Triumph Repair: Ignition light delay, bosch alternator, lucas alternator

Question
Jim, I replaced the original Lucas alternator with a Bosch drop-in replacement in my '79 Spit. It works beautifully except that the ignition light stays on for a good while or until I rev the engine. When it finally goes out, it stays out. I read here previously that the light stays on until the alt output matches the battery voltage but if the alt output drops below the battery voltage, wouldn't the light come back on (say, at idle?). I just don't understand the long delay.

Answer
Hi Tom,
The connection to the "IGN" light is the "exciter" circuit that gets the alternator to start charging. I can't explain the delay but if one quick rev of the engine puts the light out you should do that each time. Also make sure your belt is tight as that is a normal symptom on the Lucas that the belt is loose.

Also look at the diameter of the pulley on the Bosch verses the Lucas. If the Bosch pulley is larger that would account for the delay. If the light keeps coming back on at idle I would contact the supplier of the Bosch alternator to see what they have to say.

However, if the light does not come back on at idle after the initial engine rev, and the charge rate is OK (13.8v to 14.5v @ 1500 RPM) I wouldn't worry about it that much.

Howard