Has anyone tried putting a variable resistor in the down circuit?

1992 XJS Convertible

The problem is that sometimes the motor winds the window down so tight that it cannot get turning again to raise the window. I took the motor apart but found very little wrong, only a black streak on the commutator, possibly from a speck of grease.

The window was wound down very tight against the stop. Compare it to a bolt, you can spin it easily in the thread but after you crank it tight with a wrench, it can later require more torque to loosen it than when tightened. I am thinking about putting a variable-resistor between the thermal-breaker and the down-relay. Then the power can be throttled back going down but will be full power going up.

Am I crazy or will this work?
If so what resistor should I buy?

 

I reassembled the motor and bench tested it. I could not make it get stuck. So maybe that black streak on the commutator was the problem.
It is reinstalled in the car and is working. If it ever gets stuck down again I will try my idea.

 

I had a failing bearing.... I have a post with pictures detailing my repair!

 

Vee
Can't find your post.
You also referred to it in another recent post about the rear quarter window, but also failed to include a link or the title of the post. It is difficult to find specific posts on this topic because the keywords that would work for a search can vary drastically and mix in with unrelated posts.

 

https://www.jaguarforums.com/forum/x...tached-162775/

 

When a window lift motor will not reverse it usually because of what is mentioned below in a post from the archives.
This is a reply to a post from Timeisrelative
So I think both of my window motors basically crapped out. I have them on switches, and they worked fairly well for a while, then my left one started acting up. It ended up stopping all together, then my right one just decided to stop working too. I'm getting power at the connectors, but no motion. I did have an issue with my right one before, and removed the rubber buffer inside, and it started working fine before. I put them on switches because they worked intermittently at best in auto. Has anyone found a new motor that will fit and work, even if it means adding a different connector?

Answer:
The drawback with all window lift motors is they do not have limit switches to remove power at the end of travel.
It wouid be quite tricky and an unwanted extra expense for car makers to provide that feature.
So the motor is designed to withstand stalling at end of travel until the timer or the driver removes power.
Running current is around 2 or 3 amps, stalling current around 8 to 10A.
10A X 14V on a fully charged battery is a fair bit of heat energy so not good to keep it on for long.

There is another problem with the worm and wheel drive system to convert high speed low troque from the motor into low speed high torque to raise the window.
At the end of travel the motor stalls and the worm can bind in the wheel. When the motor reverses it has the same torque to "unstick" the bind as was originally used to cause the bind. So it is 50/50 that the motor will be able to do that.
The answer is to put a rubber spider on the wheel. When the motor reaches end of travel the spider compresses to some extent. When the motor reverses it has not only its electrical torque, it also has the extra torque from the rubber to unbind the wheel.
That rubber spider is essential for reliable operation of the window lift motors. When I overhauled the motors on my 1988 convertible I found out the hard way why the spider is there. I added rubber grease to help preserve it and help its action.
I guess instead of rubber other motors may have a spring of sorts to unbind the wheel.


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1979 XJ-S coupe with rebuilt HE V12 + Supra turbo 5sp manual box.
1988 XJ-S convertible, stock standard except for wheels.
2003 XJ350, given to stepson.
2017 XF25t portfolio ( 2L petrol )