[Jazzbass]

We get the old "my power windows don't work" question every so often here, as it's a pretty common failure point on these old cars. The culprit is usually worn out window switches. This circuit is a bit convoluted as Porsche electrics are wont to be, but pretty easy to troubleshoot if you have the schematic and basic knowledge of how the circuit works.

The schematic (for 84-89 Carreras - SCs, see post post #9):



This is from the factory manuals for a 1985 911 Carrera, but all Carreras should be similar. I'll dig into the SC diagrams later and update if there are any major differences.

How it works:
The Power Window Relay (53) at the top controls power to the whole circuit. This is a timer relay (hence the circuitry they are helpful enough to show you inside) - it's job is to switch power from pin 30 to pin 87 whenever voltage is applied to pin 86 (from bus X), and to stay energized for a certain amount of time after the pin 86 control voltage is turned off. This is what allows you to roll the windows up and close the sunroof for a couple minutes after you've turned the ignition off. It switches power directly from bus 30* (pin 30) to the power window fuse (fuse 1), which connects to the relay pin 87 via a 12ga black wire.

From the other side of fuse 1 we go to the actual window switch circuit via a huge (8 ga IIRC) red/blue wire, which gets pared down to a 12ga red/blue wire after the junction point with the power seat circuit. From that junction it goes to a 6 pin connector (god knows where in the car) and then to the window switches in the driver's door via a 16ga red/blue wire.

Quick note - there are three switches: two for the passenger window (PW) and one for the driver's window (DW). Two of the switches are in the driver's door (DD) and one is in the passenger door (PD). Combining all the abbreviations, we have the following three switches: driver window, driver door (DWDD), passenger window driver door (PWDD), and passenger's window, passenger door (PWPD). I've noted these switches on the schematic. Porsche part number 911.613.621.03 for 74-89 cars.

So, this red/blue wire is the main power feed to the window switches and goes to pins 3 and 5 of both the PWDD and DWDD. The passenger window switches in 911s are wired in series, with the power going to the PWDD first, then to the PWPD, then to the window motor itself. Pin 4 of all switches is ground via a 16ga brown wire.

Let's take a minute to look at how the switches themselves work. All three switches are exactly the same. They are modified double pole, triple throw switches - position 1 is momentary up, position 2 is the center (this is not an "off" state), and position 3 is momentary down. Unlike a momentary DPDT switch, the center of these is not off, as the input terminals are still connected to the output terminals. The in/out connections for these switches in all three positions are as follows:

Up: Pin 2 to Pin 5; Pin 1 to Pin 4
Center: Pin 2 to Pin 5; Pin 1 to Pin 3
Down: Pin 2 to Pin 4; Pin 1 to Pin 3

Back to the schematic. According to the diagram, pins 1 and 2 of the DWDD are wired directly to the motor via 16 ga black (pin 1) and 16ga green (pin 2) wires. According to my car, pins 1 and 2 are wired to the motor via 16 ga blue/black (pin 1) and 16ga blue (pin 2) wires. YMMV. The window motor is simplicity itself - a simple DC motor and you just reverse the polarity of the voltage going to it to change its direction. This is all the windows switches do. Push down on the DWDD switch and you should see +12V on pin 1 and ground on pin 2, and the window should go down. Push up on the switch and you should see +12V on pin 2 and ground on pin 1. In the center position there is +12V on both pins 1 and 2.

The PWDD switch is wired the same as the DWDD switch. The difference in the circuits is that the output pins of the PWDD switch (1 and 2) don't connect to the passenger window motor directly, but the to PWPD switch first. Troubleshooting in the driver's door, however, you should see the same voltages on pins 1 and 2 as you did for the DWDD switch: UP = +12V/pin 2, ground/pin1. DOWN = +12V/pin 1, ground/pin 2. Center = +12V on both pins 1 and 2. Having the +12V on both pin 1 and 2 of the PWDD switch is important for PWPD switch to work.

The PWPD switch is thew same switch as the others, so it's connections are the ones listed above. The difference in wiring is that the driver's door switches have their pin 3 and 5 inputs connected to +12V directly. The pin 3 and 5 inputs on the PWPD switch are connected to the outputs of the PWDD switch. This is where you have to pay attention to the schematic to find problems.

Output pin 1 of the PWDD switch connects to input pin 3 of the PWPD switch, via a 16ga red/white wire (according to the diagrams). Output pin 2 of the PWDD switch connects to input pin 5 of the PWPD switch, via a 16ga red/black wire (again, according to the diagrams). From here it's switched just like the driver's door switch, and output pins 1 and 2 of the PWPD switch connect directly to the passenger window motor, reversing the polarity of the voltage going to it to move the window up or down.

So, looking at the whole passenger window circuit, you see that in the center off position PWDD the output pins 1 & 2 are connected to +12V, which puts +12V on the inputs of both pins 3 and 5 of the PWPD switch. So with the PWDD switch in the middle, the voltages at the PWPD switch should look exactly the same as for both driver's door switches.

Troubleshooting
If neither window is working, start easy - check fuse 1. If the fuse is OK, put a VOM on the output side fuse 1 and, with the key in the "run" position check for +12V. If no voltage, then either the fuse or the fuse contacts are bad, or the power window timing relay is bad. Less likely but still a possibility is bad wiring.

If only one window is not working but the other is fine, or the test above shows +12V on the output of fuse 1, go to the driver's door window switches.

Driver's Window:
Pop out the DWDD switch with a flat head screwdriver and make sure all of the wires are connected on the back. They do like to wiggle off the connectors over years of use. On the back of the switch are the pins, and they are labeled on the switch with their pin numbers (you may have to look close). You should see red wires going to pins 3 and 5, a brown wire to pin 4, and whatever-color-Porsche-felt-like-using colored wires to pins 1 & 2 (mine are blue and blue/black). With a VOM make sure you have +12V from pin 5 to pin 4, and +12V from pin 3 to pin 4. If not, and you have +12V at fuse 1, then there's a problem in your wiring. If that's OK, with the switch in the center position, measure voltage from pin 1 to pin 4 - this should be +12V. Measure voltage from pin 2 to pin 4 - should also be +12V.

Now hold the VOM on pins 1 & 2 and push the switch DOWN. You should see +12V between pins 1 & 2 (1 is positive, 2 is ground). If you see this, and the motor does nothing, then the either motor is bad or the wiring from the switch to the motor is bad. Pull the door panel off and investigate. If you push the switch UP, you should see +12V from pin 2 to pin 1.

Passenger Window
Go to the PWDD switch, measure voltage between pins 3-4 (+12V) and pin 5-4 (+12V). If no voltage, you're not getting voltage into the circuit from fuse 1. If good, with the switch in the center position, measure voltage from pin 1-4 (+12V) and pins 2-4 (+12V). If nothing, then the PWDD switch is probably bad. If these voltages are fine, go to the passenger door.

Pop out the PWPD switch and measure the input voltages on pins 5 (measure +12V from 5-4) and pins 3 (+12V from 3-4). If these aren't there, there's a problem in the wiring. If the voltages are OK, measure the voltages on the output pins (1 and 2) of the switch. In the center position, you should see +12V from 1-4 and +12V from 2-4. In the UP position, you should see +12V from pins 2 to 1, and in the DOWN position you should see +12V from pins 1 to 2. If not, the switch is bad. If you see +12V on across the output pins but the motor does nothing, then either the motor is bad or the wiring from the motor to the switch has failed. Pull the door panel off and investigate.

Note that because both the PWDD and PWPD switches are in series, a failure in either switch will make the window inoperable. Start at the beginning of the circuit (fuse 1) and work your way through (to the PWDD switch, then PWPD switch) in order.

* Bus 30: Hot all the time, direct from the battery
* Bus X: Switched from the ignition, hot in "acc" and "run" but not "start".