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MG TD TF 1500 - Head Light switch problem

Just when I thought I had everything working like an "MG TD" clock, something else goes wrong. I started the car everything fine then I turned on the lights to check and make sure they were working. Well the smell of smoldering wire is not good. One of the wires started to melt, so I turned everythiing off and disconnected the battery.
My question is, what caused the short?, it worked fine the other evening with the lights on. One wire got real hot and lots of insulation is missing. How do I trace this problem, is it in the switch, or somewhere else.
Hope someone can help during this holiday weekend.
Louis
Louis Levin

Louis,

You must have made some modification or change, even a very tiny one.

Things don't happen without a reason.

The smell of burning insulation suggests a high current, so start by checking your high current devices, like the lights.

Pull the connector to just one light, then the other. At least start, by isolating the culprit light circuit. Then each tail light, then each parking/turn lamp; then each horn and so on.

Check your foot/dip switch, horn, etc. If no fuses are blowing, then it must be between the device and then the actuator

Gotta start somewhere!!

Gord Clark
Rockburn, Qué

Gordon A Clark

Turn the lights back on for a second and see what doesn't light. That might get you going in the right direction.
Jim Northrup

One other thing to check for - a bad/dirty connection inside the switch, making a high resistance connection that heats up enough to heat the wire connected to it.
Cheers - Dave
David DuBois

This is where a clip-on DC ammeter would come in handy. With one of those you just clip it around a wire. Turn on the battery, if that wire is the one drawing too much current then you have found the circuit that is drawing too much current and can trace the wire to find where it is touching the chassis and re-insulate it.
Cheers,
Bob
Bob Jeffers

Louis,

What color was the wire insulation before it started to burn? If you can see the color, you can tell what circuit it was. In general, Blue wires (with various stripes) are for the headlamps, red are for the panel lights, parking lights and the wire (red with yellow) that goes up front for the fog lamp. Green is for the brakes, fuel sending, wipers etc. White is for the ignition (and on cars with turnsignals, from the relay to the rear flasher/brake lights). As it is, only the brown circuit (horns) and green circuit are fused. The white circuit is switched to the ignition, then goes to the fuse box and goes through a fuse and comes out as the green circuit. So we say that green is switched and fused.

If turning off the car stopped the burning, it's not the brown or blue circuits, but could be the white or green circuits. If turning off the lights stopped the problem it is the blue or red circuit. If disconnecting the battery stopped the problem and nothing else would, it is the brown circuit. A switch with an internal resistance problem would melt the wire close to the switch, and not far away from it.

If a lot of insulation is missing, you will probably have to replace the wire, unless you can be sure to reinsulate it the entire length of the burn. Once the source of the short circuit is found, for small areas of burn you can clip the wire, slip on some heat shrink tubing, resolder the wire together, and then shrink the tubing in place with a small heat source (a match, or hot gun works fine). Since the wire has to melt before the short, if you clip the melted area, and isolate the ends, you will be able to tell which circuit is bad (ie. what doesn't work anymore). From there simply look for a short in the wire, or bypass the wire all together.

As others have said the problem will be a wire shorted out at the switch(es) or to the body when going through a sharp edged hole, or a wire worn through shorting to the ground side of the body or device. You will need to make your disconnections at the source, i.e. the switch, and not the device (light, motor, etc.) because a short will be before the device, and disconnecting the device from its wire will not change the short. All black wires or colored wires with black tracers are ground, and should be attached to the body from from the device, except the green with black to the fuel sender, which is sort of the ground side of the fuel level warning light.

Hope this helps,
dave
Dave Braun

Louis, you may have made the same non-smart mistake I did years ago in the lighting system. I did not pay attention to the two connections in the rear license plate lamp when I connected the two wires. I very unskillfully connected them in reverse and ruined the rear harness.
Jim Merz

But before you retest, disconnect the battery and replace with a battery charger with an inbuilt circuit breaker. That way you can test everything with no danger of burning the wires any further.

Cheers,
Lew Palmer
Lew Palmer

Hi All: Thanks for your tips. The wire is(was) red. It appears that the red wires #41 are the ones that are very hot. These come from the stop lamps and number plate light and the side lamps.
It is very hot next to the switch. I turn on the key and no heat, then I turned on the lights and very hot, but no lights. I have no idea what happened, I have not done anything to the stop lamps or the side lamps. The only thing I did was to put a plastic clear washer under the chrome screw that holds the headlamps in the chrome ring.
I am very corned that something will burn up while I am tracing the short.
Please I need help.
Louis
TD #17600
Louis Levin

Louis, I suspect that there are four red wires connected to the T contact of the switch. Can you identify which of them is the culprit, or are multiple wires burned?
Bud Krueger TD

Just make sure you didn't alter something. Use the charger/breaker/fuse as above or you are likely to burn it up- no more "heat" testing!! One branch of red also goes to the panel light switch (or rheostat). You can remove all of the red wires from the swtich, and try one at a time- from the diagram. it looks like one each goes to the front sidelight, one to the rear of the car (which divides in the connector bundle inside the left rear framerail and goes to each tail light and the tag light), and the last to the panel light switch. when narrowed down, then inspect the harness for chaffing, rodent damage, etc. Also check each lamp/socket for damage-thetag light especially is pretty cheesy in construction. George
George Butz

Thanks to all for your input. I did use my charger, and tested one wire at a time until I found the problem. No smoke or heat. It was (is) the left side lamp, the wire is broken, burned under the fender below the unit. I will replace the wire and should be good to go. I will also shrink wrap the wires under the dash that melted.
This Thread has been a real help to me, and my car thanks you guys as well.
Louis
Louis Levin

Hi Guys,

Been in Maine for a week, now back and have made another summary of this thread, "Headlight Switch Electrical Troubleshooting" Again if you want the original document request to; jneel43 at hotmail dot com since some of the formatting is lost here.

Headlight Switch Electrical Troubleshooting

I started the car everything fine then I turned on the lights to check and make sure they were working. Well the smell of smoldering wire is not good. One of the wires started to melt, so I turned everything off and disconnected the battery.
My question is, what caused the short?, it worked fine the other evening with the lights on. One wire got real hot and lots of insulation is missing. How do I trace this problem, is it in the switch, or somewhere else.

You must have made some modification or change, even a very tiny one.
Things don't happen without a reason.
The smell of burning insulation suggests a high current, so start by checking your high current devices, like the lights.
Pull the connector to just one light, then the other. At least start, by isolating the culprit light circuit. Then each tail light, then each parking/turn lamp; then each horn and so on.
Check your foot/dip switch, horn, etc. If no fuses are blowing, then it must be between the device and then the actuator.
Turn the lights back on for a second and see what doesn't light. That might get you going in the right direction.

Check for - a bad/dirty connection inside the switch, making a high resistance connection that heats up enough to heat the wire connected to it.
This is where a clip-on DC ammeter would come in handy. With one of those you just clip it around a wire. Turn on the battery, if that wire is the one drawing too much current then you have found the circuit that is drawing too much current and can trace the wire to find where it is touching the chassis and re-insulate it.

What color was the wire insulation before it started to burn? If you can see the color, you can tell what circuit it was. In general, Blue wires (with various stripes) are for the headlamps, red are for the panel lights, parking lights and the wire (red with yellow) that goes up front for the fog lamp. Green is for the brakes, fuel sending, wipers etc. White is for the ignition (and on cars with turn signals, from the relay to the rear flasher/brake lights). As it is, only the brown circuit (horns) and green circuit are fused. The white circuit is switched to the ignition, then goes to the fuse box and goes through a fuse and comes out as the green circuit. So we say that green is switched and fused.

If turning off the car stopped the burning, it's not the brown or blue circuits, but could be the white or green circuits. If turning off the lights stopped the problem it is the blue or red circuit. If disconnecting the battery stopped the problem and nothing else would, it is the brown circuit. A switch with an internal resistance problem would melt the wire close to the switch, and not far away from it.

If a lot of insulation is missing, you will probably have to replace the wire, unless you can be sure to re-insulate it the entire length of the burn. Once the source of the short circuit is found, for small areas of burn you can clip the wire, slip on some heat shrink tubing, re-solder the wire together, and then shrink the tubing in place with a small heat source (a match, or hot gun works fine). Since the wire has to melt before the short, if you clip the melted area, and isolate the ends, you will be able to tell which circuit is bad (i.e. what doesn't work anymore). From there simply look for a short in the wire, or bypass the wire all together.

As others have said the problem will be a wire shorted out at the switch (es) or to the body when going through a sharp edged hole, or a wire worn through shorting to the ground side of the body or device. You will need to make your disconnections at the source, i.e. the switch, and not the device (light, motor, etc.) because a short will be before the device, and disconnecting the device from its wire will not change the short. All black wires or colored wires with black tracers are ground, and should be attached to the body from the device, except the green with black to the fuel sender, which is sort of the ground side of the fuel level warning light.

Pay attention to the two connections in the rear license plate lamp. If they are connected in reverse it will ruin the rear harness.
Before you retest, disconnect the battery and replace with a battery charger with an inbuilt circuit breaker. That way you can test everything with no danger of burning the wires any further.
The wire is(was) red. It appears that the red wires #41 are the ones that are very hot. These come from the stop lamps and number plate light and the side lamps.

It is very hot next to the switch. I turn on the key and no heat, and then I turned on the lights and very hot, but no lights. I have no idea what happened, I have not done anything to the stop lamps or the side lamps. The only thing I did was to put a plastic clear washer under the chrome screw that holds the headlamps in the chrome ring.

There are four red wires connected to the T contact of the switch. Can you identify which of them is the culprit, or are multiple wires burned?
Just make sure you didn't alter something. Use the charger/breaker/fuse as above or you are likely to burn it up- no more "heat" testing!! One branch of red also goes to the panel light switch (or rheostat). You can remove all of the red wires from the switch, and try one at a time- from the diagram. it looks like one each goes to the front sidelight, one to the rear of the car (which divides in the connector bundle inside the left rear frame rail and goes to each tail light and the tag light), and the last to the panel light switch. When narrowed down, then inspect the harness for chaffing, rodent damage, etc. Also check each lamp/socket for damage. The tag light especially is pretty cheesy in construction.

I did use my charger, and tested one wire at a time until I found the problem. No smoke or heat. It was (is) the left side lamp; the wire is broken, burned under the fender below the unit. I will replace the wire and should be good to go. I will also shrink wrap the wires under the dash that melted.


Thanks to: Louis Levin, Gord Clark, Jim Northrup, Bob Jeffers, Dave DuBois, Dave Braun, Jim Mertz, Lou Palmer, Bud Krueger and George Butz
James Neel TD28423

This thread was discussed between 30/05/2010 and 02/06/2010

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