Speaking of fast windows in a S1… S1 window motors are definitely square-body GM/AC Delco and a window motor from a GM car of the late 60’s to 70’s will fit right in, but not all window motors that fit are the same. I had a burned out window motor in a S1 that I replaced with a front window-motor from a 1970 Cadillac Coupe I had around (I’ve had a ton of Caddy’s from that era). Fit perfectly but now that S1 XJ window is so fast it’s literally like a reverse guillotine - scary fast… About 2 seconds (or less) from open to closed!
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Wait… I can make them faster?? 
Stuart!
You would be surprised how better they get if
you spray a little silicone lubricate into the rubber
ways that hold the window in place.
That goes for the door seals also.
Walter
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I have used relays on several of my Rolls Royce windows. The result made the windows move like modern cars! The RR windows are typically “molasses” slow! Highly recommended!
Richard
Yeah ya can! I gotta warn the Trouble & Strife when she’s in the passenger seat to make sure her hand isn’t near it when I roll it up so she doesn’t lose a digit. I had intended to rebuild the original Jag supplied motor (usually replacing brushes and cleaning/greasing does it) but I kinda like that passengers are afraid of touching the window now 
BTW for anyone wanting to put a GM square window motor in a Jag that uses them the gearboxes are handed and marked L and R… BUT the S1’s use L in the right door and versa. Just make sure you get one whose gearbox orientation matches the one you are replacing.
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Thanks again for the clarification and the wonderful pictures! - Even the underside of your ski slope looks better than most parts from the top side!
Best regards
Jochen
75 XJ6L 4.2 auto (UK spec)
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A little lubrication spray like silicone along the tracks make a
lot of difference.
After spraying it moves like there is no restriction, try it.
Walter
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Mine are like this as well, motors circuit breaker and switches are all original. New felts and all and it’s still the fastest glass in town.
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I removed switches. Contacts were burned a bit, but cleaned up fairly easily. This made no change in operation – still dead slow on driver’s window. Slow on front passenger window.
Lubed tracks with silicone. Still no improvement. Looks like I will need to dig into the doors.
Hook 12v directly to the motors to see what happens.
Look for worn rubber guides and if the felt coating is gone.
Try to better adjust the windows, there is a screw on a slotted hole aft on the door, and one at the front/bottom of the door for the front guide.
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I second that, Aristides - adding cleaning and lubrication. And lookout for wear in pivot points?
Frank
xj6 85 Sov Europe (UK/NZ)