magnet to produce electricity. As car makers began installing more electrical accessories, they found that a generator couldn’t produce enough power to keep the accessories running while also recharging the battery. That’s when they switched to an alternator.
An alternator rotates an electro-magnet inside a coil of wire to generate electricity. To vary the amount of electricity an alternator generates, the voltage regulator varies the amount of power it feed to the spinning electro-magnet. If you learn nothing else from this article, remember this; an alternator uses an electro-magnet—it can’t make power without first having power to run the electro-magnet. THAT’s why an alternator can’t charge a dead battery—there’s not enough power to run the electro-magnet.
Start the alternator diagnosis by charging the battery
If you read the preceding paragraph, you know that you can’t charge your
battery with the car alternator, so you’ll need a battery charger. Once the battery charger shows the battery has a full charge, you can begin testing the alternator.
Attach a voltmeter to the battery posts
1) Set the meter to read 20-volts DC or less. Attach the red positive lead to the positive terminal on the battery and the black negative lead to the negative terminal. You need at least 12.4 volts to continue testing the alternator.
2) Start the engine and read the voltage on the meter. The old rule was that you should instantly see at least 13-volts. However, late model vehicles incorporate energy saving software that monitors battery voltage and stops the alternator from producing power if the battery is full. So don’t condemn an alternator right off the bat if you don’t see above 13-volts at startup.
3) Turn on loads. Start by turning on your headlights. The meter may dip slightly, but should return to a reading above 13-volts. Then turn the blower on full blast and increase idle speed to about 1,500 RPM. The meter should read above 13-volts. Continue adding loads like rear defogger and seat heaters while increasing RPM to 2,000. Hold the RPMs at that speed for a few minutes and watch the meter (yeah, you’ll need a helper to do that). A good car alternator will maintain at least 13-volts—usually above 13.5. A bad alternator will begin to drop below 13-volts as you add electrical loads and increase engine RPM.
You’ve just completed the first phase of alternator testing. You’ve confirmed the alternator is working and can maintain minimum voltage. However, you still don’t know if it’s causing your battery to drain.
Check car alternator diode ripple
Car alternators produce AC power, but cars require DC power. So the alternator flows the power through one-way “valves” known as diodes or “rectifiers.” Diodes can fail in three ways; they produce an AC ripple that injects AC voltage or “noise” into the car’s electrical system, the diode shorts-to-ground creating a dead short inside the alternator than can drain your battery in about an hour, or they fail in an “open” position that prevents the alternator from producing full output.
The easiest way to check for diode ripple is with the diode ripple test. Connect the positive lead from your meter to the BAT terminal on your car alternator. Connect the negative terminal to the alternator mounting bracket. Set the reading to the lowest AC voltage setting. Then start the engine and rev it to about 1,500 RPM. You should see less than 0.5-volts on your meter. The reading may go as high as 0.8 or 1.0 volts in a heavy duty charging system (like on a truck). A reading higher reading indicates a bad alternator.
Check for diode short to ground condition
Since a short to ground diode drains the battery, you’ll want to check current flow with the engine off. To do that you’ll have to use either an inductive current clamp or install your meter in series with the negative battery cable. If you choose the latter method, follow this procedure:
Remove the negative battery terminal. Then remove the wire connected to the alternator BAT terminal. Install your meter in series between the BAT terminal and the wire you just disconnected from that terminal. Move the positive lead at the meter to the mA port, while leaving the negative lead in the COM port. Then switch your meter to mA.
If you have a push button start vehicle, take your fob into the house so it doesn’t keep the electronics running. With the engine off, all electrical accessories off, keys out of the ignition and doors closed, you should see a reading of less than 0.5-millamps. If you see a higher reading, leave the connections in place and check again in 30-mins when all the computers have entered sleep mode. If the reading is still above 0.5-milliamps, chances are you have a bad diode and the alternator is bad and causing the battery drain.
What should you expect for alternator replacement cost?
Alternator replacement cost varies depending on make, model, and engine, where the alternator is located in the engine compartment, and whether you choose a new or rebuilt alternator. Here are some examples:
2010 Audi A4 Labor time .9-hrs. Audi rebuilt alternator $600
2010 Chevrolet Malibu Labor time 4-cyl engine 1.6-hrs. New alternator $568 6-cyl engine Labor time 1.0-hrs. New alternator $280
2010 Toyota Camry Labor time 1.1-hrs. New alternator $655
As you can see, the alternator itself is expensive, far more than the labor cost. If you choose an aftermarket alternator (not through the dealer), you can dramatically reduce alternator replacement cost.
Alternator replacement cost using aftermarket new or rebuilt alternators
2010 Audi rebuilt alternator $150 to $225
2010 Chevrolet Malibu New TYC alternator $200, Rebuilt alternator $150
2010 Toyota Camry Rebuilt $200
©, 2016 Rick Muscoplat
Posted on by Rick Muscoplat