Instrument Lights Won't Turn Off - 85 Carrera

[CraigDseattle]

Greeting~
Driving home last night I was messing around with the cover for light that is under the parking brake handle which has the arrows for the defrost control. As I pushed down on it, the light went out and shortly there after I smelled something burning and saw some smoke. That light housing was hot, but I couldn't see where the smoke was coming from - I think it was from there.

I poured some water on that light as I was driving, then pulled to the side of the road and fiddled with it again. I had my flashlight out by now so I could see - there was no more smoke, and the light came back on for the remainder of my drive home.

I was thinking that I may have shorted out that light to the floorboard by pressing down on it hard to close the housing (the housing just screws to the floorboard) and that moving it again broke that contact, and the light came back on.

When I got home and parked, I turned off my lights, but the instrument lights stayed on. They would not turn off regardless of what I did with the head light switch.

So - long way around to my questions. If I shorted out the lamp on the floor, could that have fried something inside the headlight switch that causes the instrument light to stay on all the time? Is there something else I should check?

Thanks in advance for the help!

[CraigDseattle]

So I replaced the headlight switch, and the situation persists: dash lights stay on with all power off and keys removed.

I looked at fuse panel and removed #3 for interior lights. That caused the dash and interior lights to act as expected - that is, turning on and off with headlight switch. However now the door switches for interior lights only work when the headlights are on. No interior lights with head lights off.

Apparently, the dash lights get power thru different routes?


Any ideas?

[CraigDseattle]

Bumping this to see if anybody has thoughts. Not understanding the wiring situation. Are there 2 routes of power to instrument and interior lights?

[DRACO A5OG]

Okay was the smoke coming from behind the dash or at the light where you were pressing?

Obviously some melted and now is grounding out the circuit. Behind the gauges the black with blue line are the gauge and console lights.

You may need to try find the burnt out wire/s. Sorry to write replacing the switch was not necessary.

[timmy2]

Red/black wires to the gauges are a possible source of power that may be shorting to the dash gauge lighting circuit.

It isn't as simple as having the rheostat for gauge illumination always on is it?

[CraigDseattle]

Thanks guys -

The reostat being on isnt the issue. The instrument lights stay on all the time - key in or out, head lights on or off.

With the #3 fuse removed, the dash lights and interior lights go off, expect function normally when the head light switch is 1/2 on or fully on. Goofy.

The #3 fuse terminal has a big red wire on the hot side, and a rather small black wire on the opposite side of the fuse.

It was dark, so I can't be certain, but I think the smoke was coming from the floor board area by the defrost light.

Any suggestions for locating a good wiring diagram for this system?

Thanks!

[GH85Carrera]

We all like to think Porsche is all knowing and wise in their engineering. The fact is that the electrical parts of our cars were ever evolving with no major redesign until the 993. The electrical parts of our cars started life in the 1960s with very few demands. As more features were added the were just appended on. The 911 was scheduled to be at the end of the line in the 1980s so the budget to do a real update was not there.

The point of that is there are several places on your car that needed a fuse and one was not there. The dash lights can and have shorted out and started fires much like the front AC condenser blower.

Search through the site for the electrical threads. There should be a fuse added to your headlight switch for the dash lights. It sound like yours is shorted somehow and providing power to the dash lights when they should be off. Electrical trouble shooting is almost as much fun as fixing a bad sewer drain. Good luck.

[DRACO A5OG]

That is why I have Fuse at the Gauges, Foot Blower, Front Condensor and Head Light Relays. Fellas, if you have not done so this thread is a good example of why fuses are needed.

Shoot Dr Gerry aka 86 911 Targa he may have a diagram for you??? He generously helped me allot on all my fuses and AC electrical poop too

You may need to tear into the Frunk compartment behind the fresh air blower, a real big PITA but that is where the wiring for the fog and defroster switches are to find that shorted wire

[kidrock]

I'd start by looking where the problem originated from in the first place: that defroster light.

[sugarwood]

Anyone know a link for front condenser fuse directions?
I couldn't find one.

[DRACO A5OG]

Search 86 911 Targa, he posted a tutorial.

[timmy2]

CraigDseatttle,
Porsche wired their cars by sending unfused power to the switching devices and then back to the fuse panel for most of their circuits. That also didn't fuse a number of circuits.
Keep
This in mind when troubleshooting each circuit that runs to the dash area.
You can pull every fuse and still have power with the battery connected.
Be careful.

[CraigDseattle]

Hey guys - thanks so much for the comments and thoughts. I am off work tomorrow and will have some time to investigate further.....starting an the defrost light as suggested. I believe the back looks a bit funky, and the screw (that is too long and penetrated the carpet) has a residue on it that smells burnt. Perhaps the wiring in the area is shorted??

Thanks again, and TGIF!

Craig

[CraigDseattle]

Sorry - another question.

I don't seem to have a wiring diagram in my Bentley that does a good job showing the instrument panel lights, defrost control lights, and interior lights. Any suggestions for where that might be found?

Thanks again

[timmy2]

You could download the PDF of the '78 schematic our host has posted on the site here at Pelican and study it.
It really isn't something that changed much other than the addition or deletion of a few lamps over the years.
Print it out and tape the overlaps together and it is fairly easy to follow the circuits once you figure out the bottom is the ground reference and the numbers on the bottom are the legend location references.

[timmy2]

I normally have Friday's off but have to work so I am envious of your time off tomorrow.

If you have questions post them and the many great members here will be sure to chime in.

[CraigDseattle]

Howdy -

I'm still fiddling around with this issue - - I've traced the fried wire, which appears to be isolated between the "defrost lever light" on the floor, and the point where that wire connects to the wire network under the dash that links all the instrument bulbs together. The rest of the black/blue wires appear to have not obvious heat damage.

I disconnected the remains of the fried wire - reconnected the battery, and now I have no instrument lights but the interior light appear to operate correctly.

I don't see any blown fuses, and I think I was mistaken before about #3 - that appears to be for the interior dome lights, but no the instrument lights. I think that fuse #7 in panel #2 is the correct one for the dash lights, but it appears to be okay (visually).

when I replaced the headlight switch, I was just reaching up under the dash, so I'm thinking I may have caused something to get disconnected up there that may be contributing. In the attached picture, you can see the headlight switch, and another "plug" that came down when I was fishing around.

If I get my big ass in a position to look up under the dash, it is obvious where the plug is supposed to connect? It has what looks like factory original wires coming off the headlight switch (visible in photo), along with some wires that somebody spliced on via the clamp on connectors that are visible (don't know where those go).

So - a couple of questions. 1) are there any other fuses that may be involved that are not visible in the luggage compartment fuse panels, and 2) Where does the big plug connect?



Always appreciate the thoughts and comments.

Have a great Holiday everybody!

[kidrock]

the big plug is a "bulkhead connector", and should be a near straight shot from the headlight switch into the bulkhead (the wall that separates the cabin from the front trunk).

Can't personally vouch for any fuses and stuff, not my strong suit. I'd be searching for fried and/or disconnected wires, and checking the fuse panel closely. That's about the extent of my limited experience.

[redranger]

Plug connects just above and to the left of the headlight switch. Get under there on your back, Shine a light and you'll see it. Does the light on the left of your hvac controls on the dash work? Ashtray light work? All gauge lights work? They are all on same line. I think you have a short somewhere, lucky you didn't get more smoke and fire, usually at the switch. You want to follow the blue/black wires and see where they might be touchng a brown wire (ground) or bare metal. The blue black wire runs in a daisy chain across all gauges, it also runs down to the floor heater control.

I went through this same thing and had a short bejind my clock. It had melted pretty far back so decided to run a new set of wires from the headlight switch to each component. Only way I could be sure it was not going to touch ground. I also put in a 3amp inline fuse so I don't get a fire.

One thing I haven't done is replace bulb at floor heater control. How does that come out?

[Neilnaz]

There are 2 plugs just to the right and behind the position of the headlamp switch in my 1971 911. One of those connectors goes to the headlamp/indicator switch/stalk. The wiring from that switch/stalk also goes to the headlamp switch. This is normally achieved via 2 sleeved bundles of cables.

I wonder whether it is possible that someone has bypassed some of this or modified this in some way. I attach a picture of what a 1971 headlamp/indicator switch looks like. You can see the connector which looks like yours. Some of the other wires connect to the headlamp switch.

This is just a thought.

Regards, Neil