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MG MGB Technical - Electric gremlins

Where to start?
79 B all stock with no mods.
Engine cranks and runs but no gauges other than clock and OP. no running or head light. Interior light works sometimes. No turn signals or flashers. No horns. Passing light does come on when lever is pulled back. No wipe or wash. No fan. No radio.
Where to start looking ?
Car sat for 23 years. Don't know if these items worked in 1992 or not.
Cleve Crews

The first place to check is the fuse box, located on the passengers side inner front fender. Make certain that all six fuses are making good contact and that all of the spade connections are free from corrosion. After that, check the junctions where several wires come together. RAY
rjm RAY

Cleve. You start with a copy of the wiring diagram. Have it enlarged to about three times the standard page size, then, have it laminated. At that point, you can use a water soluble marker to trace out all of the malfunctioning circuits, looking for a common point. Sometimes, if you are lucky, there is a single problem and fixing that problem makes all the others go away. Often, however, this is not the case and it is simply a matter of tracing each circuit, finding where you have power closest to the battery/fuse box/ignition switch, then tracing forwards to see where the problem is. Frustrating, time consuming, and it requires a lot of patient testing. But, many of us have done this in the past and you can too if you are willing to put in some hard work. Good luck with the project.

Les Bengtson

Sounds like a lot of smaller different problems all due to connector corrosion. I would just start with the battery and clean every connection. Yes all of them. The one you skip will be the one to let you down next.

For a quick result, just start with the fuse box (and inline fuses) as Ray mentions and all the earth points in the engine bay area. A quick google should get you an image of where to look for them.

You might have a duff light switch too, or it just might be the earths and the rheostat. Aside from bodywork, electrics are the most time consuming and thankless task with cars.

Start with cleaning all your fuse contacts and battery terminals. Use the conical bit on a dremel to clean the fuse box contacts. You may want to start by replacing the terminal ends as well.

I had the same issues with my 79. Replaced about 20 terminal ends, all the fuses and cables. Disconnect the battery and work your way out from there. You will probably find most of electrical issues will be done.

Let us know how it turns out.


79 MGB
gary hansen

Found the first issue. Corroded switch. Cleaned it up and now I had lights.

Cleve Crews

Did a quick switch pull on my 78 and presto, now I have lights on it too.
Funny that both cars didn't have lights due to corroded switches. Must be a common problem.
Cleve Crews

Yes, multiple problems. Horns and interior lights are probably the bottom fuse of four (not six) - brown to purple. However the headlamp flasher comes off the same fuse.

Indicators/turn flashers, wash, wipe, heater fan (if that's the one you meant) and gauges will be the second one up - white or white/brown to green.

Radio could come from anywhere, depending on who fitted it.

If your local climate is such that switch corrosion can be an issue then it's as likely to affect the fusebox as well, and maybe any number of bullet connectors. But having sat for 23 years it's perhaps a little unfair to claim it is a common problem.

The fusebox has several possible points of failure - where the spade connectors on the wires go to the spades, where the fuses sit in the holders, and there is a rivetted connection underneath as well. And having sat that long it's possible damp has got into the fuses themselves and corroded the wire and its terminations.
Paul Hunt

The light switch on this car on this car is the only thing I have found with corrosion. It was stored in a heated area, so it was not subjected to moisture. I think it may have gotten left with the top down in the past and wet the switch. While I had the covers off I looked at all the other wiring on the column and under the dash.
I'm still working on the other electric devices.
The 78 has moisture issues...
Cleve Crews

This thread was discussed between 18/03/2015 and 19/03/2015

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