How to test an alternator voltage regulator

How to test an alternator voltage regulator

Wondering if your alternator voltage regulator is why your charging voltage is off, or completely gone? There are several tests out there that call for just using a multimeter to check resistance. But these only work if you have a voltage regulator that uses the exact same internal construction as the regulator the instructions were written for. You can't count on this if the regulator was built differently - and it may be, even if it fits the same car.

Testing a voltage regulator with an adjustable power supply is a more sophisticated way that works regardless of the regulator's internal construction. This method works with almost all external voltage regulators, and even some internal designs. It won't work with regulators internal to an ECU or other ones that require an engine RPM signal to work, but usually if the regulator is a single discrete part, that isn't the case.

You will need the following tools for this test.

  • Adjustable power supply
  • 12 volt test light
  • Alligator clips
Tools for testing a voltage regulator: Power supply, test light, and alligator clips.

You can find a good enough adjustable voltage regulator in the $50-60 range at many sites that sell low cost electronics. With test lights, a LED type test light will have positive and negative sides, while an incandescent light can be connected either way. I will still refer to the wires as positive and negative based on which wire is being supplied positive volts and which is grounded.

The main challenge is figuring out how to wire up the regulator. You will connect the negative wire from the power supply to the ground (often, this is a bolt hole in the regulator) and the positive wire to the battery voltage connection on the voltage regulator.

If the regulator supplies ground to the alternator field, like our V101 voltage regulator, the positive wire of the test light will go to the power supply positive wire, and the negative wire goes to the field output. In the example below, the red clip is on the battery wire, as is the yellow wire connecting to the test light positive wire. The white clip connects to the test light negative wire, and the black clip connects to the power supply ground.

Connecting a power supply to the voltage regulator.
If your voltage regulator supplies power to the alternator field wire, like a 1969 or older Mopar voltage regulator, the test light positive wire goes to the regulator field terminal, and the test light negative wire goes to the power supply negative terminal.
Now, set the power supply for about 14 volts and turn it on. Move the voltage set point up and down. The test light will switch on below the regulator's charging voltage set point, and turns off once the power supply voltage goes over the set point. Often, the voltage will need to come down by a bit more before the light turns back on, to prevent the regulator from switching too fast.
Test light on, showing the voltage regulator has switched its field output on.
If the test light never turns off, regardless of the voltage set point, your regulator is locked full on and will overcharge the battery. If the light never turns on, the regulator output is dead, as long as everything in the setup is wired correctly.
We also have a video demonstrating this test:
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