1977 MGB low voltage when car is running

Technical MGB discussion
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Posts: 1
Joined: Thu Jun 11, 2020 8:49 pm
Forename: Richard
Surname: Benjamin

1977 MGB low voltage when car is running

Post by rbenjamin » Wed Jun 24, 2020 2:04 am

1977 MGB low voltage all over, across battery terminals, and at the fuse block. 12.5 volts, when the electric fan comes on it drops to about 11.5 volts, goes back to 12.5 volts when fana turn off. Ground at battery terminal was cleaned and mounted, no improvement. The battery is not dying, I’ve been driving about an hour for the past few days, car starts up and drives ok. I think the alternator is weak. Anything I should be looking for before pulling the alternator out and getting tested? Appreciate any suggestions. Thanks.

Vic Butler
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Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: 1977 MGB low voltage when car is running

Post by Vic Butler » Wed Jun 24, 2020 11:58 am

Check the voltage across the battery terminals with the engine running at 2,000 rpm. It should be around 13.5/8 volts. Turn on headlights and see what the reading is.
Are the battery terminals clean? Dirty ones can cause a voltage drop. Have you checked the alternator plug?. I use electrical switch cleaner to clean terminals, it works very well.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

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