Thanks for your input!

I can definitiely see this being an issue depending on the resistance of the coil of the relay you choose and the variability of thresholds in the BCM.

Another approach would be to cut the return wire to the BCM and insert a simple rectifier diode, like a 1N4001 in series with it, with the cathode facing the switch, and the anode facing the BCM. This might be a more universal solution because it would be guaranteed to work with any coil resistance.

 

You seem to share my problem. I have a failing (oops, now totally failed) switch. Removing the connector from the switch and grounding the 'Up' wire results in perfect operation. Completely disassembling the switch and cleaning = no improvement. Swapping the carbon pads between the up and down switches inside = no improvement. Best connection ever measured when assembled = ~160Ω. To me, it's crazy. The traces and their connections to the connector pins exhibit no resistance and the contacts inside the switch obviously double as the solder lugs. Resoldering the switch to the pcb = no improvement. Since I've built a custom built double-din radio surround I'm going to replace the switch with two metal-disc switches.

IMHO, while you came up with a workable solution be prepared for inevitable failure. I'd recommend watching eBay for a used one, they typically sell for ~$15 vs. the ~$70 at the dealer.

edit: just saw White's reply, it makes good sense to use a diode with the factory switch as protection for thecarbon pad switch.

 

Hi all.
Modified my rear windows pr. instructions found here. Thanks all of you. Less wind (not that kind of wind), but especially less noise. I also think it looks a lot better Winter come. Looking forward to race around with the snow blowing above my head.

 

Hi, guys. This is a great thread. Thanks for your research and head scratchings on behalf of the electronically challenged, like me. Now that you know this sector of the wiring backwards and forward, how about a TWIST to the know-how challenge?

My Scenario: 1) I beep the car closed and go away. Come back to rear windows down; 2) at other times, I open the unlocked car via door handle and rear windows go down; 3) I drop driver window via door switch while driving, rear window also goes down. All drops are occassional, random, and totally unpredictable - makes finding the problem frustrating.

In all cases, to correct, I go to the trunk and short the up-relays to get the rears back up, but it means emptying the trunk each time to get to the relay box. Grumble, grumble ensues!

Can you think of a reason why the door lock, door handle, or window switch would trigger the rears? Is rear window wiring in the door or elsewhere (like the center tunnel) where a short might be? Other ideas? Mucho thanks.

 

One question first Boris...did you implement the rear window control in this thread, or is yours a problem on an unmodified car?

 

Unmodified, to my knowledge.

 

OK, I understand now. Since it is happening to both rear windows windows at the same time, it almost certainly is the control signal from the Body Processor Module (BPM) that is causing the problem.

This is a Green/Purple wire which runs from the BPM behind the glove box all the way to the rear window down relays in the boot fusebox. When this signal is at a ground level, the windows will go down. If it is open circuit or a positive voltage nothing will happen. So if this wire is shorting to the chassis somewhere along the way it definitely could be causing your problem.

The wire could be difficult to inspect without removing a good deal of the interior of the car.

If you know how to reliably splice a wire, I think the easiest fault finding strategy would be to cut the wire near the BPM (it is terminal 77, Green/Purple), making sure you leave enough wire to splice it back later.

If the problem goes away, most likely your BPM is at fault. If the problem persists, most likely the wire is shorting somewhere. If it turns out to be the latter (a shorted wire), it may actually be quite a bit easier to run a new wire front to back than try to find the short.

If it turns out to be the BPM, you probably can have it repaired or replace it with a salvage unit.

 

Thanks, Steve. We'll give it a try, but first a question or so given that the problem is sporadic and the test could take time:

1) If I cut the wire and thus disable the down-function, will the constant lack of down-function interfere with normal top operation;
2) Is there a continuity/voltage/other quickie test that might help;
3) Can the wire be severed at the relay end (where access is simpler and easier on the back) with same result;
4) As an alternative to cutting the wire, can I simply remove the wire from the BPM connector or whatever mechanism is used to make the connection;
5) If I am to take the interior apart, I may as well install the window control switch discussed on this thread, or would you suggest resolving problem 1 before venturing with improvement 1?

Yet again, thanks

 

1) Yes, do not use the top while running this test. When the rear windows are disabled, the convertible top seals or rear windows may be damaged when the top is raised.
2) Do to the intermittent nature of your problem, I do not think a continuity test will be conclusive.
3) No, severing at the relay will not clearly differentiate whether the problem is with the wire or the BPM.
4) The BPM controls dozens of functions in the car, and unplugging it will render the car not really usable. If you can extract the single terminal that would work, but I do not recall exactly what type of connector is used and how difficult it would be to extract the terminal.
5)I never recommend doing mods unless your car is working properly to begin with.

 

Very interesting stuff! I may have a new mod in my future.

I actually stumbled across this thread with a short term need in mind. I'm getting the windows tinted and want to run the rear window tint all the way to the top. What's the best bet for getting the rear windows up with the top down on a 1-off basis? I tried manually dropping the top (pump tap turned open, latch manually opened) with the rear windows up, but there was quite a bit of resistance in trying to open the top - I was worried I was going to crack a window or worse.

Any suggestions on a quick one-off solution? Was I indeed on my way towards trouble in trying to pull the roof down with the rear windows up?

Thanks

 

https://www.jaguarforums.com/forum/x...-closed-71304/

Check post #2 in the thread above, and thank Steve!

 

For a makeshift 1-off you could manually trigger the rear quarter window relays. Disconnect the green-black wire that runs from the convertible top control to the relays first. You can do that either in the rear of the center console (pin 16 of RH12) or, probably even more easily accessible, in the trunk right next to the amp/ navigation modules (pin 3 of BT3). Then connect the relay side of this wire to ground to trigger the relays and by extension lift the quarter windows. Disconnect immediately after the windows are up. A quick look at the wire diagrams in the electrical guide will be helpful (Fig. 15.2). Hope this helps!

 

Hi Steve
I have had my rear windows mod for a while with the only issue being the hood micro switch preventing the ability to raise the windows some time after the hood has been dropped. We spoke some time ago about an electronic control unit. Any further thoughts on doing this. :-)
IanP

 

Thanks! I manually jumpered the relays and got what I wanted. Assuming my first effort at attaching a picture works, you can see in the attached that I've now got the rear windows tinted up to the top.

 

REv. Sam,
This video seems to have been taken down. Would you have another location that i might check it out. Thanks.

 

Rev. Sam sold his car and does not come by too often anymore.

Here is a link to the video:

 

many thanks to both you and rev. sam. You guys are not only informative but you are also entertaining. You guys should have a tv show. Also, thanks for the quick response. sam auger

 

Hi

Ive been following the thread for this and see that you have the diodes available.
Have you been able to source the other pieces from the UK or have you had to get them all from the USA - if so did that cause any problems
Regards

John R

 

Hi John
I got my timer units form the US via eBay I think or it might have been via Steve either way it not big bucks and reasonably quick. The lid side is all the colour coding is right for the wiring diagram. BIG plus. It's a relatively Easy mod.

 

Excellent read, I will be doing this to my XK8 this weekend