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Does somebody out there knows how to test generator output with a multimeter? I believe mine is not putting out.
Thanks,
Frank

Multimeter isn't the most useful tool to check a generator, you really want an ammeter in line with the output to check, one that goes to atleast 35 amps. Old Motors and other manuals describe good proceedures.

About all you can check with a multimeter is for a voltage increase at the battery as engine rpm increase. For example for 6 volt, the battery at rest should be about 6.6 volts; at about 1500 engine rpm with a load on (put on the headlights) you should get at least 7.2 to 7.6 volts. If you take the generator apart, you can use a multimeter to check for grounded fields or a shorted armature segment.

Put the multi meter on the battery and see how many volts you have. Let someone start the engine. When the starter is going it should suck the voltage down. When it starts it should come back up and possibly go over what the unassisted battery had. If you rev the engine to equal a road speed of 30 in high voltage should definitely come up, to 7.2 on a 6 volt car or up to 14.5 volts on a 12 volt car. Then it should level off no matter how fast you rev it.

There are other tests you can do. For example if you put a 12 volt jumper to the field when running the voltage should go higher than 14.5 caution don't do this too long.

This will tell you whether the generator is working well, if it is a question of whether the generator or regulator is at fault.

Your old repair manuals should have a chapter on electrical testing for the more intricate diagnosis.

If you want to just see if the generator is doing anything at all, you can start the car and then disconnect the battery. The generator should keep the car running without relying on the battery.

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Yes, Just measure the terminal voltage at the battery. With the engine shut off the battery, with the surface charge drained, will read pretty much 6.3 volts. The fact that 3 lead acid cells puts out 6.3 volts is why the filiment voltage for all 6 volt vacuum tubes was designed for this(6.3 volt) voltage. With the engine running at say 1500 RPM+ it should read between say 7 and 7.4 volts. If the Battery terminal voltage is much above the 7.4 volts it's too high. If you hook the meter to the battery and note the voltage then start the car and run it up the battery voltage should slowly climb up to as much as 7.4 volts. Any rise of .5 or more means the battery is charging.

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I disagree with this. A quick check of the battery terminal voltage will tell you if the battery is charging or not and you don't have to unhook anything.


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This works as a go no go check but you really can't tell much about the charging rate. The generator/regulator may be putting out just enough to keep the car going and still alow the battery to run down. Maybe to further this test you should turn on everything that there is with the battery disconnected.

Owens recommendation to check amperage is the best idea. The generator exists to keep the battery charged.

The best cursory initial test is to start the engine and then disconnect a battery terminal. Connect an ammeter between the battery post and cable that was just disconnected.

Now, watch the ammeter as accessories are turned on. Mite have to rev the engine up or set idle a bit high.

Ammeter should always show a charge or break even. Should not show a discharge.

If ammeter shows a discharge then go from there to more invasive or difficult tests with ammeter and voltmeter at generator.

Again, the generator exists to keep the battery charged.

BTW, just about any 6v or 12V automotive generator will produce as high 60 to 80 volts. It depends on how fast the generator is turning. So using strictly a voltmeter for testing does not indicate charge or discharge of the battery (direction of current).

Voltmeters are often prefered only because they are easier to connect than an ammeter.

Actually the generator exists to run the electrical in the car when the motor is running. The battery is there to start the car, and to buffer the generator at low idle or when the engine is not running.

Thank you for all the replies and information. I removed the generator to examine the brushes and found one of the springs that holds the brush against the armature was broken and not making contact. Trying to find a replacement will be a real adventure. I also found out that there are no shops locally that can test a generator anymore.
Frank

We're all glad you found the apparent problem, and I'm a bit surprised at no generator shops in your area, they are fewer in number here in NJ, but still one every few towns and most of them have piles of generator carcassas to take a spring from.

Though it might be a bit pricey, there are plenty of places you can send it to for a thorough rebuild, AceWichita comes to mind, but there are others - check your Hemmings Motor News to find some if you wish to go that way. At the least, while it's out of the car, I'd check/replace the bushing and bearing and most importantly check the armature for shorted segments, and a possible armature turning.

Also, generator carcassas seem to plentiful at flea markets for $5 or so, and I guess that Delco used many of the same internals for years in different models.