Rear heated window wiring diagram for a tourer
 

Hi folks,

The rear heated window on my tourer has never worked, the light on the button comes on but the window doesn't clear. I've read that breaks in the element in the window is a common problem and usually only fixed with new glass. However, I thought I'd check power to the window.

Checking in the fuse box under the bonnet, it looks like the rear window uses yellow relay 2 and green fuse link 7. I tried pulling fl7 out but it's tight and didn't want to break it with pliers. Should these pull out?

Next I thought I would check relay 2. I didn't have a trusty assistant on hand to operate the switch with me feeling the relay so thought I would check the power to the socket terminals with the relay unplugged. I get 12v across the switched contacts (87 and 30) which i assume means fl7 is OK. but I don't get anything across the coil contacts (85 and 86) whether the switch is on or off.

First off all, am I testing this correctly? secondly, if I am, where does the supply for the coil come from? Does anyone have a wiring diagram of the circuit?

Assuming I'm being a jumpy and power is getting to the rear window, where do I check this? I see convoluted rubber tubing running from both the top left and right hinges, which side to the HRW wiring come from? Is there an easy how-to on what to check before I break out my Torx head socket set?

Cheers,
Rich

Rich, I have no assistance to offer but mine is the same and I will follow your investigation with interest, someone will be along shortly to help us both.
As for the higher powered side of the relay doesn't it need to be 'energised' by the lower powered side to allow a current to flow? Not sure myself but think this maybe the case.:shrug:

You should get a voltage drop across the relay coil contacts when the HRW is switched on. Did you have the engine running at the time? This always used to be necessary to protect the battery.

Yes.
From the fused side of link 7.

Rich; start the engine and place your finger on the relay case. Now ask an assistant to switch off the HRW (if the outside temp. is 10 degrees or less it will have turned on automatically). You should be able to feel it operating. Alternatively place your voltmeter between terminal 87 and earth to check that the relay is operating and supplying voltage to the HRW.

Simon

Have a read of this

http://www.the75andztclub.co.uk/foru...09&postcount=4

Remove the trim around the rear screen you will see a large black plug with heavy duty wires these are the HRW connections. Check for 12v at this point.

Dave T

With the aid of a helper, I tested the RHW with the engine running. I couldn't feel any noticeable click on the relay wen turning it on or off. I swapped with one of the other relays and still could not detect switching. So it seems I have an issue somewhere between the switch and the relay coil, does anyone have a wiring diagram?

Cheers,
Rich

Hi Rich,

The relay coil is earthed by the BCU so it's not a simple switching arrangement. This is because the BCU is programmed to switch on the HRW automatically for a timed period at temperatures of 10 degrees and below.

You can, of course, test the relay another way by removing it and applying 12v across terminals 85 & 86. I doubt that it's faulty though.

I notice that you also reported that you placed your voltmeter across the relay contacts (30 & 87) and got a reading of 12v. This indicates that the contacts were open at the time (relay not energised) and crucially that there must be continuity through the rear window element to earth. You could confirm this by placing your multimeter (switched to the lowest resistance range) between the engine compartment fusebox pin 1 (brown/green wire) and battery negative with the ignition off. The reading should be zero, or a fraction of an ohm.

Simon

Thanks for the info Simon.

Where would I find pin one in the fuse box? Do you mean on the diagnostic connector? I can't see any wire colours from the top of the fuse box.

Just some more clarification... I see 13v on pin 85 of the coil contacts but open circuit wrt earth when checking pin 86, I guess this is where it is earthed by the BCU?

Also, I see 13v on pin 30 of the switching contacts and 19 ohms to ground for pin 87 which I assume means the resistance of the element, so looking ok there.

So how do I check the circuit on the BCU side? Could it bee a faulty dash switch?

Cheers,
Rich

Have you actually checked for voltage on the strips either side of the rear window on the glass? Could save yourself a lot of trouble if you have volts there.

Ohms law tells us that 0.6 amps will flow through your HRW element. I must admit that I've never measured the actual consumption of a heated rear window but I was expecting much more than that.

However, let's not jump the gun. We haven't established that the relay is operating yet. Can you get your voltmeter onto terminal 87 of the relay somehow? With the engine running and the HRW button illuminated, you should see battery voltage there (which, by the way, should be above 12.5 volts for the BCU to activate the relay, according to MGR).

Simon

Just checked the voltage with the engine running, zero at both sides of the screen wrt exposed bodywork earth.

So I pulled the relay and checked again terminal 30, and found it was live at all times, even with key off. Sooo, I briefly wedged in a thick jumper wire between 30 and 87 and checked the voltage at the screen again... 12V wrt ground at the passenger side of the screen. Result!

I wondered if the BCU has some interrogation voltage across the relay and would not control the earth side properly if it detected that the relay was missing, or fault. Anyway, I can't get at the terminals with the relay installed unless I make up some extension wires and I don't have the bits for that at the moment. But I think my test above proved the 'high current' side of the relay through to the rear screen is OK.

I have verified that the relay is OK by applying 12V across the coil and can hear/feel it operate. I also checked the voltage with the engine running and it is around 14V or above, so should be fine for the BCU.

I think I've now checked everything apart from the earth side of the coil controlled by the BCU, and would need a wiring diagram for that :(

Cheers,
Rich