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MG MGB Technical - Main lights on at ignition

Hi,

Starting to put my 'other' beast - 1972 Bgt back into comission after a year out of MOT.

Got the car started up ok after cleaning up the main connections etc. - seems very little needs to be done for MOT Test. Hooray!

However, one funny thing happening. All the side lights come on without the light switch being in the on position but no front light i.e. low/main beam. Is it a knackered switch do you think?

Ta in advance.

Iwan
Iwan Jones

Check the fusebox connections. The 2nd fuse up (white at the front, green at the back) is live when the ignition is on, and the 3rd and 4th fuses up (a single red/green on one of them at the front, four reds at the back two on each) are for the sidelights. If fuses 2 and 3 come into contact for any reason the parking lights will come on with the ignition. And you won't be able to turn the engine off if you have switched the lights on!

If the parking lights are on all the time the battery is connected then that probably is a faulty main lighting switch, or its brown and red/green wires shorted together.
Paul Hunt

Hi,

Thanks for your reply Paul.

I've looked for a fault at the fuse box - can't see any.

If I remove all of the red cables from the 3rd and 4th fuse terminals both red lights at rear parking light still on.

If I remove all the wires from the light switch at the dash, rear red parking lights still on.

However, if I remove the green cable at third fuse - light still on if I remove the twin green cables behind lights go out. Hope this helps as I do not have a clue!

Cheers

Iwan
iwan

Iwan...
Check to see if the fuses are really in their proper clips.strange thins happen if the fuses have been installed (squeezed) betweeen the clips.
By any chance has the fuse block been installed upside down?
Sandy
Sandy Sanders

Hi,
Thanks for the reply Sandy. The answer is that the fuses are all fine not between any clips etc. Fuse block has not been moved since last MOT.

The problem was not there before its lay-up. Nothing has changed to the electrics it has now appeared - that is what I can't understand.

Ta

Iwan
iwan

Hi,

If its on the green twinned cables - could it be that the brake light swich is frazzled?

Rather than parking lights on as this is all in the rear?

Iwan
iwan

Iwan - Check that the fuse block has not been installed upside down. It seems that this is a rather popular thing for DPOs to do. Cheers - Dave
David DuBois

If the four red wires are removed from the fusebox and all the parking lights are still on then it can't be anything to do with the fusebox.

If you are saying that if you then remove the single green connector from the fusebox they stay on, but when you remove the other connector with two green wires they go off, then there is some connection between the one of those two green wires and the red wires. Of the three greens one feeds just the brake lights, one feeds just the reversing lights, and the third feeds everything else like the instruments, indicators, and heated rear window. Without parting the two greens from the single connector you aren't going to find out which of those two is involved.

But it isn't even as simple as that, because each of those four red wires feeds just one corner of the car. If all four corners including the number plate lights are still on when all four have been removed from the fusebox then there is a lot more going on than just a simple connection between a green and a red. If the car has been laid up then I'd be looking for major rodent damage of the main harness in the area of the fusebox and the mass of connectors where the main, rear and gearbox harnesses all join together.
Paul Hunt

This thread was discussed between 27/07/2009 and 29/07/2009

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