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MG MGB GT V8 Factory Originals Technical - Alternator Wiring Question

I have the stock alternator on my factory 75 GT V8.

Does it matter which of the lower two fatter spades I plug into?

Thanks. I should have looked before I removed the wires.

Bob Scott

No, they are the same piece of metal inside the alternator.

I used the spare one to connect direct to the fan relay (4-terminal type, not the original 3-terminal) which, with making local grounds for the fans instead of that long run of thin wire, resulted in about a 25% increase in voltage at the fans and a noticeable improvement on the temp gauge. Now maintains it mid-way between N and H even in ambients of 40C+
Paul Hunt


Interested in your comment of getting 25% more voltage to the fans. Was this because the normal earthing is so poor or was it because you are now using full alternator voltage as well?

Laurie Webb
L Webb

A combination of both. Originally the fans got their voltage from the green circuit, even though it used a relay, which adds 10 amps to the fusebox and ignition switch and is not a good idea IMHO. But a PO had replaced the relay with one of a different configuration which meant the fans were being supplied from the brown at the fusebox, albeit unfused. Even so I was getting nearly two volts lost bit by bit between the alternator and the fans. The ground path was also losing up to a volt as it is a long run on standard gauge wire and is shared with the headlights. Both the headlights and the fans have 12v supplied to them on a heavier gauge than standard, which makes the standard gauge earth return illogical if not downright wrong. I checked and cleaned all connections but got very little reduction in the losses.

I left the original grounds connected and provided an additional short tail on heavy gauge wire from each fan to a bolt-through tag which I secured between the mounting brackets and the bonnet slam panel, after cleaning it to bright metal and assembling with copper grease. I left the original ground connected so my new grounds would help the headlights as well as the fans. And even when the first fan had its new ground, which was assisting the 2nd fan, adding another local ground to the 2nd fan resulted in an audible increase in fan speed of the 2nd. The final step was to add a heavy gauge brown from the spare alternator output spade to a spare input spade on my non-standard relay. Overall I now have almost full alternator voltage at my fans instead of something about 3v less, and I'd imagine the original wiring from the green circuit would result in even less.
Paul Hunt

Thanks Paul, I'll print out your fully detailed explanation for my "V8 tips" file.

Laurie Webb
L Webb

Thanks Paul for your detailed wiring notes. I am about to upgrade the coolong fan on my 3.9 and had aready planned in the relay and earth but hadn't thought of the alternator feed idea to maintain a high voltage!

R Weston

This thread was discussed between 21/10/2004 and 01/11/2004

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