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MG MGB Technical - Mother of electric shorts

I have a huge short that I can't find...The battery will go completely dead overnight unless disconnected,Shows 12 volt load between + battery terminal and + battery cable with everything turned off. No electric acessories including gauges will work unless headlight switch is on(parking lights or headlights will do) side marker and front parking light bulbs blown. Engine starts and runs ok. Car is 2.8 V6 conversion of a 79 B. Changes to wiring were to convert to GM alternator and starter. This is a new problem, the car has 2000 miles on it since the conversion with noproblems. Alternator volts check out OK, starter works fine...couldn't find a problem like this one in archives...PLEASE HELP...Thanks
Glenn Johnson (Georgia)

The good news is that it *isn't* a huge short, or the car would catch fire. The bad news is that you have a drain, even headlights left on for a few hours will flatten a battery.

You should be disconnecting the battery ground cable, not the 12v cable for safety reasons. If you are working on the ground cable with the 12v cable still connected and the spanner touches bodywork nothing will happen. But if you work on the 12v cable with the ground cable still connected and the spanner touches bodywork you will get a flash, maybe bits of molten metal in your eyes, and possibly igniting battery gases and blowing up the battery, which will spray you with acid.

With the 12v cable connected and the ground cable disconnected, connect a meter on its 12v scale between the battery ground post and the ground cable. If you see 12v you have a drain. First thing to do is always unplug the alternator. These always cause a very slight drain, in the order or micro-amps, which typically registers as just a few volts on an *analogue* voltmeter (not tried a digital, and they will probably vary anyway). At this level it is harmless and the battery shouldn't drain for weeks.

With the alternator unplugged the voltmeter reading should have dropped to zero. If it has, then the alternator is causing your drain and is faulty.

If you still have 12v shown then it is something else. If you have any after-market kit in the car like alarm, stereo, and some clocks you may still see a drain, so disconnect these as well.

If you still have a drain remove the fuse between the brown and purple wires in the fusebox. If the voltage drops now something on the purple (fused, always powered) circuit is taking current. Normally this is things like horns, courtesy lights, cigar lighter, headlamp flasher, clock, and 'fasten seat-belt' warning control-box.

If you still have 12v shown then it is something on a brown circuit, so you will have to disconnect the brown wires from the main lighting switch, ignition switch, hazard flasher in-line fuse, starter relay, ignition relay until it goes off.

If still showing 12v you will have to remove the brown wires from the solenoid. Reconnect the battery cable and if the 12v has gone it is on the brown wires leading up to the lighting, ignition etc. circuits above. If still showing 12v, which goes when the battery cable is removed from the solenoid, then it is the solenoid. If it still shows 12v with the battery cable off the solenoid then it is the battery cable, which would be worrying, as that *could* turn into a major short and a fire.

The other thing is that being a modified car anyone could have connected anything to anything, so to a certain extent you are on your own.
Paul Hunt

Thanks Paul for the info. That gives me a series of things to try to find the problem.... I forgot to mention that the 2 top fuses in the panel have no power until the light switch is turned on.
Glenn Johnson (Georgia)

That's correct, they feed the parking lights at the corners of the car when the main lighting switch is in its first position. Possibly the wiring is incorrect on the fusebox, if at least one of the green wires were connected to the top two fuses instead of the 2nd one up that could cause the instruments and accessories issue. The fuse box should have brown on the front of the bottom fuse, purple on the back. White or white/brown on the front of the 2nd fuse up, green on the back. A single red/green on the front of either of the top two fuses, and two reds on the back of each of those fuses.
Paul Hunt

This thread was discussed between 27/09/2008 and 28/09/2008

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