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MG MGB Technical - Lost most lighting - I've done something wrong!

OK guys, my electrical abilities failed me again!

Help, please!

I was trying to fit a centre rear brake light (a Thule 9902 removable one) to my roll over bar - well I fitted it but the wiring has beaten me.

I wired it into the rear brakes (one long wire into the green & purple near the steering column UJ where the loom goes to the back of the car and the other to earth). Pressing the brake peddle with the ignition on the brake lights came on briefly, pressing a second time I now have no brake lights, no indicators anywhere, no rear lights (but the front side ones work) and the fuses are all OK (1964 MGB, only two fuses).

Before I realised the indicators and rearlights were out I checked all the rear bulbs and they are are OK.

Any ideas what I have done? It seems to have caused a pretty drastic fault.

I'm about to spend ages over wiring diagrams working out what might have gone pop...

John.
John Prewer

John,
After a quick look at the wiring diagram I'd go back and check the fuse which has all the green wires on it. Take it out and ensure it and the holder are clean and try again. This fuse feeds all the ignition fed items like the indicators, fuel gauge, fan etc.
Then check all the connections at the loom where you attached the new wire. You may have disturbed a couple of connections and my main reason for thinking this is that the rear side lights are not fed by the same fuse. There could always be a second fault or someone has changed the wiring in someway. Are you running a standard set-up eg dynamo, separate regulator and +ve earth?

Best of

MGmike
M McAndrew

Mike,

I am standard but negative earth, the car has a manual tacho so the switch over from +ve was easy & done a few years back.

I had checked the fuse & put it back it but there is obviously enough muck on it to stop it working, so I just moved it around a bit and it is now fine.

The red wires for the rear lights had worked loose when I fiddled with the bundle of connectors down near the steering UJ, so they are now OK.

The brake light problem still persists.

For some reason there are a couple of unconnected wires coming out of the loom that goes under the car, and which of them is green & purple is difficult to see now even with an inspection lamp so I think the next stag of the fix waits until morning. I've tried all combinations to no avail, and I've also made sure the wires on to the switch in the brake pipes are good.

The fuel pump ticks, so the only other think down & back there which might be affected is the overdrive & I'll test that when the brake lights work!

Thanks,
John.
John Prewer

John, good that the rear lights are working. One problem solved...
First moral in wiring is:-
Don't move anything until you know what could go wrong!
"just moved it around a bit and it is now fine" yep been there, got the tie shirt..... clean it properly annually or use some Vaseline...
Forget the unconnected wires for now. these were there before the problem and will be there after.. My take on these (with out looking at the CONNECTION diagram) is simply these are "through" connections on the harness i.e one loom to the other.
Hang on a bit, don't get carried away and go looking for problems with the overdrive!! Stick to one problem at a time....
Your problem is almost certainly with the brake light circuit and more likely the connections but could be the switch. Get a volt meter and check what you get out of the switch.
BTW it's good practice to use a relay to power additional lights as the stock switches and fuses are marginal at best and any additional load will cause problems at some point.
So to summerize; disconnect the new bits. Make sure the old bits work and then add the new bits via a relay with a new fuse in the new "brake light" circuit.

best of

MGmike
M McAndrew

This thread was discussed on 28/09/2012

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