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MG MGB Technical - Battery Drain Question

I have 73 MGB with a serious battery drain. A voltmeter between the battery ground and chassis (cable disconnected) shows a full 12 volt drain. First thing I did was disconnect the wires to the alternator and found that the alternator housing was quite warm. Still had a full 12 v drain with alternator wires disconnected. This surprised me. Could it be the voltage regulator?

I'm hoping this provides a important clue to what is going on. Can anyone point me in the right direction. I need to solve this in the next couple of days.

RES Schultz

If the alternator got warm it is likely that one or more of the diodes in the alternator is bad, this will drain a battery quickly. Anything that consumes power when the car is off, such as the memory in a modern radio, will give you a reading on the voltmeter. Put an ameter in series with the battery lead and look at the current draw. If you read more then a couple hundred milliamps due to things like the radio mentioned above you have a problem. Start disconnecting items from the system one at a time, when the ammeter drops, you have found the cause.
John H

I will get the alternator checked out. I'm afraid to use my ampmeter because anything more than 10 amps will fry my meter. I think I have more than one problem. With the alternator plugged in and the purple wire fuse unplugged I get 12V. When I reverse these connections (unplug the Alternator and put the purple wire fuse back in)I get 12 volts again. It is only when BOTH the alternator and purple wire fuse are unplugged do I get Zero volts.
RES Schultz

Be careful of what you are measuring - 12V is not draining the battary as such, it simply shows that there is something in circuit draining the battery, this could be a few microamps on a wet plug or 10amps on the headlights, both will show up as battery voltage using the method you are using.

So if you have a radio pulling 2mA from the purple wires then you will see what you are seeing - it's not for certain a bad thing - but also the same thing whould show if your boot light was permemently on, which would be bad.

Use an ammeter, connect one meter lead to the battery post and touch the +ve wire to the car against the battery post, the connect the other meter lead to the car wire, finally seperate the battery and the car wire using the meter as a bridge. Doing it this way ensures that anything that spikes on connection (CD player, capacitors, door locks) has done it's spike already and the current will have settled down.

TBQH the alternator sounds like the culprit, the alternator should have a diode in it that protects the output from becomming an input, with this fried the alternator is continiously taking current (I would also expect that your charge light does not come on with the ignition anymore)

A full 12v when you put a voltmeter in place of the ground strap does show a significant drain. With an alternator connected in my experience only a few volts is shown, which indicates an insignificant drain of micro-amps from the alternator diodes and can be ignored. Dynamo/generator equipped cars should show zero drain. Other than that a drain of anything from a few milliamps to many amps will all show 12v, and is wrong so should be found and fixed.

The reason for using volts rather than than amps is that an ammeter will allow a large current to flow, which may be too much for the meter and indeed cause arcing which could ignite battery gases (I realise one has already removed the battery ground strap which if a large current were flowing would cause a spark anyway). There is another very good reason which I shall come on to.

If you unplugged the alternator and the 12v is still shown then there is a drain *in addition* to anything from the alternator, you will have to find that before you can determine if your alternator is faulty *as well*.

If you have an alarm, some stereos, and some clocks these can cause a drain so should be disconencted. Next port of call is to pull the purple fuse to disconnect horns, courtesy lights etc., then the hazard warning circuit in-line fuse. If still there you will then have to start disconnecting browns from the main lighting switch, ignition switch, starter and ignition relays and finally the solenoid until the reading goes and this is where using a voltmeter rather than an ammeter is another advantage in that you can't draw too much current iof the browns you are working with happen to short to earth/ground. If it's still there with the browns removed from the solenoid then it is either the solenoid itself or the battery cable.
Paul Hunt2

This thread was discussed between 08/05/2008 and 12/05/2008

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