Classic/Antique Car Repair: 1976 MG MIdget Alt., voltage drop test, light glows

Question
The alt. light on my MG came on the other day without the key in the ignition. When I started the car, it went off. Repeated several times to see if I was crazy. On the way home, The Alt. light came on.  It no longer is on when the ignition is off.  Let me explain. At a low idle, the light glows softly, but definitely on. The higher the idle, the brighter it glows. If you turn on any electrical, it glows bright. Battery seems to be holding its own, it has not gone dead.
Any ideas.

Thanks,

Jay Laros

Answer
Jay,

I guess this was you first question. I received the 2nd already.
I first will explain how the light system works on your car if you have the orginal Lucas alt. on it.
When you first turn the Ign. key on, power is sent to the bulb. The bulb will not burn unless the alt./regulator grounds the bulb. Since the alt is not producing anything it will be a ground for the bulb and the bulb lights. As soon as you start the engine, the alt starts charging and becomes a power source. Now both sides of the bulb are power thus it will not burn.

Your tests should be as follows. If it is a two wire alt, turn the key on but do not start the engine. Look at the light, it should be on, if it is, unplug the (small) wire on the back of the alt. and note that the light goes out.
If that is OK, Have your battery load tested and do a voltage drop test on BOTH battery cables and your Engine to frame ground strap. Don't overlook the condition of the large wire on the alt. and belt tension. If all that checks OK you may have an alt and or internal regulator going out. A fast idle, lights, wipers and fan motor running voltage test will usually show a bad reg or alt. Be sure the battery tested OK first as a bad battery can make an alt./reg fail a test in some cases.

I hope this helps,
HMF