Need wiring diagram help (speedo and defroster)

[ohecht]

I have poured over the wiring diagram for my 82 SC for hours trying to diagnose an intermittent electrical fault, but I cannot find any way the two components in question could be related:

Sometimes, but not always, the speedometer will stop working when I turn off the rear defroster. Never when I turn it on. What I do when I need the defroster is move the switch to the off position after shutting off the car, and it never causes a problem that way. Once the speedometer stops working, it will sometimes come back by itself in a few minutes, hours, or days. Often, I can speed up this process by repeatedly cycling the defroster switch between the right turn and off positions. THe speedometer will also come back to life only as the switch is moved into the off position.

The defroster works, but only on several bands in the middle of the glass. There is one continuity break in the grid near the middle that I have been unable to repair.

The only other electrical issue with the car is the cruise control, which I have traced to a clutch switch, servo, or control unit, but I am still troubleshooting slowly (I don't really have a need for it).

I have rerouted the ground for the speedometer to its own location to eliminate any grounding issues, and the problem persists.

I cannot find any link in the electrical system between the rear defroster and the speedometer. Could it be related to the defrosting of the rearview mirrors? I've been trying to see if there is any relation to the car being in the rain and the problem happening.

I'm hoping someone with a similar experience or more electrical knowledge can give me some leads here.

Thanks,

Olivier

[Roland Kunz]

Hello

Very strange, still head scratching, will just boost it up again.

( And add my subscription to it to find it easier )

Grüsse

[Roland Kunz]

Hello

Did you checked the fuseblocks for corrossion ?

Grüsse

[ohecht]

Yes, one of the first things I did when I got the car 18 months ago was to replace all the fuses and check the contacts for corrosion. There wasn't very much at all.

Olivier

[Roland Kunz]

Hello

Can you tell me what is the problem with the cruise ?

Did you checked if something changes when you removed the big connector on the brain from the cruise unit ?

Grüsse

[ohecht]

I have only been troubleshooting the cruise control sporatically as I have been near the controller for other reasons. I followed the troubleshooting procedures and found that the clutch switch was out of spec, but when I checked the swith itself, it seems to function fine as a simple switch and it was adjusted to operate correctly with the pedal. The cable from the cruise servo has been broken since I got the car. I replaced the cable with a home-made version and it still did nothing, so I think the servo itself may be bad. If I remember the troubleshooting steps correctly, there was an "either/or" relationship with the servo and the clutch switch based on the test for the clutch switch at the controller. I'm not sure how the cruise would effect the speedo operating or not with just a signal wire, and especially considering the fact that the entire problem is somehow related to the defroster switch. Nothing has ever gone wrong with the speedometer when I have sometimes activated the cruise control switch in all directions while driving.

I'm hopeful you can think of something, as this has bugged me for months now.

I've been thinking of rerouting the power to the speedometer to a totally independent run-power source in order to eliminate any power issues as I did with the grounding issues, but I'd rather find the real problem.

Could it be related to the fact that only part of the rear defroster grid functions? I have continuity across all the lines except one, which I have tried unsuccessfully to repair with one of those liquid metal kits. Continuity everywhere else seems to be fine among the 3 separate sections of the grid.

Thanks,

Olivier