MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - More electrical problems

I have completed installation of a complete new harness in my 77B. Everything has worked well to this point, but now something is amiss. The fuse to the green wire circuit keeps blowing - instantly. I insert a new fuse - and "poof" the minute I switch on the ignition. Of course it takes with it the brake lights, turn signals, fuel gauge, temp gauge and tach. In the changeover I also replaced the voltage stabilizer and brake light switch - so I know there are no problems there.

There are actually two greens coming out of the fusebox, and I tried the ignition with only one of the wires connected. The one that containes the handbrake light circuit does NOT blow the fuse, but when I try with only the other one - "poof" again.

I am relatively knowledgeable about electronics - enough to use a multimeter to diagnose problems, but I am at wits end for the answer here. I've run through voltage drop tests from the Lucas Fault manual and haven't found any problems.

I could use some input here - possible locations of shorts, some testing suggestions, perhaps a checklist I could go through to pinpoint the problem. It's KILLING me to have the car running so well and not be able to drive it because of such a simple glitch.
Scott

Scott. Paul Hunt is back with us, although somewhat under the weather. He would be the best one to contact. If it were me, I would disconnect every green wire terminal on the circuit that is blowing the fuse, then install a new fuse. If it blows, it is a wiring problem. If not, I would begin to connect each item one by one and then check to see if the fuse blows. When it blows, the last item corrected has a short to ground. But, Paul will have a better idea on how to handle it in the easiest manner. Les
Les Bengtson

I assume you have disconnected the green from the following:

Stop Lamp Switch,
Direction Flasher Unit,
Blower Motor Switch,
Reversing Lamp Switch,
Instrument Panel Voltage Regulator,
Windshield Washer Pump, Hazard Flasher Switch

With a meter on Ohms/resistance between the green feed at the fuse box and ground? what is the resistance?. If its nearly a short, and the above are disconnected, then try splitting the green at the joins in the harness, one should be in the engine bay by the rear bulk head.
Tatty

I'd connect a high-wattage 12v lamp i.e. a headlamp with both filaments paralleled in place of the fuse. When the light glows at full brightness the short is present but will do no harm. Start disconnecting greens around the car and when the light goes out the fault is further along the wire you have just disconnected. There are several 4-way bullet connectors in the green circuit forming branches like a tree, so these will help you track it down. According to my diagram the wire at the fusebox that *doesn't* feed the handbrake switch only feeds the washer pump, induction heater and reverse and brake switches. The one that feeds the handbrake switch also feeds the wiper motor and switch, instruments etc.
Paul Hunt

This thread was discussed on 25/06/2003

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.