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MG MGB Technical - 1979 B electrical issue

I am having problems with the third fuse blowing.
This is the fuse that covers guages, brake lights and turn signals. I have changed the brake light switch because I thought it was the problem. Anyway, I can get every thing to work with out blowing the fuse until I start the car. The car starts and runs well, but blows the fuse and will continue to blow the fuse when replaced. The third fuse has 2 wires runnig off it. If I disconnect the wire that works the brake lights, I can run the car without blowing the fuse and have working turn signals and guages but no brake lights.
Since I have already changed the brake light switch, and the brake lights will work and not blow the fuse until I start the car, What shoud I do next?
Thanks, JoeF
Joe Frucci

Check the wire that goes to the carb heater mine came off and caused the fuse to blow This is the black thing between the carb and the manifold that has a wire going to it.
pat in Tehachapi
patrick bailey

You have done well to narrow down the problem so much - take out the brake double filament bulbs and check whether the problem persists - check the lights and their connections at the rear - then the connections where the rear loom joins the front - If you haven't found the prob by this stage - check the cables for an earth.

UK cars have their fans on this fuze - as they get older, the fans tend to draw more current and blow the fuze. The answer is a separate relay and fuze.


Thank you for you quick and helpful answers!
Pat, you were correct! It was that wire to the carb heater. I cant believe I didnt see it! I connected it and the electrical problem is fixed! No more blown fuses, and every thing works.
I do have one other question, what does the carb heater do exactly? The engine seems to run much smoother with the carb heater NOT hooked up. The car seems to idle rough with the carb heater hooked up, and dosent accelerate smoothly either. I really didnt let the engine heat up. Will it run smoother when it does warm up? Its 15 degrees out today, I do not have a top on the car so I will not be able to find out for myself today. Thanks again.
Joe Frucci

Glad it worked out ok I don't know what it does but it DOES blow fuses!! I have seen a modern fuse block with new spade fuses on sale on Ebay at times that splits up the circuits into 8 fuses instead of 4 It is supposed to be a direct replacement,has anyone done this?Losing brake lights aint good!
Pat in Tehachapi
patrick bailey

Pat, I haven't used one of the fuse blocks with blade fuses to replace the Lucas fuse box. Most of the low cost fuse boxes seen in eBay and local auto parts stores have a single power input terminal. If you try to replace your 2 or 4 fuse Lucas box with one you will have all circuits hot anytime the battery is connected. You could take one of those boxes and cut the center feeder strip and wire one side for ignition hot operation. That would work but it's not a plug and play change. If I switched I would get a Painless wiring "7-CIRCUIT AUXILIARY FUSE BLOCK
#70107 - 3 constant-hot and 4 ignition-hot circuits. Uses one 30 amp relay" and adapt it to fit the MGB.
Dan Masters sells a fuse block with four fuses and a relay to have ignition on only fuses.

For my MGB electrical system I added another four fuse Lucas box and wired headlight relays, a Hayden cooling fan and the fuel pump and distributor through the added fuse box. I have also installed a fuse in the overdrive circuit and installed a fuel shut off inertia switch in the fuel pump feed. It's a quality unit, made in the UK, it came from a Ford Tempo and cost $5 at a salvage yard.

FWIW, Clifton
Clifton Gordon

In the UK I rewired an 80 (FWIW) and fitted a bigger blade fuse unit at the request of the owner. I found a six-fuse unit in an auto electricians that mounted to the same points as the old four-fuse unit and was almost exactly the same size. Each fuse was independant both sides. I wired them to give him two fused always on(i.e. purple) fused circuits, two fused ignition (i.e. green) circuits, the other two being wired as normal for the parking lights.
Paul Hunt 2

This thread was discussed between 19/02/2006 and 23/02/2006

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