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MG MGB GT V8 Factory Originals Technical - V8 Electrical Maladies

I have a 77 B with the 215 V8 conversion. My problems are electrical gremlins that seem untraceable. I'll try to list them in order of appearance.

* Drivers side back up lamp remains on independent of ignition being switched on, or car being in reverse, in fact even without any wires connected to the reverse light switch.

* I replaced a dead fuel pump with a new Lucas OEM pump and the pump will not tick when the ignition is switched on. I have confirmed that it is working by running a lead from the battery to the hot lead and fuel is then pumped to the carburettor. I have checked relays and they are all working fine. WHAT should I check next?

* The number three fuse from the top keeps blowing and this controls the Tachometer, fuel gauge, temperature gauge, wiper switch, turn signals, and heater motor switch. I 've checked all these items and cannot find any obvious reasons for the problems. I will confess that diagnosing and finding electrical faults is not my strong suit, so I need lots of help. Thanks in advance to you fellow V8 MG'ers.
David Cousins

If the reverse light is on when the ignition *isn't* it must be getting power from the purple or the brown circuit. If you pull the bottom fuse (purple) and it goes out start looking for a cross with the boot/trunk light purple wire. If it stays on then there is a cross with a brown somewhere else. If the other reverse light works as it should then I would say the problem is in the boot.

If the fuel pump runs when you hot-wire it then it is not getting power through the white circuit. Originally on a 77 this could come through the inertia switch which I think is on one of the toe boards and has two whites on one side (ignition) and one on the other (fuel pump and o/d). Check this switch and wiring.

The 2nd fuse up also powers reverse lights (ping!) brake lights and washers. Have you checked these too? If you can't find anything obvious use a high-wattage 12v bulb, like a headlight, solder two wires to it and use that in place of the fuse. It should only glow dimly, if it glows brightly the short is present. You can connect this bulb in series with each of the green wire connectors on the fuse block in turn, this will allow you determine which of the many green branches is causing the problem easier and cheaper than popping fuses all the time.
Paul Hunt

One can never diagnose at arms length but I would guess that you have got some wires crossed in the engine bay where the two looms meet and the leads from the gearbox switch joins. You need patience and a wiring diagram.

The problem with the third fuze is common - it takes a heavy load and if the terminals are not tight and freshly cleaned the grease you put around the fuse will have turned to varnish and cut the supply. In many cars the weight of the loom tends to play on this fuze and the terminals on the fuzebox work loose - if this is the case - chuck the box and get a new one.

Don't lose confidence - they are very reliable so long as you accept that certain parts have a limited life and have to be replaced before they give trouble (Plug leads/distributor cap/rotor/coil and ... wiring loom .. the latter only at 15-20 year intervals)

Safety fast


I had a problem like yours which turned out to be a shorted out wire due to earthing pump connection in the boot (Rubber bumper type). This wire (white) runs right back to ignition swtich and made a fine job of melting plastic at several points along length.

Just a thought

I agree with Roger about the problem probably is in engine bay where the two looms meet and the leads from the gearbox switch joins.
When I did my V8 conversion I had similar problems, only after carefully studying (and working with) the electrical diagram, was I able to figure it all out. The end result was crossed wires. And I thought I had been so care-full marking them. I had trouble with my turn signals, even now it doesn't' make any sense but I stated at the rear turn signals and made sure the wiring was correct, checked the front turn signals and made sure all was correct there and then checked the wiring into the steering column and made sure all was right there (using a light tester where required) and then checked the fuse box and where the looms join, the point I am getting at is that only a systematic search will solve your problems. Do one problem at a time, check every thing, check grounds, check connections. I

Good Luck

Thanks to all who have responded, but I still have problems. Now wires from the ignition switch loom have fused. They are the brown power wire and a white wire. I have taped these off but still no tach, turn signals, wiper motor, or temperature gauge. I do not know what else to do.

David Cousins

The most common cause of burning the white and the brown is the fuel pump or overdrive feeds shorting out.

Tach, turn, temp etc are all fed from the 2nd fuse up. With the ignition on (making sure the cause of the short has been found and fixed first!) check for 12v on the white and green sides of that fuse. If you have white/brown on the fuse-box then you have an ignition relay, so check that if you don't have 12v on the white/brown. If you do, and on the green at the fuse-box then it is a matter of checking all the 4-way green connectors (there are at least three) and the steering column multi-plugs for continuity on to all the wires.
Paul Hunt

This thread was discussed between 14/12/2001 and 19/12/2001

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