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MG MGB Technical - Electrical Drain Help Needed

Hi, 1974 1/2 MGB GT...

I've had an issue with the battery slowing draining, so I did the following troubleshooting:

This was all done with all doors closed.

1. Disconnected negative side of battery and measured voltage of 6 VDC between negative battery terminal and car ground.

2. Disconnected all browns at the alternator, no change, 6 VDC.

3. Started pulling fuses at the fuse box on the right side fender. No change until I pulled the purple fuse, voltage dropped to 2.7 VDC.

4. So with the alternator and the four fuses pulled I still have 2.7 VDC at between negative terminal and ground. Is this normal? Any ideas or thoughts?

William Gardner

Radio or alarm system with a keep alive circuit?
David DuBois

No alarm installed and radio is orginal that doesn't work.
William Gardner

You need to look at the wiring diagram and find what is still connected when the fuses are out. Looking at Advance Auto Wire diagrams, you have the starter solenoid, starter relay, hazard flashers, headlight switch and ignition switch. Try pulling wires from these one at a time and seeing what makes a difference.

The trouble with these cars is that they have usually been through lots of owners and who knows what mods have been made to the wiring over the years.
Mike Howlett

Just because something does not work does not mean it is not taking current. What do you measure with the heavy gauge cable disconnected at the starter solenoid. The insulation in those cables can leak some current, they are available new as spares.
Stan Best

Ordinarily around 6v measured with a voltmeter in place of the battery ground strap shows a microscopic drain that is just down to the alternator diodes and can be ignored (verified by unplugging the alt). But that is with a typical analogue instrument, digitals may be different. However these are usually *more* sensitive than an analogue, so even a microscopic drain should measure higher.

The fact that unplugging the alternator made no difference makes me think that either you were measuring 12v and not 6v, or you had the meter connected incorrectly somehow (amps and not a voltage scale?), in either case you probably do have a significant drain as the alternator made no difference.

The purple fuse supplies interior/courtesy lights, horn, headlamp flasher and cigar lighter so removing the purple feed from each of these in turn will show if it is a component fault, if none of those either something else is connected to the wiring or (less likely) the wiring itself has chafed and is partially shorting.

After that the sequential seat-belt system is fused off the brown so that should be the next thing to disconnect, then brown feeds from ignition switch, lighting switch, fusebox, starter relay and the hazard flasher fuse.

If you still have a drain disconnect the browns from the solenoid, and if it goes then as before either there is something else connected to the wiring or the wiring itself is chafing.

If the drain is still present disconnect the battery cable from the solenoid and see what happens.
PaulH Solihull

This thread was discussed between 12/02/2011 and 14/02/2011

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