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MG MGB Technical - blown fuse

71 roadster.
The indicators had stopped working and the fuel guage and rev. counter also. I found a fuse had blown so replaced it. Shortly after this also blew so there must be a short somewhere. can anyone suggest an obvious place to start my search. Thanks Trev
Trevor Harvey

Hi Trevor

I had the same problem recently, and discovered the fuse was blowing when I selected reverse. Unfortunately before I could trace the problem it "cured" itself when I was tracing the wiring at the joints below the fuse box. As soon as the weather is warmer I will disconnect the joints and find the problem. This may help you.

Ronnie
RA Potter

If it's blowing straight away it's easier to find than if it's intermittent.

Replace the fuse with a 12v bulb and if it glows brightly the short is on, so remove first one then the other green wire from the fusebox until the light goes out. Put the other wire back on and replace the fuse, then trey all the ignition powered stuff to see which things don't work, and the short is on that circuit.

On a UK 71 there should be three green wires, two of then are in the same spade connector so can't be separated. One of the wires feeds the instruments and the flasher. Another feeds the brake light switch, and the third feeds the reversing lights, which might help.

Two common problems are things in the boot that have damaged green/purple (brake lights) and green/brown (brake lights), and another is the sub-harness that connects the brake light switch to the fusebox and the rear harness.

If it's intermittent you will have to put a lower value in-line fuse (say 5 or 10 amps) in series with one of the green connections to the fusebox. If that fuse blows, then you know that circuit is causing the problem. If the main fuse blows, then you know that it is the other circuit i.e. the one without the inline that is causing the problem. Then depending on which it is, you may have to put the in-line in series with various green wires that are in the 4-way bullet connector behind the dash. But my money is on the reversing or brake lights.
PaulH Solihull

I had the same problem. It was driving me nuts until I got under the car and noticed that the reverse light wire was rubbing on the cross member. Most of the time, the insulation kept the wire from shorting out to the cross member, but if the chafed wire shifted just a little bit,it contacted the bare cross member and blew the fuse. RAY
rjm RAY

Thanks to those who responded to my inquiry.
I have had a chance to try to track down the cause of the fuse blowing.
The blown fuse takes out the fuel guage, rev counter, indicators, brake lights, and reversing lights.
I put in a lower amp fuse and tested each thing that had not been working,as soon as reverse was selected the fuse went.
I have had a look under the car to see the reverse switch and had a visual at the wiring in the boot but can see nothing obvious.
Is there a way to pin point where this problem could be coming from other than trying to follow the wiring back through the harness.
Also as the reversing switch is towards the top of the gearbox, is it more accessable from the top, once the gear lever is removed.
Thanks Trev
Trevor Harvey

The fuse will take out all the fused ignition circuits as there is only one fuse on that year.

Most likely in the boot, but. Disconnect the green/brown where the gearbox harness joins the rear harness by the fusebox and try again.

If the fuse blows it's the gearbox harness.

If not it's the rear harness or lights, in which case replace that connector, check that the fuse still blows (could be intermittent and you have disturbed it) then disconnect the green/browns from the 4-way by the right-hand rear light and try again. Of the two green/browns there one comes in from the front of the car, and the other goes across to the left-hand light.

If the fuse blows it's the rear harness under the floor. If not, and the fuse blows when you reconnect that bullet, its the wiring across the back of the boor or the lights. To isolate which reconnect the feed from the front of the car to the 4-way bullet, then the other one, and disconnect the light unit tails from the bullets one at a time, making a note as to what blows the fuse and what doesn't. If you put a high wattage 12v bulb in place of the fuse you can tell from the brightness when the short is on or not to save all those fuses.
PaulH Solihull

After spending most of the day trying to trace this short useing Pauls tips, I think and hope I have found the problem. It appears to be a faulty reversing light as with this disconnected the short seems to have dissapeared but I wont know for sure until a new light is delivered.
Trev
Trevor Harvey

This thread was discussed between 22/02/2013 and 05/03/2013

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