Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - Lighting Switch Overheating

I was replacing a bulb in my front parking light and I smelled wire burning. I checked the lighting switch and the input brown/blue wire was smoking. I turned off the lighting switch. After looking into it, I determined 1, the switch is bad - you have to wiggle it to get it to work, and 2, it overheats when the lighting switch is turned on to the parking lights position. Another thing is none of the parking lights work when its in the parking lights position, the instrument lights work, but not the parking lights. Is this the result of a bad switch, or do I have a short somewhere, or a bad ground?
TBP Thom Patrick

I would suspect a short somewhere on the parking light wire. A bad switch shouldn't cause the brown/blue wire to overheat. However, since the lighting circuit is not fused, a short could cause the switch to overheat and damage it so that it only works if wiggled.

The strange thing is that the panel lights still work. They should be wired to the same switch terminal as the parking lights, so a short on the parking light wire would also affect the panel lights. Perhaps the wire to the parking lights was shorted and is now burned in two. Does the brown/blue wire still get hot?

Jeff Schultz

The brown/blue wire still gets hot. Its the only one that gets hot. It takes it about 30 seconds before it starts to smoke, right at the switch. The panel lights are attached to the same switch as the parking lights and the fog light switch (I don't have fog lights attached). I briefly checked the connections at the parking lights and everything is connected. I am putting it back together after a repaint, and I labeled all of the wires before I dis-assembled. Could I have a set of wires backwards?
TBP Thom Patrick

Dissconnect all the wires from the swith and then check for a an internal short to ground in the switch. Using an ohmmeter check between each terminal and ground. Repeat for each position of the switch. The insulation in the switch may have developed a carbon track to ground.
J Heisenfeldt

It sounds like you have a bad switch, but the question is whether it was helped along by a short somewhere. You'll probably have to try various combinations of removing wires from the switch to see if any of them keep the input wire from heating / smoking. From your description of the initial problem, it sounds like the wire was heating with the switch in the Park position only. If true, that would seem to eliminate the headlamp circuit as the problem area. You should have four Red wires going to the Park position of the Light switch. They feed (a) Park Lamps; (b) Map light; (c) Panel Lamps via Panel Lamps Rheostat; & (d) Fog Light Switch.
1)I'd remove all of those wires first and confirm that the input wire isn't still getting hot.
2)If the input wire still gets hot, remove the Blue wire that feeds the headlamps. If OK, check the headlamp circuit. If not OK, suspect the switch.
3)If you make it through the first test, then I'd add each of the other wires back one at a time and see if you can find one that causes the input wire to heat. The one that causes the heat probably has a short circuit in it that you'll have to find. As Jeff said, the park lamp circuit would seem to be the culprit.

One other thing that I thought of while writing this: It's always a little suspicious when doing one thing seems to cause another thing. Had you replaced the bulb and then it started smoking? If so and as strange as it might be, try removing the bulb to see if the problem goes away.

Hope you find your problem,

LP Pittman

Thanks for the help. I'm still having the problem. I've removed all red wires from the switch and turned the switch on and the headlamps work and nothing gets hot. I then added the red wires for the panel lights and fog lights back to the switch and its fine. Its only when I add the parking lamp circuit, then the brown/blue wire gets hot. The switch is fine because everything works fine, except when I add the parking lamp circuit.

I've disconnected the red wires from all of the parking lamps and the brown/blue wire still gets hot. So I've eliminated the switch, narrowed it down to the parking lamp circuit, eliminated the wiring at the parking lamps. What's left? Is it a short in the wiring loom?
TBP Thom Patrick

Have you tried disconnecting the number plate lamp, it is on the tail lamp circuit?
Keith Morris

That says the red wire to the parking lamps is shorted to ground somewhere. There are a lot of bullet connectors in the red wire for the parking lamp circuit. You need to start disconnecting them to isolate exactly where the short is. I would start with the connector over the front passenger side wheel well. It has 3 red wires and is where the circuit splits between front and rear. Then in the front is another connector with 3 wires that splits the circuit between left front and right front. At the rear is another connector with 4 wires that splits the circuit between left rear, right rear, and license lamp.

Jeff Schultz

It sounds like a lot of people need to understand basic electricity. Some of the above explanations are combining short(low resistance high flow) with High resistance low flow. Both create heat but are VERY different.
High resistance creates heat. When the heat is localized around a single connection the connection has too much resistance. A bad switch can heat up because of high internal resistance. The connections to the switch can be bad (dirty or loose)and cause a good switch to heat up. Additionally each bullet connector can have it's own poor connection problems that get hot. These type of poor connections reduce the flow of current and lower the amperage flowing. They don't melt wires in the looms.
A short is a different problem. A short will melt wires. A short is the uncontrolled grounding of a energized wire. In a shorted situation there is too little resistance in the circuit. The amperage goes up when the resistance goes down.
To answer the original question. Having to wiggle the switch to activate the lights combined with the heat makes me think you are correct in your diagnosis. If connecting across the back of the switch with jumper wires allows the lights to operate properly and the switch area remains cool you can be quite sure the problem lies inside the switch.
R J Brown

"It sounds like a lot of people need to understand basic electricity"

I hope you were not talking about me. I am an Electrical engineer and I do know electricity. I have re-read all the posts from everyone and have not seen any from anyone that would merit a comment like that.

His last post said that he has the red wires disconnected at all of the parking lights but that when he connects the red wire for the parking lamp circuit to the switch, it still causes the brown/blue wire to overheat. That pretty much says there has to be a short somewhere in the red parking lamp circuit. The next logical step is to start disconnecting the bullet connectors to identify which wire segment is shorted.
Jeff Schultz

A real SHORT in the running lamp circuit will cause the other lights (dash, map, fog etc.) to stop working at the same time. A high resistance short will add more current in one of the four lighting circuits and could cause the switch to heat wiring. Either way, all lights should not be working.

You know you have a problem. You must replace your lighting switch if it is intermittent. This can cause heating by itself but not the lighting symptoms you describe.

I must admit there is some missing info in the scenario described. I find it difficult to see the brown/blue wire to melt and not any of the red wires getting hot.

OK so I just looked at the wiring diagram. IF the brown wire is melting at the light switch and NOT at the ignition switch end, This would indicate a bad connection at the lighting switch, either internally or externally. The intermittant switch is suspect here.

As I recall, fitting all the red wires into the switch is a bi**h. It is very easy to have operator error in this area. A stray strand of wire could be finding the chassis somewhere.

I would start at the scene of the last change, and remove the lamp that was replaced and inspect the socket and all of its local wiring. After that start removing all the individual bullet connectors in the circuit one by one.

C Schaefer

R.J. Brown is right on one thing -- the over heating at the switch is most likely caused by a poor contact. If not at the switch then in it.
David Werblow

My opinion is it's the switch. I had the same (similar?) problem, no lights without jiggling the switch. When I pulled the switch out I found the insulation melted just at the switch because I believe the high resistance in the switch was causing enough local heat to melt the insulation that was right up against the contact.

I was able to disassemble the switch and clean up all the contacts. No problems since.

- Ken
Ken Doris

I found it tonight. The light over the license plate had a short in it. I fixed it and everything works fine. I need to fix the light switch on the dash, but for now, I'm just glad to have all of my lights again. Thanks to everyone for their help!!!
TBP Thom Patrick

This thread was discussed between 01/11/2007 and 05/11/2007

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.