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How To Test an Alternator Using a Voltmeter


This video shows how to check for correct voltage at the alternator and diagnose if a diode has gone bad internally.

In the 1960’s, our Classic British cars started coming with alternators. That was a quantum leap. Alternators were less expensive to build than generators, more effective at low RPM’s than generators, smaller than generators, lighter than generators, required less maintenance than generators, and came with their own voltage regulator. All this created a dichotomy. Looking at the differences between generators and alternators, we find the stator windings and the armature windings have swapped roles. The voltage regulator lives inside the alternator. Generators never had a part called a diode bank (or a rectifier). Alternators do, and it also lives inside the alternator. This is a better package, but it means we need to relearn how to test our charging system. That relearning leads to a pleasant surprise. You can test virtually everything you need to test on this with a volt meter.

A person might sound surprised and ask, ‘How am I going to diagnose and service something like this without a box of tools and test equipment?’ The question is valid, but it overlooks the realities of our modern world. With the cost of labor, and the difficulties of finding original internal replacement parts for the original alternators, it’s usually faster, and less costly, to simply replace a defective alternator. So, all we really need to know is this; is my alternator faulty or not? We can determine that with a volt meter.

Let’s do this. We suspect our charging system has a problem. Maybe the headlights are dim. Maybe we had a dead battery this morning. Alternator is in the car. The wires and cables are still connected. If your meter has a dial to set a voltage range, set it to 20 volts DC. If your meter has buttons to set the voltage range, again 20 volts DC. If your meter automatically sets its own range, be sure it’s on DC and let it work on its own. Connect (piggy-back) the red lead of your meter to the big lead on the alternator where the current comes out. Connect the black lead from your meter to a good ground. That’s it. Two wires.

Because your alternator is connected directly to your battery, your meter will be reading battery voltage. That will be about 12-and-a-half volts. Start the car. Bring the RPM’s to about 1500. The meter should be reading in the 13 volt range to the 14 volt range. If she does that, she’s fine. A fully charged 12 volt battery should yield 12.6 about volts. So, a fully charged battery in your car that’s neither charging nor discharging will be around 12.6 volts. When we star the car, that will change. Meter still connected as above. When your engine is running, the ignition system is drawing power from the battery. If the car is running and you are seeing 12.2 to 12.4 volts, (that’s less than 12.6) that usually means the alternator is doing nothing. The car is running off the battery. If the battery is showing more then 12.6, the alternator is involved and charging. However if it’s not up into the 13 or 14 volt range, it usually means the alternator is weak. We’ll address that in a moment. Imagine: If you get a reading (at alternator) in the 13 to 14 volt range, the alternator is doing its job. However, if we had a reason to be concerned about our alternator in the first place, there may still something wrong someplace else. (Lights weak.) Leave the car running. Take note of your voltage at the alternator (13.8).

Remove your meter from the alternator. Now, connect your meter to your battery. Remember, the alternator is connected to the battery. The voltage reading at the battery should be very close to what you have coming out of the alternator. If it is, you are fine. If it is not (12.8), you probably have a dirty or damaged connection between the alternator and the battery.


  1. If the alternator is not putting out enough voltage in the first test, the alternator has a problem.
  2. If the alternator is putting out enough voltage, but the battery is not getting it, cable issue.
  3. If we have a cable issue, a Voltage Drop test is an easy way to find the problem using the same volt meter.


  1. An old test for generators was to disconnect the battery with the motor running. This made perfect sense. If the generator was making electricity, the motor would run on that. So if we disconnected the battery and the car kept running, the generator had to be making electricity. DO NOT DO THAT to a car with an alternator. The voltage regulator inside the alternator can be damaged by doing that test. Use the test I just gave you with your volt meter. Fast (two wires) and it’s safe.
  2. Lot’s of people have pulled an alternator from a newer car and put it in their Classic British car. The belief is that alternators for newer cars produce more current. They do. A newer car has computers, electric sensors, air conditioning, rear window defrosters, power windows, and lots more that all need power. They need a powerful alternator to feed them all. The original alternator in your Classic British car probably produced 35 to 45 amps. An alternator from a newer car can produce 60, 70, or well over 100 amps. The alternator in the car belonging to this technician’s wife is rated at 200 amps. Putting that excess power into your classic car leads to two problems. First, the wires in your car were never made to carry more than a certain amount of current. When excess current is introduced, things can get hot and damaged. Second, an alternator takes a lot of power from the engine to make electricity. If you put a high output alternator into your classic car, the power drain can be so significant there will be times when it will feel like you are towing a boat. If more powerful alternators are so power hungry, how can small modern cars use them? Computer control. The computer can turn lots of things off when it senses you need more power. Classic car can’t.
  3. Lastly, the alternator makes alternating current (AC). Our car wants and needs direct current (DC). The diodes in the alternator filter the current so only DC gets out to the car’s electric system. If AC leaks out of the alternator, the battery might not be able to charge as it should. A failed diode can let AC leak out. This can be a little tricky to diagnose, because your alternator has three current generating windings in it. If one of the diodes goes bad, you lose the benefit of one of the three windings. The other two windings are still functioning, so you can run the car with the crippled alternator. Because it’s still charging, the red light won’t come on. However, it produces 1/3 less current than normal. In these classic cars the electric load on the system is low. Unless you drive with your lights on, a car whose alternator has a bad diode might run fine except for a possible battery charging issue. How do we test for a failed diode? Get your volt meter again. Connect your volt meter to your alternator. Just like before. Set your meter to read 10 volts AC. 5 volts AC would be better. If your meter can’t go that low, 20 volts AC will serve. Start the engine, bring it up to about 1500 RPM’s. Turn the lights on. Alt working. Note: we set our meter to AC. We usually use DC when testing our car. By selecting AC, your meter will ignore the DC voltage and focus on AC. If AC is leaking out, your meter will see it. The diodes are supposed to block the AC from getting out of the alternator. If a diode goes bad it will let some of the AC get out. For the simple alternators in our classic cars, anything over 0.5 volts AC is too much. You will probably see a little AC. The diodes are not perfect, so a small amount of AC will always get out. Sensitive meter. Typically, it’s 0.010, 0.020, or 0.030 of a volt. Again, check with volt meter.

Today We Learned:

  • Different technology and a different era call for different testing.
  • Volt meter. Ideal tool.
  • No disconnecting the battery to test.
  • Your volt meter will tell you if your alternator is working as it should.
  • Your volt meter will tell you if the alternator’s electricity is not getting to the battery.
  • Your volt meter will help you do a voltage drop test if you have a cable problem or a dirty connection.
  • Your volt meter will help you diagnose a failed diode.
  • All we need to know is if the alternator is functioning as it should or not. Your volt meter will do that.

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