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MG MGB Technical - Lights not working

My 79 MGB has developed a strange problems. The signal lights work with the ignition off as do the windshield wipers. The courtesy and trunk lights work with the ignition off as they should. The headlights and tail lights do not work at all. I disconnected the ignition switch but this did not have any effect on the problem.

Has anyone had a similar occurrence which would give me a heads up as to where to start my trouble shooting? I do have a wiring diagram and have crawled around with it but have not been able to isolate anything that seems wrong.

Thans,

Jim
JMcHugh

Have you carried out any repairs recently?
Dave O'Neill

I cannot relate the two problems, but I would remove the fuse in the white/brown to green circuits, I believe it's the second fuse from the bottom. With the fuse out and ignition switch off there shouldn't be any voltage on either side of the fuse holder. I assume there will be 12 volts on one side. If it's on the white/brown side check the ignition relay W1 terminal (it should have a white wire on it) for 12 volts. It shouldn't have 12 volts unless the ignition switch is on. If nothing is there check the ignition relay. The C1 & C2 contacts should be open with ignition off. Have you checked for 12 volts at the brown wire input to the light switch? Does the position of the handbrake change anything? There is a diode in the handbrake circuit fed by a white/red wire on one side. The W/R wire goes to several places including a seatbelt module if you still have it in the car. The module is fed by a purple wire. These are some of the areas I would check, not sure if it's the best way but it should help you find where the stray voltage is coming from.

Clifton
Clifton Gordon

I had a similar problem on a 1979 MGB. I replaced the complete wiring harness and my lights did not follow normal switching. I had installed the fuse panel upside down. Turned it over and installed wiring as it should be and all was well.
Sandy
Sandy Sanders

Check for a crossed wire in the steering column.
Check the brown wire connecting to the headlight switch for voltage. Check the headlight switch for outgoing voltage.
Somewhere in the wiring the brown or purple circuits have crossed with the white or green circuit.
Installing the fuse box upside down would cause the brown or purple circuit to cross with the green circuit, but would only affect the parking lights on one side of the car and shouldn't affect the headlights.
Have you recently done any work in the dash, ie. install a radio?
Kimberly

Jim, It sounds like an earthing or a shorting problem to me - you need to check all bullet connectors - sometimes just pulling them and putting them back works.Also look for the wiring harness being chafed and worn through - this can cause all sorts of strange lighting effects - a similar problem on a club member's car turned out to be just that - but took ages to find it. good luck Cam
Cam Cunningham

Have you installed the fusebox upside down? Or the wiring on the wrong spades? The fusebox is handed in that the front of the top two fuses should be linked internally, and this powers the running lights. Upside down it will connect the purple to the ignition and so power some (in theory all except the ignition warning light unless the wiring has been altered from standard) of the ignition circuits all the time.

Bottom front should be brown
Bottom rear should be purple
2nd up front should be white ot brown/white
2nd up rear should be green
3rd up from should be red/green
3rd and 4th up rear should be 2 reds each
Top front should be empty.

The problem with the headlights will be something else altogether, check power through the main and dip switches. Do the main beams flash when you pull the indicator stalk towards you? They should, at any time, they are powered off the purple circuit.
Paul Hunt 2

Thanks to all for the replies. My fuse box was changed recently and is wired the way Paul details. The flasher for the headlights does not flash the headlights.

I will look at the headlight switch and the wires in the steering column over the weekend.

Jim
JMcHugh

If the headlamp flasher doesn't work it could be because the purple circuit is incorrectly wired. But if the horns and courtesy lights work the purple circuit would appear to be OK. If the headlights don't work from either the main lighting switch or the flasher then there could be a common problem out towards the lights, in the blue/red and blue/white wires. If the main-beam tell-tale on the dash glows with the headlamp flasher pulled or the main lighting switch fully in the the indicator stalk pushed forwards, then it would seem to be this headlamp wiring. But if the tell-tale doesn't light either then you need to look at the switches and their immediate wiring, like round the steering column multi-plugs.
Paul Hunt 2

Wiring diagrams are available at http://www.advanceauotwire.com click on stock schematics. Make yourself an enlarged copy of the wiring diagram for your car. Use a meter or test light to verify that the fuse box is installed correctly. Highlight everything that is working correctly. Use your meter or test light to find to trace the problem areas. Be methodical.
Kimberly

Thanks to all, but I'm more confused than ever. The headlight problem was apparently due to my loosening a connection when I had the steering column off. They work now but the taillights still don't. Paul was right. I installed the fuse box upside down. However, when I turned it upside down and had the jumper on the top two front connectors and wired the way Paul described, I was back to my original problem. The lights work but the ignition won't. The signal lights won't work at all. When I turned it upside down it worked again as described above. Another thing to be considered is that I have the electric fans wired directly so they are always on and they will be on with the ignition off as well as the signal lights.

Hope this description rings a bell with someone.



Jim
JMcHugh

I am not certain as to the amperage of the fuses in a 79. On my 70 it requires 35 amp. One time I installed the wrong fuse and some of the lights would not work. When I installed the correct fuses all the lights worked as they should.
Lee

With the fuse box installed correctly, use a meter or test light to see if you have continuity through all the fuses. Make a couple of enlarged copies of the wiring diagram, one for reference, and one to make notes on as you check the wiring and components.
Start at the starter and check for voltage and make sure all the connections are clean and secure, then following the wiring diagram move on to the next component.
Kimberly

You have the fans wired so they are always on *and* will be on when the ignition is off? In that case they *will* be on all the time and flatten the battery double-quick. But there is no need to wire it via a thermoswitch such that the fans can still run with the ignition off, much less bypass the thermoswitch but run it off the ignition such that they are running all the time the engine is. Largely defeats the object of having electric fans in the first place.

With the fusebox the correct way round i.e. linked pair at the top front I suspect that the ignition fuse i.e. the 2nd one up is open circuit either in the fuse itself or in its connections to the holders. Connect it up as I described, and measure the voltages.

With the ignition and lights off there should be voltage on both sides of the bottom fuse and it shouldn't drop more than a tenth of a volt if you sound the horn or pull the headlamp flasher, and these should sound/light as appropriate. There should be no voltage on any of the other fuses.

With the ignition on but lights still off there should now be voltage on both bottom fuses, both sides, you should have the ignition warning light on, and again negligible volt-drop when you turn things on like the brake lights which should light. Still nothing on the top two fuses.

With the parking lights on, irrespective of whether the ignition is on or off, there should now be voltage on both sides of the top two fuses and all the parking lights should be on.

Turning the headlights on should make no difference to the above.
Paul Hunt 2

Well, I think everything works now. The fuse box was turned upside down. Cleaning some connections seemed to get all the lights working the way they should.

Paul, I probably mis communicated the way the fans are wired. I have them wired to work when the ignition is on. After two new sensor/connections failing and causing the car to overheat I re-wired them to work with the ignition. The problem with the fans was by being live with the ignition off they were working to draw down the battery very quickly.

I'd like to have the fans work as designed but the car will very quickly overheat in traffic in the summer when they don't come on and I have found that the replacement sensors aren't very reliable.

Thanks for your help,

Jim
JMcHugh

Jim, Good to hear you found the problem.

Clifton
Clifton Gordon

This thread was discussed between 24/01/2007 and 01/02/2007

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