[Trak Ratt]

Originally Posted by jazzbass

basically there is not enough excitation current to the alt at low RPMs

What do you mean As previously stated alternators require outside stimulation before they can produce the correct response. Should I have touched briefly on how the whole system is based on switching back & fourth between AC/DC

[BlackTalon]

Originally Posted by david riley

What do you mean As previously stated alternators require outside stimulation before they can produce the correct response. Should I have touched briefly on how the whole system is based on switching back & fourth between AC/DC

By "switching back & forth", do you mean "alternating"?

[Jazzbass]

Originally Posted by david riley

What do you mean As previously stated alternators require outside stimulation before they can produce the correct response. Should I have touched briefly on how the whole system is based on switching back & fourth between AC/DC

Right... not sure what the question is - I think we're saying the same thing. The excitation current is the outside stimulation. An alternators is a 3 phase AC generator, in essence, with a set of diodes that rectify the AC into DC. The stator of the alt is what produces the 14V+ of AC (then turned into DC) the car runs on.

The rotor needs an excitation current of up to 7A to produce the rotating magnetic field necessary to induce the current on the stator. Once the alternator is spinning high enough, it will feedback current from the output to "self-energize" the rotor. However, this feedback condition needs a "bootstrap" current to get started, and this is the current supplied through the alternator warning light. This is the reason why your system will not charge if that light is burned out. The issue with the older cars was that the resistance of the bulb was too high to produce enough pre-excitation current. So, putting a resistor in parallel with the bulb lowers the overall resistance of the pre-excitation circuit, producing more current (because, of course, I = V/R).

[Jazzbass]

BTW, here is a thread from Pelican that describes the resistors needed for early cars that have the low pre-excitation current issue:

http://forums.pelicanparts.com/showt...light+resistor.

BTW, if your 912 has a generator, then none of this alternator crap applies. Like David said. generators don't need excitation currents.

[DonNewton]

The poor guy's ears are smoking now. . .

[Eli]

Jim is correct, it's most likely your belt tension. The belt should only give about 3/4th of an inch under slight thumb pressure.
I have the same problem with my '66 912. Don't have a 36mm socket to fit the generator pulley nut . Have been too busy to go pick one up. Probably will this week....

Originally Posted by Jim Richards

The prior owner lied. That light comes on when the charging system isn't doing its job. Maybe you just have a loose fan belt, or maybe it's more substantial. Best bet is have it checked. Or, you can monitor your battery voltage with the engine running. I don't know the specs for your car, but it should be 13.something volts or more.