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MG MGB Technical - Electrical question

I have a 66 MGB with positive ground. The other day I replaced the battery (using one 12V battery) and made the brilliant move of hooking up the terminals wrong (i.e., positive to negative lead, etc.). Don't know quite how I managed to do this as I have replaced multiple batteries in positive ground MGB's over the years, but Murphy was helping out that day. Did not notice the lead swap and started the car and it ran OK except for no tach and the purple circuit equipment (horn, wiper, etc.) would not work.

Took a quick look and determined that purple circuit fuse was burnt out and replaced it. Ignition was turned off and I looked at the tachometer and noticed that the ignition warning light was lit, even with no key in the ignition. Looked further and realized that I had done the stupid battery trick. Proceeded to reverse the battery leads and polarized the generator. Looked at the ignition light and noticed that it was still on with no key in the ignition. Turned on the ignition and the light went off and the car started and ran just fine. The ignition light stayed off while the engine was running which leads me to believe that the generator is charging correctly. Tachometer was OK and all purple circuit equipment worked OK. Ran engine for a bit, turned off car and ignition light still stayed on. I immediately detached the ground lead from the battery to avoid any fire from a short circuit.

Any ideas on what may be causing the ignition light to stay on when the ignition switch is off? It seems like the light is working in reverse as far as when it should be off or on (i.e., light on when ignition switch is in off position, light off when ignition switch is in on position). I am thinking that the problem may lie in the control box (possibly one of the armature coils may have fused??), but before I start tearing everything apart, I would appreciate some input.
Ronald Kluwe

Ronald - The cut out points in the regulator may have welded themselves together. With the battery disconnected, remove the cover of the regulator and and check the contacts, using a screwdriver to lift each one. If this doesn't work, I would suggest that you send a query to Bob Jeffers at bobj50@comcast.net Bob is the resident regulator expert. Cheers - Dave
David DuBois

Dave;

Thanks for the tip. I pulled the regulator cover and took a look and the "B" armature points were not only welded together, they were almost non-existent. I have ordered a new regulator and also a new generator just to make sure that I did not short circuit that also.

I will let you know if this is the fix once the new parts arrive.

Regards;

Ron
Ronald Kluwe

The generator probably survived the incident. They are much more forgiving then alternators with reversing polarity. I think you will get by with replacing just the regulator. RAY
rjm RAY

I agree with Ray regarding the survivability of the generator. Further, I would have suggested that you send the regulator to Bob Jeffers for conversion to solid state, a very worthwhile conversion. Cheers - Dave
David DuBois

Replaced the regulator and the generator and everything is running perfectly now. Even though the generator may have survived the incident of reverse polarity, there was some charring on the main generator terminal from the forced short circuit of reversing the polarity and I did not want to take the chance of replacing the regulator and then finding out that there was a short in the generator and having to start all over again.

Did not know that there was someone out there doing a solid state conversion of the regulator. Will file that away for future use.

Regards;

Ron
Ronald Kluwe

This thread was discussed between 21/05/2010 and 25/05/2010

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