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MG MGB Technical - Panel Light Switch - Melting Insulation

My panel lights weren't working on my '64 MGB, so I removed the switch/rheostat and cleaned the terminals. The dimmer still wouldn't work properly (the wire coil inside had broken), but at least it worked as far as on/off, and wasn't worth the $70 for a new one. But when I reattached the courtesy light and panel light wires to the switch terminals, and turned on the headlights, smoke came out from the panel light wires. The wire insulation had begun to melt!

So I need help on two questions: 1) Could the melting have been caused by my reversing the courtesy light and panel light connections?

2) Can I simply use electrical tape to cover the wire where it was melted, or do I have to do a major rewire for the length of the circuit? I want to do the right thing and be safe ....

Brian

Brian S

Brian - I don't understand "courtesy light" in a 64 MGB. The 64 MGBs didn't come with courtesy lights. If someone wired some kind of courtesy light into the car, it shouldn't come through the dimmer rheostat, but rather be wired directly to 12 volts through a door switch.

The wires for your panel light overheated because somewhere along the line, there is a wire shorted to ground. The very first order of business is to find that short to ground and rectify it. Once that is done, then you can go about repairing the melted insulation. The first choice for the repair would be to replace the wires. the second choice would be to cover the entire length with shrink tubing (about the same amount of work as replacing the wires). Electrical tape over the melted areas is an inventation for future failure. Electrical tape has a habbit of coming loose over time. Good luck - Dave
David DuBois

Dave -
Sorry - I called it a "courtesy light", but meant the map light. I'll trace the short back, sure hope it doesn't go back to the harness. THAT won't be a fun job.

Brian
Brian S

Your short probably blew the rheostat winding. fix it before you drive it again. the short could have melted other wires inside the harness as well.
R. L Carleen

Brian - Someone obviously miswired the panel lights/map light if they both come off of the same side of the rheostat. The map light should have full 12 volts going to it, thus it should be wired to the opposit terminal from the panel lights. I think that I would be looking at all of the wiring in the car to see what else the PO may have miswired (plus finding where the short circuit is that melted the wireing). Good luck - Dave
David DuBois

Dave -
Actually the map light and panel lights were on separate connections (as opposed to the same connector) on the rheostat. I may have reversed them when I reattached, but it still shouldn't have caused the melting.
In rechecking the wiring diagram, it would appear I need to rewire from the headlight switch to the panel switch (and maybe back over to the map light). I think that would alleviate having to go back through the harness.

Brian
Brian Stevens

Hi Brian.

It is definitely worth working out exactly why the wiring melted, as this will help you to identify the wiring that is likely to be damaged and prevent it from happening again, as David says.

I would certainly unwrap any part of the loom that the affected wiring goes through, and inspect each wire.

The wire that carried the excess current is obviously the prime candidate for damage, but the heat from this wire may have damaged the insulation of adjacent wires.

The wire that carried the excess current may have damage to it's copper conductor as well as its insulation, I would replace it.

Don
Don

The panel light wires are red/white and go one side of the rheostat, the other side of the rheostat should have red wires which come from the main lighting switch and go on to the map light.

If the *panel* light wires melted when you put everything back together, and worked before, then then sad to say you caused a short somewhere when working on the wiring. This will be on the red/whites out to the gauges, but will have overloaded the red wires, main lighting switch, and brown wires back to the solenoid.

If the red/whites were on one connection of the rheostat (which probably has two spades on each of the two connections) and the reds on another then the short circuit current will also have gone through the rheostat. If this was on its brightest setting, and is of the wire-wound variety, then you may be lucky and not have damaged the rheostat. But if it was just off its brightest setting, or you have a printed circuit type of rheostat (all the ones I have seen are like that) then you could well have damaged that as well.
Paul Hunt

This thread was discussed between 16/05/2005 and 17/05/2005

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