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MG MGB Technical - alternator misbehaviour


I am facing a problem with my B alternator. It does not charges at all. It could be an alternator problem. But what puzzles me it reading I get at Pos. alt gives 12V and D+=7V. This explains why alt indicator light is dim whith switch on and stays dim. I would like to know if it could be some other problem with wiring or a faulty alternator.
Jean Guy Catford

The only experience I had with poor charging and difficult starting was traced back to dirty and loose battery terminals, cleaned the posts and put on new terminal clamps and solved my problem. I don't know if it is the same problem you're having but it's a place to start.

I also have an alternator problem. It bench checks perfectly, is only two weeks old, but does not charge on the car. New Battery and cables as well. My fear is a grounded hot wire somewhere on the chasis. If so, God help me finding it. Does anyone know of another reason. (73 mgb roadster)
S. Akins

What does the ignition warning light do? Does it come on when the ignition switch is turned to the run position? Does it go out when the engine is reved to about 1,000 rpm? There is a wire, running from the alternator to the ignition warning light, then to the white wire circuit. If the light is not working, or this wire is not connected to the alternator, the alternator may not charge.

I have not seen Paul Hunt around recently, but, you can go to his website, The Pages of Bee and Vee and send him an e-mail asking him to drop in and respond. He is one of the best people I know for complex electrical problems--and a great teacher.

Les Bengtson

Hi Les,

When switch turned on it light dim and I measured 7 V at that litle wire. It is like if something wrong in that circuit.

Jean Guy Catford

JG. Yes, something is wrong in that circuit. You should have full battery power, about 12.5V going to the light. Les
Les Bengtson

If you're only reading 7 volts, and if you still have the 2 six volt batteries, maybe one of your batteries has an internal short. Have both batteries checked at a reliable parts store. Interstate makes them, group 17HF.
As Lewis says, be sure to clean the terminals. If they're the add-on terminals where the cable bolts into a clamp on the terminal, clean that connection too.
Make sure the ground cable is making good contact to the body.
Many times what appears to be a starting or charging problem is actually a bad battery connection at the terminals.
Safety fast,
Ken T
Kenneth Thompson

Jean - If your alternator has an internal regulator, then it is bad and will need to be replaced. May as well get the brushes replaced at teh same time.

S. Akins - You won't need God's help locating a grounded hot wire if you have one, just follow the smoke! I believe that the earlier cars had an external regulator. If your car is wired lke that, then your problem may well be in the regulator.

In both cases, go to Paul Hunt's web site as Les suggests, at , click on Spanners, electrics, alternator/dynamo. Paul has done a great job of explaining how the circuits work and how to troubleshoot them. Good luck - Dave
David DuBois

This thread was discussed between 27/06/2005 and 03/07/2005

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