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MG MGB Technical - Electrical - Lights - Diagnosis path

Putting my car back together now that it is back from body shop and need to diagnose a couple lighting probs.

headlights and parking lights work when turn on light switch on dash.

No turn signals front or back and don't hear flasher unit clicking.

No tail lights at all and no brake lights and they worked on the way home from the shop when I re-connected them so I could drive legally.

What I have done...connected ground that hooks to the license plate bolt in trunk...ground paint off the ground point, and wire wheeled the circular piece on the wire that mounts there and used dielectric grease.

Found wire on right side of trunk that was broken and reconnected it (bullet connectors and connection sleeve)...after testing (touching it to broken other side )indicated it would make rear lights, light.

CAREFULLY connected everything up front so have right color wires going everywhere. Screwed that up once previously.

Plan to:
Replace flasher unit with old one that I have that works and see if that helps the turn signal problem.

Wire past the Hazard Switch ( I have the radio console apart in order to get at the heater as I am replacing that after restoring it) to see if that is causing probs.

Check my grounds

Check my wiring at the rear end to be sure I don't have something mis-routed when I "fixed" the broken one.

Any other advice??
J.T. Bamford

oh....and it all worked properly after fixing 1/2 dozen small issues before taking it to the body shop.
J.T. Bamford

It is pretty much a step by step sort of thing. You need one of those $4.99 Wal-Mart circuit testers if you do not have one.

The first step is to verify that each lamp assemble is grounded. I had the most problems with the backup lamps.

Next go to the Fuse box and check each side of all contacts. You should clean and polish each connection. I ended up replacing the fuse box on my 77 after seeing the corrosion and intermitent operation.

From this point, go to the switch and just follow the circuit for each lamp. Just because a wire is plugged together does not mean current is flowing through. Clean each contact and recrip as needed.



R Hill

JT. Either work backwards or forwards. I prefer to work backwards. When working backwards, do as R suggests and start at the fuse box and check for power into the fuses, then through the fuses. Take your wiring diagram and keep working backwards through the system until you get to a point where there is no power. The problem is between the last good power check and the first bad power check. Then, continue on through the system until each component has been checked good and is working.

Working forwards is exactly the same, but goes in the opposite direction. Start at the grounds, then work forwards until you get power. At that point, the problem is somewhere behind you (which is why the working backwards system makes more sense to me).

If you have not had a copy of your wiring diagram expanded and laminated, you may wish to do so. This allows you to trace the various circuits with a Vis-a-Vis marker (they come in colors and are washed off with water) and check off the areas you have checked to be good. Makes it much easier for me to follow the wiring diagram and keep track of where I am.

If your flasher is a US one, you may wish to consider replacing with a Lucas one. I installed a US flasher on my GT several years ago on the advice of a parts tech. Never worked well. Replaced it with a Lucas unit and it has worked fine for several years. Les
Les Bengtson

Update:
Thanks guys....yep...time to break out the circuit tester.

Put in the clear back-up lights last night and in addition to them working I suddenly have power to tail lights and brake lights. Could I have found a ground through them that affected the others? Hate it when I accidentally fix something.

So I am down to the turn signals/flashers/hazard switch which is more usual and more logical. So will go at that tonight.

Actually my electrical system has been working quite well till the disassembly and re-assembly but I still see a new wiring harness in my future...perhaps when I remove the engine for a rebuild in a year or two.
J.T. Bamford

Everytime I have turn signal problems is tends to be the connections behind the radio console. Hope it's an easy fix. I've been there twice in the last couple of months.
Brad Batchelor

Brad....you are probably onto something there. I have console completely apart and wires everywhere in order to get at the heater cable, etc. I might just check all the connections there first.....ground wire connection, hot wire connection, etc. Thanks for pointing out what I should stinkin know by now.
J.T. Bamford

This thread was discussed between 28/06/2004 and 29/06/2004

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