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MG MGA - Tail light fixture gone bad

After much reading of the archives and using Barney's troubleshooting guide (Thanks Barney) I have come to the conclusion that the tail light fixture itself was faulty and causing the fuse to blow when the brake pedal was held down for more than a few seconds. This is a 1500 restoration in progress. (work sure gets in the way with progress on this resto) The fixture looks Ok with all the paper pieces in place. What goes bad? Where is it shorting out? Is it common for the tail light lamps to light up even with the ground wire disconnected?
David Holmes

Hi David. The tail lights should NOT come on if the ground wire iside the tail light unit is disconnected. It sounds like something is miswired in the lighting circuit. Check your wiring with the colour coded diagram in the MGA manual. Additionally, corroded male and female bullet connectors can and do cause high resistance which pops fuses on a continuing basis. If your wiring harness is old, you might want to think about pulling each and every bullet connect apart, (one at a time) and thoroughly cleaning each one before reassembly. This is a lot of effort, but can work wonders when the electrical system is giving you fits! A 22 calibre gun cleaning brush is ideally suited to cleaning the inside of female bullet connectors. You will need to make a crude handle for the brush in order to use it. (epoxy the brush into the end of a wooden dowel, with a hole in the end) After cleaning the bullet connectors, a dab of silicone electrical grease will help keep the contacts clean and prevent future corrosion. Best of luck! Glenn
Glenn

I am trying to think of a basic test here...

If you take the wires off of the taillights and let them hang free, then attach a bulb for testing each one, it might give you a better idea of where the problem lies.

It does sound like you have a shorted wire in the brake circuit, so if the taillight fixtures are not the problem, you can go through the loom that goes to the rear to look for bare wires or replace that wire and run it back alongside the others.

I don't think the brake switch itself would be the culprit, but it wouldn't hurt to check it with an ohmmeter for continuity. But it must be conducting at the proper time to blow the fuses.

The taillight fixtures get a ground from being connected to the metal of the car, if there's a ground wire too, that's just an extra one for insurance. Taking the fixtures loose from the body may tell you more about this.

After reading what I've written, you should know for sure if it is the taillight fixtures when you disconnect the wires from them and press on the brakes. Still blowing fuses? That tells you it's not the fixture. It could be you need new taillight fixtures, and they're not too expensive. Try to think of all the possibilities of what happens in the circuits.

Bad grounds can cause all kinds of wierd stuff to happen. And clean all your bullet connectors with a little brass brush that is meant for cleaning gun barrels. Hope this helps.

Tom

To test the fixture, use a test meter to check that there is no short between the wire terminals (remove the bulb and wires to do this). If there is no short, the fitting is fine, and the problem is in the wiring.
dominic clancy

Remember the relay box on a 1500 will go bad, Barney as a check list on his website (IT HELPS). Also check color code green/brown looks a lot like brown/green etc.

Good Luck:Cliff(SC)
Jones

I replaced the fixture and that solved the problem. I did have the brake wire connected to the tail light connector which is a good thing. When I hooked it up correctly the red wire which is unfused immediately became very hot risking the release of the precious smoke contained inside the wiring. I might have burned up this part of the wiring harness had I hooked it up correctly in the first place with all the messing around I did in trouble shooting the problem. Although the tail light lamp was brighter on the right than on the left I thought I had a ground problem and so did not recognize my mistake. Anyway, my original questions still stand:
The fixture looks Ok with all the paper pieces in place. What goes bad? Where is it shorting out? And one more question... have any of you fixed a faulty fixure? The only reason I care is that it is an original Lucas part that looks pretty good for being 50 years old.
Now, on to the turn signals. I have right but no left.
David Holmes

This thread was discussed between 26/11/2006 and 28/11/2006

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