QuestionI have a 1980 spitfire. The Ign light is on but the car is charging the battery (13.2 to 15 volts) The car has a lucas alternator. what could the problem be?
AnswerHi Frank,
The "IGN" light is a charge light and it is powered when you turn on the Ignition key and the bulb is grounded by the regulator inside the alternator which is a ground when the Alternator is not turning. As soon as the engine starts, the flat spade connector on the regulator becomes positive and with a positive on both ends of the light, it goes out. That is how it is suppose to work.
On your car when the light is on when the engine is running but a volt meter says the charge rate is ok, you need to do a couple of simple tests.
First turn the key to Ign. Is the light "on"? (it should be on) leave the key on and remove the small spade connector on the back of the alternator (Brown w/yellow tracer wire). Did the light go out? (it should go out) If it did not go out, you have a short in the wiring somewhere on that brown w/yellow tracer wire.
If it did go out, and the light stays on when the engine is running, you have a bad regulator if the charge rate at the battery is 13.8v to 14.5v at 1500 RPM.
Almost all of the Lucas alternators work this way and I have tested many over the years I worked in dealerships. I did run across one that was different. I tested (at the battery) 14v at 1500 RPM but the light stayed on and after running several tests I found that the alternator had a leaking diode not yet failed and it was allowing a small amount of AC current to run into the system which kept the Ign light on dimly. I found it by putting my scope on the charge light spade and on screen was AC current which should not be there.
Most of the time a failed alternator or regulator will cause the light to stay on and the charge rate to be below or above specs. If your voltage test was at idle and you got 15v you need to run the voltage test again at 1500 RPM
Howard
