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MG MGB Technical - Electrical Fault Diagnosis

I am a new owner of a 1977 MGB that has an electrical problem. The car, I'm afraid, suffers from some "Shade Tree Mechanic-itis". Previous owners have "jury rigged" the headlamp switch (replaced by a pull-knob located under the dash) and the ignition (they inserted a push button on the dash to activate the starter motor). My problem is that the battery will go from a full charge to nothing in a 24 hour period if I leave the terminals connected. I have removed all 4 fuses to try to isolate at least the circuit that has a problem, but the discharge continued with the fuses removed. I have a new alternator and a new battery, but have not yet replaced the battery cables.

Can anyone offer some practical advise as to how to find the problem? I hope that someone has "Been There/Done That" in this regard. I am new to this message board and am really impressed by the depth of knowledge out there and the willingness of enthusaists to help one another. What a great resource for new guy with dumb questions like myself.
Mike W

Mike, connect an ammeter (10 amp or so, not a 300 ma) between the positive battery post and the (disconnected) positive cable end. This will tell you how much current is being drawn. Then you can start disconnecting things while watching then meter. Places to start would include the new alternator (yours wouldn't be the first one to have a bad diode), the 'Bubba' starter switch, and the trunk light.
Ken Lessig

I would recommend connecting an analogue *voltmeter* in place of the battery ground cable, set to read 12v. I say this because if you have a large drain, using a voltmeter is much safer than using an ammeter as you won't get heavy currents or sparking.

You really ought to remove the ground cable first anyway if you intend to remove the hot cable to avoid shorts to ground. And even if you propose to use an ammeter you should put it in the ground lead, it will display exactly the same thing and you won't have to remove the hot lead as well. This is this the case for both negative and positive ground cars.

Anyway, to get back to detecting the drain. On an alternator-equipped car you will see a few volts registered on the voltmeter, this is the reverse leakage current of the alternator diodes and is completely normal. However if the voltmeter displays a full 12v then you have a significant drain. It is now you start pulling the alternator plug, the fuses, and then brown wires, all the while keeping an eye on the meter, to isolate where it is going to.
Paul Hunt

Ken & Paul:

Thank you very much for responding to my plea for help. My 77B is in an unheated garage but this weekend here in St. Louis is supposed to be 60 degrees, so I should at least be comfortable while trying to find the problem. And besides, with no more pro football to watch, I'll be looking for something to do.

Thanks again, gentlemen.
Mike W.

Mike , I get to chase this kind of problem all the time - I use an old turn signal flasher or buzzer , lightbulb etc. to indicate the current drain .

I prefer the flasher or buzzer as I don't need to watch it to know when I've found a short , it makes noise .

If I ever found any current leaking through and alternator I'd swap it out quick ! one volt is a huge parasitic drain .

Just set up whatever current indicator you desire on the ground cable and start disconnecting things - first the alternator then go for the light switch , etc. pay close attention to what goes where so it'll all work when you're done . take your time and remember you'll need a good small flashlight to look under the dash with .

I've had the memory in modern radios go bad and run the battery dead overnight so check that too .

-Nate
Nate

A light-bulb or buzzer is far less sensitive than an analogue voltmeter and will only indicate a sizeable drain, smaller drains can be present that are still discharging the battery over time. A few volts shown on a voltmeter in this configuration represents micro-amps which is normal for an alternator but is not enough to give any noticeable loss of cranking power even after several weeks of inactivity.
Paul Hunt

This thread was discussed between 08/02/2002 and 10/02/2002

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