O.K. Here is my question. On a vehicle with an alternator light (GM),power from the accesory side of the ignition switch goes through the alternator light bulb to the field terminal of the alternator. On my 1936 Pontiac with OHC6 and a standard internal regulator Delco alternator, I have to rev the engine past 2000 RPM to get the light to go off. Why? This is not the first time I have run into this. My 54 GMC is the same way. I have been a mechanic for a lot of years and I have seen this several times and I don't know the answer. Also, changing the regulator dosen't help. I hope there is someone out there smarter than me. Thanks, Joe

Sounds like you have a pulley ratio that turns the alternator slow.

The "coming on speed" of the alternator is a relatively fixed speed at the alternator. If you have the pulleys to where the alternator is turning slower (relative to the crank) than oe setups it will take more engine (crank) RPM to bring the alternator on line and extinguish that light.

If it's causing a problem you can use a smaller diameter alternator pulley, or a larger diameter crank pulley.

From the sound of it, you don't need one that's much different than you currently have.

Do you have a diode in the wire to the alternator it needs to have 12 volts to start charging at idle, if not it will not start to charge until the RPMs reach a higher speed usually between 1200 and 2500 depending on pulley size. Also without proper voltage they will stop charging if the RPMs drop below a certain point usually lower than the start RPMs.

It's not the alternator speed, because I can hook up a 12V wire to the alternator and it starts charging immediately. None of the factory wiring diagrams I've seen shows a diode. It is hooked up the same as the fatory wiring. Joe

I had this very problem on a car of mine and it had an easy fix.

The alternator uses the dash idiot light to tell it when the car has been turned on and the alternator should produce current. It seems to be somewhat sensitive to the amount of current through the dash light circuit. I substituted an LED lamp replacement for the light bulb, and the super low current consumption of the LED wasn't enough to "start" the alternator! I too had to rev it to 2000 or so. It was very repeatable and predictable. Once I'd revved it and the lamp went out, it would charge correctly, even down at 700 rpm idle.

Switching to a higher-current dash lamp ought to fix it. Sounds silly, but easy to do!

I'll try it. Joe

The GM internal regulator has two wires in addition to the large output lead. The smaller of the wires on the plug connects through a light (which serves as both a resistance and as a indicator) and serves only as a signal to the alternator to turn on.

The larger plug wire is connected to unswitched 12V and serves as a source of power for the field when alternator RPM is low. An internal shunt power source within the regulator provides field power when RPM is high, which allows the unit to generate even if the field source wire is not connected.

Not enough power can pass through the dash light to actually power the field, so the regulator is designed to separate the signal function and the source function into two different wires. Both are needed for correct operation.

It would be possible to bootstrap the field source wire to the alternator output, but GM has never done this; I suspect that a few feet of wiring is needed to filter out the switching noise of the rectifier set to preclude the regulator going nuts. The field source lead can be connected to a switched 12V source, but is not neccessary as it is blocked by diodes in the regulator.

I assume you are talking about a 10SI or 12SI alternator. You looked at the wiring diagram for a '70s car and you did not and should not have seen a diode. You should have seen a resistor. In most cases it is a resistor wire, much like a ballast resistor wire but without the cloth insulation. Sometimes the wire is just listed as "Res. wire" with no symbol.
The resistor serves two purposes. It allows more current to flow to the Alt so that it begins to charge without reving the engine. The second purpose of the resistor is in case the bulb should burn out, the exciter circuit continues to function.
In the original wiring, one side of the resistor is connected to a Acc. connection on the ignition switch. On most retro-fits on older cars, you don't have an extra connection on the ignition switch.
Go to Radio Shack and get a 10 ohm, 10 watt resistor and wire it in parallel across the Gen lamp.
This will duplicate what GM did and your problem will be solved.

If you look at a wiring diagram for the older 10DN external regulated alternators and you will see the same resistor in the exciter circuit.

On the 10si type GM alternators, one the two smaller connections are not quite as described above...

The 3rd wire is not field power, it's the "sense" line -- the alternator measures the battery voltage with it. This compensates for voltage drop in the fat charging wire; the alt will raise it's output as necessary such that the voltage at the sense terminal is the set point voltage (14V nominal, varies with temperature I think).

The alternator would work fine if you just jumped it to the big terminal, but if you had voltage drop in the wire(s) between the alt and the battery, the battery wouldn't get full charge.

The sense wire should be as close to the battery "+" terminal as possible, electrically-speaking.