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MG MGB Technical - DPO and Me - Reverse polarity

Confession is good for the soul - or so they say.

Today is a beautiful day in Michigan and I thought I'd take the convertible out. The battery was flat (it is somewhat old) and I attempted to jump it. (1972 'B with a single Group 26 battery.)

As I don't visit the battery but once in a while, I had completely forgotten that my ground wire is RED (as installed by a previous owner). You know what happened - installed the jumpers backward and turned the start key. Quickly realized things WERE NOT good. Turned key "off" and removed jumpers. There was some smoke underhood, and the battery kept venting until I pulled off the ground and positive lead. It was then I realized that the red wire led me astray.

1) I assume the battery is fried. As a point of information - what happened to it? If I were to reattach leads "properly", what would happen and why?

2) What permanent damage was done otherwise to starter or alternator?

John Z

John - You probably fried the diodes in the alternator. Your best bet at this point is to remove the alternator and take it to someplace like NAPA or Auto Zone and have them test it. If you have electronic ignition that is probably, along with the radio if it was on when you turned on the ignition (if it is a modern radio with a keep alive circuit, that may have been damaged regardless if the radio was on or not. I would also inspect the wiring harness very closely, smoke could also indicate melted insulation in the harness, not only on the wire carrying the reverse current, but also on the wires surrounding it.
Cheers? - Dave
David DuBois

If you have a polarity sensitive fuel pump the internal diode is likely cooked. May have also cooked the regulator in the alternator. If you jumped it from a modern car, it may also display some problems.
J Heisenfeldt

And when you're checking all that FIX THE RED WIRE :)

At the very least get some black heatshrink tubing and put that over it.

Simon Jansen

Hell, even paint the damn wire black.

Almost certainly has shorted the alternator diodes, which was why the battery kept bubbling until it was disconnected. You also need to check carefully the harness leading up from the solenoid to the alternator as the brown(s) between the two will also have been cooked to some degree. The battery is probably least likely to have been destroyed, but reverse jumping won't have extended its life!
Paul Hunt

This thread was discussed between 04/11/2008 and 05/11/2008

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