One of the common issues that can cause headaches with the 968, and consequently the 944, are faulty electrical switches. The most common ones seem to be the window switches as they happen to be the ones that are most often used, although other less used switches cause problems as well. Owners have reported that cleaning materials seeping into the switches can cause issues as well. In my case, I have a couple suspect switches, and a couple that I made the mistake of not testing before removing the interior. In hind sight, it would have been better to fully troubleshoot all the switches before beginning work. For now, I have most of the switches out so I decided to test each one by checking continuity between all the pins. I did so by hooking up test leads to each of the pins and then, using the audible continuity check on my multimeter, I tested the continuity between each pin. I did this for all the switch positions (on/off, up/down, etc.). Most of the switches have a wiring diagram printed on the side of the switch. I used this to help confirm my results. Most of the switches have fairly simple diagrams that are easy to understand. Others, such as the central locking and fog light switches, are more complex with designated terminal numbers. For these I also relied on the electrical wiring diagram from the workshop manual. One thing that I did not account for is the resistance when continuity was found. It is possible that a switch will have continuity but the resistance will be too high for adequate current flow. In the end, if there is continuity but the resistance is high, there is hope for the switch. Corrosion may be the culprit at that point.

To fully understand the electrical diagrams and switch wiring, it is important to understand the Porsche logic. One document that helps to understand this was created by Alan Moore on  928 Electrics website. The “free” section of the document explains the fundamentals of the Porsche wiring diagrams. Most of the electrical symbols are fairly standard. A couple key circuit markings are listed in the following table.

Light Dimmer Switch

The first switch I wanted to test was the internal lighting dimmer switch. One of the issues I had found was that the internal lights did not work. I suspect the cause might be the dimmer switch. The dimmer switch is a basic potentiometer that varies the resistance, which limits the current flow to the lights. The higher the resistance, the dimmer the lights.

I measured the resistance at the high, middle, and low dial positions. The results were all over the place and were not consistent. The switch works on a very simple principle where an arm runs up and down a coiled wire. I went ahead a sprayed the entire switch with electrical contact cleaner, CRC QD Electronic Cleaner, hoping that corrosion is the primary issue. After using the cleaner, the resistance measurements were consistent. I could not find what the correct resistance range is but since the results were repeatable, I suspect these are reasonably close.

 

Wiper Speed Control

The wiper speed control works just like the dimmer light switch and likely suffers from the same issue as the dimmer. The resistance measurements were all over the place. A spray with the contact cleaner again solved the problem.

Window Switches

The window switches worked intermittently when installed on the vehicle. Since the alarm system had spliced into the window switch wiring, it’s hard to say if the switches were faulty or the wiring prevented proper operation. The switches tested out well, but one of the switches does not click like the other. This will be an area I will check closely when I reassemble the interior.

 

Sunroof Switch

The sunroof switch was not tested when installed on the vehicle but the results were positive.

 

Mirror Selector

The mirror selector switch worked when installed on the vehicle and the results confirmed that.

 

Mirror Adjustment

The mirror adjustment was partially tested when installed on the vehicle as the motors in the mirrors seem partially faulty. The test showed that the adjustment should work without issue.

 

Rear Wiper

The rear wiper switch was not tested when installed on the vehicle although it tested well.

Fog Light

The fog light switch was tested when installed on a vehicle although it appeared to be faulty. The fog light switch circuit is much more complex than the previous switches as it contains LEDs, which are represented by the triangular diode symbols. Pin 1 is the ground pin that links up to the two LEDs and accompanying resistors, shown as rectangles. The LED at pin 3 is controlled by the internal lighting dimmer and will light up the fog light symbol. The other LED will light up the indicator light when the switch is switched on. Matching up the test results with the circuit diagram on the switch and the Porsche electrical diagram, it appears the switch is working correctly. That being the case, there is possibly a fault in the system beyond the switch.

Defroster Switch

The defroster switch was not tested when installed on the vehicle. The switch circuit is very similar to the fog light as one LED is controlled by the internal lighting dimmer and the other LED is lit when the switch is on. Based on the Porsche electrical diagram, when the switch is on, pin 1 and 6 should have continuity. My test did not show any continuity. Just in case, I sprayed the switch with contact cleaner, but that did not change the result.

Central Locking

The central locking switch was tested while installed on the vehicle with no effect. My initial suspicion was that the aftermarket alarm had something to do with it, but after removal of the alarm it does not seem like it would have had any influence on this switch. The circuit diagram on the switch itself is not very intuitive but luckily the Porsche electrical diagram is. Pin 3 once again powers the LED via the internal lighting dimmer and pin 1 is the ground. Pin 4 which is powered by the alarm unit appears to light an indicator bulb, which is designated by a circle with an X. The switch should have continuity between pin 1 and 6 when switched on. Based on this, the switch appears to be working correctly. The connection through pin 4, which came up intermittently, shouldn’t be an issue as it is linked to the bulb and shouldn’t affect the function of the switch. Based on these results, I will be checking the connection between the switch connector and the alarm when reconnecting all the switches later on.

Main Light Switch

The main lighting switch worked when installed on the vehicle, as the headlights worked. I decided to test the switch to see if this switch might also be a contributor to my interior lighting problem. On the light switch, pin 58 powers the internal lighting. The Porsche electrical diagram shows that in the off position (the black lever arm up) the indicator bulb is always on, pin 30 (seems to be power to the high beam flasher), and pin 30b (not sure what that controls) are powered if the ignition is on (powered by X). In the second position (black arm horizontal) the turn signals (L and R), the interior lighting (pin 58), and pin 57 (which seems to go to the instrument cluster) are powered. In the third position (black arm pointed down) any pin on that line is powered. Knowing this, it should be easy to find continuity across many of the pins, especially between the X pin and the ones that should be powered. I had no such luck. I was able to get one pin to show continuity but it didn’t match the diagram. I sprayed the switch to contact cleaner, which seemed to help but again the results were poor and not worth sharing.

 

Ill be able to fully confirm all of these results when I reassemble the interior.