Will a right side window regulator/motor assembly from a 1986 XJS Coupe fit my 1989 XJS Coupe? I can get mine working for a brief period but when I run the window up all the way, it is as if the motor binds and will not work again. I have the opportunity to get a used motor and regulator for $150.

It should but are you sure that you really need it ? - the window lifts are one of the most poorly maintained aspects of most cars, out of sight out of mind and the inside of a door is a pretty hostile place when it comes to heat or humidity.

It could be simply that the mechanism is in need of TLC, cleaning up, regreasing - especially the track under the window itself that the lift runs in. The motor itself is probably in need of some TLC also, and then you need to be sure electrically that the motor is getting the juice it needs.

Personally I'd start there, you don't know what state the replacement is in so you'd need to apply TLC to that before installing anyhow - well that would be the course of action I would follow, TLC does not involve the use of WD40 ...

Don't forget to clean up the tracks in which the glass runs too - these may not be helping - many lubricate these or at least try to but this really shouldn't be necessary, if you absolutely feel the need dry lube only, try lowering the window and thoroughly clean them with some nice soapy water and something like a paintbrush.

Oh I have been all over this issue. I've pulled the regulator assembly out and cleaned, lubed, etc. I disassembled the motor to polish the commutator and clean the junk out of the brush holder assembly. I've cleaned and greased the little gearbox and replaced the bushing to the output shaft (see a previous post of mine). I've inspected, cleaned and lubed the glass tracks. It is just not reliable. I can get it to work a half dozen times and then it doesn't. I've seen the part number break between '88 and '89 so my thinking is one out of a '86 will not work.

Sounds like you've touched the bases, I've not seen that parts break but I'm focused on mine which is an 87 and only start digging if I need to - I'd be interested to know the difference - may prove useful. You probably have a weak or dead winding - did you check the motor windings while you had it apart ? - this may explain the reliability and erratic failure if it just happens to stop in the bad place.

I'd be surprised if the overall mountings were different - perhaps new motor supplier is the reason for the part number change like the wipers - who knows.

I did not check the motor windings and this would explain the way it is acting.

I think if you get bo the motor and reg from '86 it will work.

I had this issue years ago, so I hope my info is correct.

The motors changed, and although they appear to match, the teeth are different. I recall putting a newer motor on an older reg, and it bound up. But putting a mtached pair in worked, even the plug is the same.

Rob

Running the regulator on the bench I discovered that the sheet metal cover, red X, was bowing out allowing the nylon ring gear to disengage from the worm drive gear. I pried the cover off, cut a piece of thin rubber to fit between the gear and cover effectively taking up the space between the cover and gear. I then refit the cover, peening it in place and then adding a safety wire wrap around the casing and metal cover. I cut two notches into the case, black dots, to locate where the safety wire fits. Working so far. Several cycles up and down with success.