75ZT Community

Window regulator adjustment ?.. Is there any.

My rear passenger window goes down by itself when ignition turned on.
Either rear door or drivers window switch has to be flicked several times to raise window.
Ideas anyone??

Doubt it's an adjustment, probably a switch or wiring problem. Try disconnecting the rear switch, and that will rule that out. Could be the drivers switch, or more likely a chafed wire somewhere between the rear switch, and the BCU (Body Control Unit), or maybe water in the connector somewhere, most likely where the door wiring connects to the car (at least I think there's a connector there).

Edit:

Yes there's a connector in the B pillar. It's a similar design to the one for the front doors, and they are know to get water in and corrode. I'd start there.

Thanks I will try that

Connector in perfect condition.
Other windows work ok. Do windows have individual relays or are
they controlled by just one ?

The windows are all controlled by the BCU which is above the glovebox. Blocked plenums can cause this to get wet, but symptoms tend to be worse, and more widespread. The switch in the rear door is connected to the BCU, and then the BCU is connected to the motor in the door. The drivers door pack is connected to the BCU, by a communication bus.

If I change the BCU does it require a session on a T4 (Testbook version 4.Computer Diagnostic System) to be programmed?

Yes, but the BCU is the least likely culprit. If it's not the switches or connector theres likely a wire rubbed through somewhere.

Thanks .Will check wiring first.

Swapped BCU but still no difference. Not looking forward to trying to trace out wiring loom.

I assume you did do the first step of disconnecting the switch in that door, and the drivers switch pack?

Yes, swapped rear window switches ( offside works ok) No difference . Disconected drivers door switch, again same result

You could disconnect the switch wires from the BCU at the BCU end. That would prove it was wiring if the problem went away. Drivers switch would still work, the rear one wouldn't.

I'll come back with the wire colour / pin number at the BCU.

Edit:
OK at the BCU it’s the middle connector, the one with 54 pins. You are looking for pin 39, blue green, and pin 40 green blue (main colour first, trace colour second). One is up, one is down, but I can’t tell from the circuits which is which. Do one at a time, or make a note of which one is which otherwise you might get them back in the wrong holes and the window switch will work upside down.
If you disconnect these at the BCU, then the problem is with the wiring between the switch and the BCU. To isolate whether it’s in the door, or further down the car, put the pins back in the BCU and disconnect them in the door B pillar connector. Pin 8 Blue Green, pin 10 green blue.

You could also use a multimeter to check for short (or partial short) to earth. With the switch not operated, and the wires disconnected at the BCU, there should be no reading between the wires and earth. If there is, that confirms a wiring problem.

I seem to have a permanent earth on pin 37 ( Grey/Blue) Do you know what colour that is on the door pillar connector??

Mm, interesting. According to the circuit diagrams thats for the left rear window. But then the diagrams have been found with errors before now.

According to the diagrams though, pin 37 blue / grey is connected to pin 8 in the door connector, and changes colour to blue / green at the door connector. Though it does say that's in the LH rear door!

Edit: try removing that pin from the BCU connector and see if the problem goes away. They are fairly easy to remove, just use a small screwdriver to press in the tang on the terminal.

Rear nearside is faulty

Removed front passenger footwell carpet to trace wiring loom,Now everything works as it should.
I am assuming that the insulation must have worn and was earthing somewhere in the loom and by pulling the loom it is no longer touching the body. Will have to dissect the wiring in daylight
to try and find fault.
Thanks for your help.

Anyone need a spare BCU?

Ah, sorry my bad. Even having re-read your original post I was convinced it was your rear right that was the problem. Doh. Must learn to read properly.

Anyway, yes it sounds like it's exactly that. There's a few sharp edges in the bottom of the A post area.