Tom's 87 928 S4

I have fuse #44 that keeps blowing, and no dash lights, so I removed the two connectors to the rheostat wheel linking them with a jumper but still blowing the fuse, and I also replaced the headlight switch because the fuse was blowing without the key in the ignition, and whenever I was turning the headlight switch on. Now, I put the two connectors back on the rheostat wheel, and the fuse does not blow until I turn the wheel to the brightest location.

Do I need a new rheostat wheel, or is the short somewhere else?

Thanks guys

Ed Scherer

Rheostat is OK, given what you've said. No matter what happens inside that variable resistor, it can't cause a short (not in any way that I can think of) to ground by itself. Its failure mode would be more likely to be "open circuit", which would cause your instrument lighting to fail (without blowing a fuse) when your parking or headlights are on. Since you evidentally have a short somewhere after the rheostat, having it positioned at the "dim" end leaves about 4 Ω to 6 Ω of resistance to ground; enough to allow only about 2 A to 3 A of current to flow and keep the fuse from blowing. Move it to the other end, though, and that's close to 0 Ω, thus the fuse blows.

Something after it, though, is likely shorted to ground. That could include anything from the wires leading to/from the resistor mounted on the firewall (slightly to the left of the bracket that supports the pivot point for the brake pedal) to most of the pod switches, to the A/C control head backlight, to the clock backlight, lighter backlight, radio backlight, instrument cluster backlight, etc. Basically, anything that has one or more bulbs that the rheostat dims.

You'll be looking for wires color-coded BK/BL (black with blue stripe). Or perhaps a bulb socket has pulled out somewhere and shorted to ground (I think that's possible with the socket for the lighter backlight bulb).

BTW, I assume that the fuse only blows when you turn on your parking or headlights, right?

Don Ashe

This should not happen, so the problem is not the rheostat. Something is drawing too much current - possibly multiple dirty connections or even a shorted bulb. (yes, there is such a thing), but the rheostat sounds like it is working exactly as it should.

HTH,
-don

Tom's 87 928 S4

Thanks for the reply Ed and Don,

Actually, I can turn on the lights now without blowing the fuse until I adjust the rheostat to the brightest location, then I hear the fuse blow. The other night I moved the pod up and down to show my girlfriend, and I think maybe I did something then? I need to remove the pod anyway as the speedo only works intermittently this weekend.

Ed Scherer

All sorts of interesting events begin that way...

Seriously, though, that might be a good hint. There are a lot of those BK/BL wires in the pod (lots of the pod switches have BK/BL wires leading to them for the backlit buttons), including, of course, the one connected to the rheostat that feeds all the rest. Perhaps one of them got pinched and shorted to ground.

And... in case you weren't aware of it, you can remove the instrument cluster without totally removing the pod. Procedure is covered in Schocki's thread "15 minute instrument pod removal update and instrument light bulbs for an OK price" (and to some extent, in my guide Converting Porsche 928 Interior Lighting to LEDs). These guides, though, aren't a perfect match for your '87 (which I believe uses different connectors to the instrument cluster).

Tom's 87 928 S4

Excellent. Thank you Ed. You have been most helpful.

MainePorsche

Ed,
Now that Tom seems on his way, I have a question. What is the significance of tracking the Black-Blue wires ? I have a longstanding gremlin that I think is related to a ground feedback loop at the MP you were mentioning above.

Ed Scherer

The BK/BL wires are the high side for all the instrument lighting. Put a bulb between any of those BK/BL wires and ground, and it'll illuminate (with an intensity governed by the dimmer rheostat, which is basically limiting the current flowing to all of the bulbs on this circuit). This is only, though, with the parking lights or headlights on (with the exception of the instrument cluster backlighting; it also remains lit even with the parking/headlights off).

Unfortunately, there are a lot of these bulbs and these BK/BL wires all all connected one way or another. If you're trying to track down a short in this circuit, it wouldn't hurt to have the wiring diagrams at your side. And realize that any short to ground on any of these wires, bulbs, etc. will cause the fuse to blow, as is happening to Tom.

Craig, you might want to start another thread on the problem you're having, as I'm not very clear on what you're looking for.



BTW, Don's comment about the possibility of shorted bulb (related to Tom's problem) is an interesting possibility, too. It's probably unlikely, but I, too, have seen incandescent bulbs fail in a way where the filament breaks along its original path, but then a shorter segment of it reattaches, providing a lower resistance (and as if it continues to operate for awhile, brighter light!)

Hurdigurdiman

Has anyone had trouble with the inside courtesy light and inside kerb light not coming on as either door is opened? I have checked and the fuse and all bulbs are working. I removed the inside cluster above the mirror and bridged across a couple of contacts and all bulbs are working ok. Can anyone tell me where to locate the actual door switch which brings on theses lights. I have a 1999 C2.