75/76 Alternator Light, Trying to understand an ignition feedback issue. Options

[McMark]

I thought all 914s had a diode on the charging light, but looking at the 74/75/76 diagrams I don't see one. A certain green car doesn't shut off when you turn the key off, but disconnecting the charge light wire kills it.

[jcd914]

I am avoiding writing a convoluted e-mail at work right now, so this is longer that necessary. (IMG:style_emoticons/default/smile.gif)

If you had a diode in the charging light circuit the light could work as it should.

Normally you have current flow from the ignition through the charge light through the alternator fields (excitation charge) and to ground when the key is turned on.
When the engine starts and the alternator starts charging the voltage at the charge light connector on the alternator (blue wire) matches the voltage in the output of the alternator which matches the voltage of the battery and current quits flowing and the light goes out.

This indicates normal charging, light out while engine runs.

If the alternator output drops and voltage drops below battery voltage, current again flows from the ignition switch through the light and the charge light glows.

This indicates charging problems, in this case low alternator output.

If the alternator starts over charging, regulator failure or inter short causing over charge. Voltage at the alternator output and the charge light connection (blue wire) goes up higher than battery voltage and current flows from the alternator through the charge light and back to the battery (sort of) through the ignition switch. And again the charge light will glow.

This indicates charging problems, in this case over charging.

If there were a diode in the charge light circuit current could only flow one way and the light could only indicate under charge or over charge not both.

Jim