I know there are lots of posts about electric windows on this forum. I have read most of them, but not all of them....so apologies if this is a repeat. If you are new to electric window problems, I would highly recommend you read the other posts in conjunction to this one.
PROBLEM: Windows very slow in rising up and eventually stop before fully closing. Needed to wait a 4-5 seconds before being able to lower window.
SOLUTION (Part 1): I removed the window motor and lift mechanism (other posts cover this) and attached the motor directly to 12v supply. The motor performance was improved, but not significantly and when the window arm reaches its limit, it still cuts out and cannot be moved in the opposite direction with out waiting a few seconds.
The cause of this was the copper thermal switch inside the motor housing (it is about 20x10x4mm). This is covered in another post, so I'll not go into it here other than to say I soldered in a new thermal switch (from Amazon HERE). It needs to be mounted externally as there was is enough space inside the motor housing. I've never opened up a motor before, but they are surprisingly simple and robust inside. Mine was filthy inside and looked like it had some water ingress.
SOLUTION (Part 2): Having replaced the thermal switch and reinstalling the window motor, it was still very sluggish....but no longer cutting out. I removed the whole mechanism again, and also removed the motor from the lifting arm. Without the motor attached, I was expecting the lifting arm to rotate easily about the cog mechanism. In my case I found this to be really stiff, even after I released the coiled spring. This is not something that can be dismantled, but I did managed to work in some grease and improved it quite a lot, but to be honest, it was still a bit to stiff for my liking. After trying out the mechanism again, it improved, but not by a great deal.
What I did realise at this stage was that although the window is very slow going up, it really flies coming down......this is how I came by an easy modification.
The coiled spring on the mechanism (see photo) is intended to tighten and build resistance when the window movement goes down. This resistance is then released to assist the window going up. With the window mechanism on the work bench, I found that the spring was just about overcoming the rotation stiffness I mentioned earlier and would be of little help in actually lifting the window. To improve the performance of the spring, I used a grinder to cut a further groove (90deg to the existing one - see arrow point in photo) in the pivot point. This allows the spring to be rotated 45deg before slotting back in, which gives it much more tension than it had before.
I reinstalled the mechanism and was really please with the results...its not perfect, but much much better than what it was. The upwards lifting is much better and the downward stoke is a tiny bit slower, but barely noticeable.
To improve this further, I feel the solution is to further release the resistance when the window arm rotates about the cog mechanism. It seems far to stiff, but it as previously mentioned, it is not something that can be dismantled or adjusted, but I am sure there is a solution out there.
Anyway, I hope this is helpful.