Power window switch booster for XJ12C, 1976

There was a wonderful post several months back that showed how a lister had developed an electrical setup that provided a good boost to the current going from the switch located on the console to the power window motors. I believe this may have been specific to the rear windows in an XJ Series 2 coupe. I am unable to find this post in the archives. If the original poster is reading this, or if someone else may have flagged his post, I would be most grateful for a link.
My rear windows are so bad as to be unusable. I would consider adapting a hand crank to roll these rear windows up and down if one was available.
Thank you,

Hi Philip

My series one has electric windows and they work just fine so the basic circuit seems fine as do the motors and switches., I suggest you have an electrical problem with a high resistance somewhere or a bad earth. I don’t have a wiring diagram for the XJ12C but if you have one I’d be looking for the common point in the rear window setup.


My work has been defined. Thank you and on to it.

Another possibility is dirt in the tracks or gears or gummy rubber seals.

Yes indeed. When I fitted new track felts and seals to my Series 1 sedan the motors really struggled. To get past the “break in” friction I sprayed all with silicone spray. After a few applications I was in business. It might be worth a try on old gummed up fittings.

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You have made reasonable suggestions but the inherent problem with these rear coupe windows is the below par engineering of the mechanisms. The nearly 90 degree corner that the window channel exhibits is just too much for the motor to power the window to negotiation. The plastic wheel that follows the channel should have been designed of a better material and dimension. Larger and in tandem.
There was an earlier post from a lister that addressed this by installing a “booster” electrical box that provided the extra force needed to make these windows operate properly.
Wish I had flagged that post.

Is the windows lift wiring on the SII different from the SIII?
Because in my SIII I’ve put relays and now my windows DO go up and down !

That’s what I’m talking about! Thank you Aristides.