MGB Battery help


Asked by Mike Dec 09, 2010 at 12:44 PM about the 1979 MG MGB Roadster

Question type: Car Customization

I have a 79 MGB that was already converted to a 215 Buick V8 when I bought it. For the last few years I had a problem where if I forgot to use my kill switch which shut off the entire electric system my batter would be dead in a few days. A friend installed a new modern wire harness from Advance Auto Wire to resolve the problem. The car runs 100 times better now but my battery still goes dead after a few days. Everything is new (including the battery) except for my alternator that I replaced about the same time I started noticing the drain happening. Is it possible that the problem could be in the alternator? I just purchased a battery kill switch that I'm going to install this weekend but I would really like to be able to turn it off and the only thing i can think of is the alternator. The car also charges just fine when the car is running.

2 Answers


I have a few suggestions for you: 1. Double check that the ground strap from the block to the frame, and from the frame to the negative terminal for corrosion and any resistance. Also check the ground coming from the alternator itself for the same. If you find corrosion that permeates the wire even underneath the insulation, replace it. Personally, when I redid my grounds, I went to a local stereo shop who was kind enough to donate 2 short lengths of high-quality, heavy duty stereo amplifier power wire, one 2 gauge, one 8 gauge. Always remember that it never hurts to overdo it on the ground side of the circuit - the fatter & shorter, the better. Do bear in mind that you do not want to add additional ground straps, as you may wind up with a ground feedback loop. 2. Once you have confirmed that the ground wires are in the proper shape, check that your alternator is outputting a high enough voltage while the car is running to maintain a surplus on the circuit, thereby building the battery's charge as the vehicle runs. An easy way to test this is to probe both battery terminals as the vehicle is running and the headlights, stereo, and any other accessories off - you should see between 13.5 and 15 volts, with most cars falling right around the 14.4 mark. Anything less than 13.5 and the battery will have a hard time building any charge at all. 3. If the alternator is outputting the proper voltage, your battery may be getting weak, especially if it has been drained to dead on multiple occasions. Most major auto parts stores have the equipment to test your battery's ability to take, hold, and release a charge under load, and the last time I checked, they are happy to test the battery for you free of charge. Please remember that the testing equipment can sometimes take several hours to properly test the battery, so you may want to bring it separately in another vehicle. Please let me know if you have already performed these or any other tests, and the results, and I'll be happy to assist further.


Thanks Doran, The ground straps are questionable. As far as the battery, it's brand new. I just replaced it a 2 months ago because I thought the same as you mentioned about the old battery. As for the alternator, I haven't done a measurement of the voltage but I've drove it in the dark for about 4 hours in the dark before I finally got a new battery and headlights stayed nice and bright the entire way and it wasn't until it was parked for the rest of that week that the battery went dead again. That's why I'm thinking the only thing that is left some sort of drain going out the alternator. There is nothing electrical left that hasn't been replaced except the alternator, fan motor and windsheild wiper motor. Thanks again Mike

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