MGB: alternator keeps dying

help!! i am a new owner of a 77 MGB and I have been through two alternators and am on my third one... I can drive for about three days and then the alternator is no good anymore. what can I do to fix this problem?

Make sure the fan isn't on backwards. The fin side goes against the alternator.

How old is your battery? Have you had the alternator bench tested? Does the ignition light stay on while you're driving? Where are you getting these alternators from? What is the condition of the wiring? Are the connections correct (PO may have changed something)? How tight is the belt?

hopefully i can answer some of the questions well enough to find out what is going on here. It is a brand new battery, each time the alternator has failed i have taken it off and had it tested,the ignition light is always on while driving, i'm getting the new alternators from checker auto parts,the wiring looks ok, I'm not sure about the connections being correct and the belt is plenty tight.
thanks

If the light is on all the time and you're sure the alt works, then you've got a connection problem. Check your wiring diagram and make sure the wire from the ignition light goes where it's suppose to on the back of the alternator.

Have the new alternator tested before leaving store (make sure Checker didn't get a bad batch). Sounds like a faulty connection (shorted wire) or a bad diode or a sneak caused by a bad ground. Electrical problems can be tedious and frustrating to diagnose. Photocopy the wiring diagram for your car and enlarge it. This will make it easier to read and follow.

Forgot to mention above, the regulator is also a possible culprit.

Check the wiring diagram on the front page of this web site under DIY. Wiring is fairly simply, two brown wires both go to the same terminal on the starter solenoid. The brown/yellow wire goes to the alt warning light. Make sure the 3 wires are connected to the correct blades at the back of the alt.

Is it an Endurance brand alt? That could be the problem.

Had the same thing with my first B. I'd be driving along and all of a sudden I'd loose everything, as though you turned the key off. Went through 3 alternators, 1 from Moss, and 2 rebuilds at a shop specializing in that sort of thing. Had the last 2 checked before we put them in the car. Changed battery thinking that was it. New starter, still not it. Ended up being a bad condenser. Ended up being an expensive way to get to a cheap easy fix. The prince of darkness surely applies.

I found a diode?resistor? right behind the alternator that is no good. what is this for and could it possibly be a source of my problem?

Sounds like you may have found the problem. Diodes were once decribed to me as a "one-way" or check valve for electricity.

thanks kimberly. I'll replace that today and hopefully it will solve the problem. At least it's a very inexpensive part and easy to replace.

I was able to fix it by changing the diode that is on the wire behind the alternator. The alternator is charging and everything is fine again. Thanks all.