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MG MGB Technical - Fuse Keeps Blowing

Hi All,

Fuse 4 on my MGB keeps blowing. From my understanding, this is the fuse relating to the horn and other dash instruments. Every time i put in a new fuse, it blows immediately. I've been using 35A fuses.

Any ideas?

Thanks in advance,
Rudi.
Rudi

There's a short circuit somewhere. It's useless to keep wasting fuses. It's never going to work until you sort out the problem. Get the wiring diagram and then disconnect everything that is wired through that fuse. Put the new fuse in, and then connect up the circuits one by one until the fuse blows - bingo, that's the culprit.
There are brilliant colour coded diagrams here http://www.advanceautowire.com/mgb.pdf Make sure you choose the right diagram for your car's age.

Mike
Mike Howlett

Connect a 12v bulb in place of the fuse. When the short is on it will glow at full brightness, when it dims or goes out altogether the short has gone. Easy-peasy.

You don't say the year but the purple circuit (bottom fuse) feeds horns, map/courtesy/boot/hatch lights, headlamp flasher, cigar lighter, brake test. If you have a radio it may also feed the 'keep alive' circuit on that.
Paul Hunt 2

Hi Paul. Thanks for your answer.

Are you saying to keep the bulb until the short circuit is over? Then all should be fine after that and replace the normal fuse again? Or should i permantly keep it in?

Car is '72 and it is the purple circuit. Radio keeps working although fuse blows, so i'm not sure if its connected, unless its 'keep alive' as you say?
Rudi

No, only use the bulb while you are diagnosing. The bulb has resistance, so while the short is present it keeps the current to a safe level while you disconnect each branch of the purple circuit. If you left it in permanently the resistance of the bulb will affect how well things work depending on how much current they draw. I.e. it wouldn't affect a 'keep alive' circuit at all, but it would make the courtesy lights dimmer, and probably stop the horns working altogether.

On a UK (?) spec 72 it just feeds horns, courtesy/boot light, headlamp flasher, and optional cigar lighter, plus possibly any aftermarket stuff like radio 'keep alive', clock etc.

Normal radio power will be from the accessories circuit (white/green unfused) or ignition circuit (white unfused or green fused) depending on how it is connected. Only some radios have this 'keep alive' circuit, it seems to be those permanently fitted or with a removable face-plate. Completely removable radios use another method.

You can isolate the rear of the car at a 2-way bullet connector in the mass by the fusebox. There should also be two separate spade connectors on the fusebox, one with one wire and the other with two, I don't know what circuits each of these feeds. There is also a sealed 3-way connection behind the dash where the feed to the headlamp flasher comes off, and a 4-way bullet connector, possibly behind the centre console, where the courtesy light and cigar lighter tap off.
Paul Hunt 2

This thread was discussed between 01/10/2007 and 03/10/2007

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