VCCA Home Forums TECHNICAL - GENERAL Electrical - 12 volt Alternator Output requirements

I have a 1934 Chevy that was converted from 6V to 12V. I'm trying to determine if the alternator is adequately functioning. What should be the minimum required amp output from the alternator I need to have for the truck to run correctly?

It will depend on the accessories and load from the electrical units (lights-heater-radio etc. ) that are installed on the car. I read online that it only takes a couple of amps for the ignition system to function properly.

The 37 Amp alternator was standard equipment in 1963, first year to have an alternator as standard equipment on most of the Chevrolet cars.
That was adquate for a car with four head lights and a host of other eletrical accessories. The air conditioned cars and optional heavy duty was a 61 amp.

Excuse me for being 'electrically challenged', so with the engine at idle the output from the alternator could be 3-4 amps?

The "book" says 9 Amps.

THANKS!!

I think it says up to 9amps It all depends on the load you asking from it. On a fully charged battery, you should have a 14.4- 14.6 volts @ the battery @ 1000 rpm. Your 12-volt battery has 2.2 volts per cell giving you a fully charged 12-volt battery13.2 volts with the engine off, so to charge it you need one or more volt going in the battery to keep up with the load. I would check the battery with the engine off look for the 13.2 volts. Then with engine on 14.2 -14.5 volts then pull all you can out with the lights radio and anything else you have on the truck. Each time you pull more load with the accessories the alternator should kick up its output as well as fall back as you decrease the load. An alternator putting out too much can be just as bad on the battery as well.

All good advice so far, and correct, but it can be a bit misleading. The minimum current (amps) output you need is whatever current the stuff on the car is currently drawing.

If the battery is charged (make sure it is before testing), and the alternator is keeping up, with the engine revved up to a fast idle or more, expect 14.2 volts (really old regulator) or 14.7 volts (most regulators you will actually see today on GM alternators).

These voltages are approximate, as regulators are temperature compensated. This is checked from the post on the alternator to the alternator case. Also check it at the battery posts. Right on the posts, in the middle. If there is a bunch of difference (loss), clean your terminals and/or fix your charging system wiring.

To see what the alternator can do for current output, put an ammeter in *series* with the alternator output terminal (current flows through it to get to the charge wire), or clamp around the wire at the output terminal if you have a clamp on ammeter.

Connect a load tester to the battery. Run the engine speed up to a fast idle or higher. Hit the battery with more load than the alternator could possibly be rated for. Read the ammeter. Release the load. The ammeter reading you got is the alternator maximum output. If it closely matches what the alternator is rated for, there is probably nothing wrong.

You can also do this trick at idle. The thing is alternators don't typically have full capacity at idle. They pulleys are set up so there is some output at idle, but not all. At least thats how it leaves the factory. That is probably the 9 amps Chev Nut referred to. You don't get full capacity until you rev it up a little.

On a converted car though, the pulley sizes may not be ideal. It might not be spinning as fast at idle as the alternator designer intended. Thats also true of any car that has had a replacement alternator from the parts store. The pulley on it is probably whatever the rebuilder had in the barrel.

If you only get 3-4 amps with the load tester loading the battery at idle, instead of the 9, and everything else is OK, I wouldn't worry about it even if its wrong. Maybe your lights dim a little at stoplights. Maybe get a faster pulley if it bothers you.

Keep in mind that without the load tester dragging the battery down, you will only see as many amps as car and battery ask for, and that is way less. Even the smallest GM alternator (like that 37 amp unit Chev Nut mentioned) is WAY more than a 34 Chevy will ever need.