MG Car Repair: mgb electrical, regulator problem, tracer wire

Question
hello - I have a 79mgb - ignition light goes off when car is started, but stays on when car is turned off - also the headlights and sidelights do not work although the high beam flasher works when lever engaged - I have replaced light switch, but same problem still exists - any help would be appreciated - thanks

Answer
Hi Dennis,

The "IGN" light is a charge light not an ignition light. Normally when the engine is not running the regulator connection where the "IGN" light is connected is a ground. But when the engine is running and the alternator is charging that ground becomes positive so that is what makes the light go off.

In your case there can be two causes to make the "IGN" light come on when the engine is off and the key is off. One is that you have a wiring problem that is making the "IGN" wire hot with the key off and the other possible is that the regulator has a problem and is hot when it is suppose to be a ground. The test is simple, just remove the plug on the back of the alternator and confirm that the "IGN" light goes out and take a 12v test light and probe the wire end of the small brown with a yellow tracer wire to see if it is powered with the key off. If it is powered, you have a wiring problem somewhere. If it is not powered you have a regulator problem. If the "IGN" light did not go out when you unplugged the alternator, you have a wiring problem.

The head lights may or may not have anything to do with the "IGN" light problem. Any time you have several electrical problems you must test and work on only one circuit at a time. Some times when you correct the one circuit it will fix or lead you to a fix for the other but not always.

You CAN NOT hope to fix any electric circuit wiring problem without a wiring diagram and a test light and a system to test.

The system I have used for many years in dealerships is to pick one circuit and with the diagram and a test light start at the load (in your case the "IGN" light). and start testing at each connection working toward the source (in this case the alternator plug) (if it turns the light off when unplugged)

The same test procedure must be used on the head lights. Pick only one circuit (like a low beam) and start at the load (the head light bulb) and test back toward the source (the head light switch) not the dimmer switch.

If you don't have a wiring diagram, let me know and I will put one up on my web site for you to copy.

Howard