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MG MGB Technical - Charge me

I mistakenly sent this to the MG-F folder.

I have a 1975 MGB with 65 K miles. It runs great, but recently the battery died. It wouldn't take a charge.
I bought a new battery, put it in and it fired right up.
After about 4 miles down the road, the car died and the battery was totally dead. I am thinking that the car is asking the battery to power the entire electrical syste. I imagine the alternator has gone out...any thoughts?


Steve in Santa Fe....


Steve. The battery alone should power the car, with lights on, for more than four miles, even if the alternator is not working at all. (This from actual experience.) The only way the problem you describe could happen would have been an almost flat (minimally charged) new battery or some form of high drain on the system.

Disconnect the battery from the system and charge it. Measure the battery voltage. Should be about 12.5 volts. Attach the battery terminal clamps and measure the voltage. Again, it should be the same as the battery voltage. Start the car and check system voltage at the battery terminal clamps. Should be about 14.5V with a good alternator.

Regardless of these test results, you may have a significant drain on the system, due to a short which, under some conditions, could cause the alternator to go out quickly and the battery to run flat.

Disconnect the ground (negative on your car) battery cable and hook a test light between the cable and battery terminal. It should only glow dimly, if at all. If it glows brightly, you have a short somewhere. Disconnect the plug at the rear of the alternator and see what happens to the test light. If it goes out, or goes very dim, the alternator has an internal short causing part or all of your problem. If there is no change in the brightness of the light, keep disconnecting circuits until you see a significant change. That would be the circuit the problem is on. Some circuits will require the ignition switch to be on. When checking them, detach the wire going to the coil or the wire from the coil to the distributor so as not to take a chance on the distributor burning up wires (as can happen if the points are closed with the ignition switch left on).

More information, after you have done some testing, would allow us to offer more exact advice.

Les Bengtson

Further to what Les posted, for info I ran 90 miles with a duff alternator on a charged battery in daylight. A long shot, but don't always assume new parts are 100%, I got caught out with a new 'duff' battery many years ago, one caused me a lot of hassle because I checked everything else first!
R. Algie

Adding to what Les has posted while a test-lamp will indicate a largish drain (which if the cause isn't a duff replacement battery it probably will be in this case) it will not indicate a small drain which could still be significant in draining the battery overnight. For that connect an analogue (digitals may give differing results) voltmeter on its 12v scale in place of the battery ground (irrespective of polarity) strap. On an alternator equipped car you will probably see a few volts registered. This is just the reverse leakage current of the alternator diodes and can be ignored. If you see a full 12v registered then you do have a drain and should proceed by disconnecting the alternator, fuses, and then brown-circuits in order to locate it.
Paul Hunt 2

I did a daylight Bonzi run of 200 miles without an alternator this summer. My passenger son was not impressed. I was delighted to make it home.
JLG Galbreath


Definitely should get more on a battery than you did. Way back in the 70's when my only car was a 49 YT (and I was young and foolish) I was going through Santa Paula on my way to China Lake for a meeting the next morning when I lost my generator. Managed to make the 160+ miles to China Lake at night and, with a charge the next day, all the way back the next afternoon.

Take your time and follow the guidance from Ron, Paul and Les and you should resolve your problem.


Larry Hallanger

Check the battery cable connectors and connections. Mine were both bad, leading to similar symptoms.
1977 mgb
randy olson

This thread was discussed between 03/10/2005 and 13/10/2005

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