Alt wouldn't charge. Fixed, but why?

[regnihe]

87, 911 race car. Late last season we pulled most of the factory wiring, gauges, basically everything we didn't need. In our last race the car ran fine Saturday. Sunday, during the enduro, we started getting weird electrical gremlins. Radio would cut in and out, AIM system started going out when I hit the brakes. Car would not start at the end of the race. Using an Antigravity battery so we hit the magic button and it started. Last month we started poking around and found the alternator not charging. Changed alternator and same thing.

After 3 hours we figured out we had pulled "the bulb" from the gauge in the dash when we pulled all the wiring. Apparently this bulb is required in the system. Something about it allowing the polarity of the regulator to behave.

Can anyone explain how this special bulb works in this system?

[ammonman]

The bulb creates the excitation current necessary to energize the rotor winding electromagnetic field. Once the electromagnetic field is established in the rotor winding and the alternator starts producing current it is a self-sustaining system. Without the bulb load to start current flowing through the rotor winding the alternator never outputs any current. You can substitute an appropriately sized resistor to draw the necessary current to "bootstrap" the alternator if you don't want the warning bulb in the system.

[spoke]

To go along with ammonman's description.

This is a 914 charging system but it's almost identical to the 911. W/O the GEN bulb, the alternator does not start up.

[spuggy]

Originally Posted by ammonman

You can substitute an appropriately sized resistor to draw the necessary current to "bootstrap" the alternator if you don't want the warning bulb in the system.

Even the 2W bulb on an '81 & earlier car may not be sufficient for a retrofitted late alternator w/ internal VR; from another thread:

[regnihe]

Thanks everyone. This has been very helpful!