Ignition Light Does not come on - anytime!

[bob.hart@talk21.com]

I recently experienced a number of problems where 'Lucy' my 1979 MGB, would cut out when running at relatively high speed in hot weather. I finally tracked this down to a failing ignition coil. Having replaced the ignition coil the problem was cured, but I've subsequently had problems starting, as the battery was not fully charged. Having recharged the battery Lucy starts well, but the headlights are dim, and the indicators are slow. I've checked the current at the battery leads, and it appears that the alternator is not working. However, I'm reluctant to just change the alternator because I've noticed that the ignition light never comes on. I read in a previous topic that the current that feeds the ignition light also excites the alternator. I am now concerned that when replacing the coil I have unwittingly disturbed the wiring that both feeds the ignition light and excites the alternator. Are my worries unfounded?

[Vic Butler]

Check the wires in the alternator plug. The thin wire is the one that goes to the ignition light. If the warning light bulb has blown the alternator can't get excited to start charging so check the bulb too.

[Charles Farran]

You may have worn brushes in the alternator. The ignition light is only on when the voltage each side of the bulb is not matched.On the alternator side the brown and yellow wire rreaches the earth side of the car through the brushes.You can check that end by removing the plug on the back of the alternator and put a test light probe on the exposed tab with the other on one end of the bottom fuse in the fuse box.If it does light up then it raises the probability that the ignition bulb has blown which is not the easiest to get at. (I have just replaced the instrument bulbs with LEDs (but not the ignition bulb) so i should know)!
You can have your alternator texsted on the car.
Cheers, Charles

[bob.hart@talk21.com]

Thanks gents, I had already ordered an alternator before finding an earlier post relating to the ignition bulb circuit feeding the alternator exciter. I'll now get hold of a replacement bulb and fit that, before fitting the new alternator. As an engineer, I can't understand why the circuit has been designed so that the exciter feed is dependent on a bulb!