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kidporsche · Posted: Sun Mar 14, 2004 11:25 pm Post subject: Electrical dramas

Instrument lights not working, replaced one blown fuse but problem remains. Had a look at the dimmer pot, by bridging the pot I get lights but it gets very hot. The pot did have a huge resistance, but after poking around to make sure everything that was supposed to touch did I got a resistance around 7-10 ohms. This did not get the lights working though until I shorted the pot. So what do people think is the problem?

TIA
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Chris
1981 924 - RoW (Australian) spec

Finbarr · Joined: 14 Jan 2004 Posts: 85 Location: Worcestershire, UK

By "bridging the pot" you mean connecting the two terminals together? That should stop any current going through it, so it shouldn't get hot at all. A resistance of 10 Ohms with 12 Volts across it will dissipate just over 14 Watts of heat, so yes - it will get hot if it somehow gets the full battery voltage across it, which sounds like what is happening.

There are normally eleven 1.2 Watt dashboard bulbs fitted, that's a combined resistance of about 10 Ohms - so if the pot has a resistance of about 10 Ohms they should light at roughly half brightness, which sounds about right. At the dimmest setting, the pot will still be trying to get rid of 7 Watts of heat, so it will still get warm at least.

With the wires disconnected, does the pot resistance vary from 0 to 10 Ohms as it's adjusted? Is there any resistance to ground from either terminal?
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1980 931 (with the odd electrical quirk...)

Paul · Posted: Mon Mar 15, 2004 1:10 am Post subject:

Probably the pot, replace it before you smoke it!

kidporsche · Posted: Mon Mar 15, 2004 10:17 am Post subject:

Finbarr, that is what I meant. It was not the pot that got hot, but the connecting wire, which is what I found so strange. There must be a lot of excess current going through there, why else would it get so hot? The resistance of the pot does change, I'll check the resistance to ground and report back.
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Chris
1981 924 - RoW (Australian) spec

Roger · Joined: 06 Jan 2003 Posts: 1235 Location: Cordova, TN

The headlight switch is a common problem. Mine melted in the on posistion. The lights still worked, however the only way to turn them off was to dissconect the battery. I was able to clean the switch up and reuse it.
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1981 924 NA

Some people are like Slinkies. Not really good for anything, but you
still can't help but smile when you see one tumble down the stairs.

kidporsche · Posted: Mon Mar 15, 2004 6:07 pm Post subject:

Update:

Pulled the clock, volt meter, oil pressure part of the dash. The blue/grey power wire for the lights has melted onto the ground in a couple of places. I suspect the two wire shieldings got cut and touched, there is one point where it looks like that may have happened, but I can't be sure. When I get a chance I'll fix the melted wires and try again, but hopefully it was just a short (because if it was something else I'm all out of ideas).
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Chris
1981 924 - RoW (Australian) spec

Finbarr · Joined: 14 Jan 2004 Posts: 85 Location: Worcestershire, UK

Ah, right - the bridging wire got hot, not the pot. That makes sense.

Once you've fixed up the wiring, check the resistance to ground from the wire leaving the pot (blue/grey) before connecting it back up and turning on. You shouldn't see less than about 10 Ohms as per my original post (it might be more like 7 or 8 as lamps have a lower resistance when cold).

It's possible that the melt was a side-effect of the real problem, which may have been a short somewhere else. That wire runs a long and intricate path and may have got snagged when gauges, dashboard or heater etc were worked on in the past.
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1980 931 (with the odd electrical quirk...)