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MG MGB Technical - Replacing Wiring Harness

OK, here's my problem. I'm rebuilding (pouring money into) a 77B. It was running fine, I stripped the exterior(lights, chrome, bumpers, etc) and took it to a paint shop. When I got it back and put everything together I somehow shorted out the headlights and fried the wiring harness - all the way from the steering column to the fuse box. I've acquired a used harness that seems to be in good shape and I'm about to install it. Also got a new headlight switch. Questions: What would have caused the short to begin with? Should I also replace the headlight wiring? Anything else? Thanks for any suggestions. PS: Would someone in England please find Mr Lucas and slap him for me? David
David B

For starters (no pun intended), all MGB wiring harnesses are not the same. Compare the numbered list of components in the Bentley manual for both cars.

Since the headlights are hot, ignition on or off, you have to disconnect the battery to work on any part of it. You also have a dim/bright stalk switch left of the steering wheel which could have been damaged.

Take a good look at Paul Hunt's site --- there's much to help you there.
Dan

Another thought for you.

Look at your old harness, and pay attention to -which- wire went Au Flambe'.... trace it back up the harness as best you can. That should give some indicator on which end went, and hopefully why.

If its -really- a mess, try to find the one of which the wire itself is really toasted. It should be the one that the copper is almost brittle when you peel the plastic back.

Hope this helps.



Gord Bestwick

Good chance you had a bare bullet connector at the front laying against a ground and the light switch got switched on or was left that way. The wire became the fuse and melted all in its path which makes a nasty mess to try to do anything with. If your new harness is from about the same year you may be ok. Even if not, the main differences will be added circuits for things like seat belt interlock and other non essentials.
Jim Blackwood

David - you made the mistake, look in the mirror and slap yourself.

The main headlight wiring doesn't go from column to fusebox, but the main-beam flasher does as others have mentioned. It is the lighting switch that is always hot, not the headlamps themselves, so if this switch is also fried the problem could have been the sidelight wiring to the fusebox or the rheostat as well as the headlight wiring. As Gord says, the wire that is burnt (rather than just collateral melting) will be needed to determine where the problem was.

When reconnecting a battery for the first time after major work involving electrics you should always do the same test as when looking for a drain on the battery while parked - make sure everything is switched off (including boot/trunk and courtesy lights), connect the hot cable to the battery, and connect a test-lamp or voltmeter in place of the battery ground cable. If the test-lamp glows or the the voltmeter shows 12v you have a drain of some kind. If the test-lamp glows at full brightness it is a heavy drain, possibly a full short.
Paul Hunt

This thread was discussed between 01/01/2002 and 03/01/2002

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