Power window 'clunks' at end of travel
 

Recently, my driver side window has been making a loud 'clunk' as it hits the end of its upward travel, as opposed to the softer stop that the other 3 windows have. It goes down nicely though.
Does anyone know how this is supposed to work? In other words, I think something shuts off the motor just before the window glass reaches the physical limit of its upward movement, so it won't 'bang' when closing, so if I knew what does this, I'd know what to fix. (and if the job is worth doing, as it does mean door panel removal first).

It probably means your regulator is going bad. If you take off the door panel you may be able to see that one or more teeth on the regulator are missing or ground down. The solution is a new regulator which is maybe $100 and a couple of hours of DIY labor, although it can be a frustrating couple of hours.

One of the screws holding the regulator to the door sheet metal serves as an "adjustment." When you install a regulator/motor assembly, using this screw you can adjust between won't work and works smoothly. I'd propose "out of adjustment" is more likely than "broken tooth."

Sounds like all the bolts on the regulator are loose.

Thanks to all for the input. When the rain lets up I'll buckle down and take off the panel to find out which theory is right, and post back results.

donbryce,
Thanks for the thanks, and if you post your findings, you will be among the few that do. My hat's off.

Good luck,

Mine did the same thing just before this part broke:
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1MB12FET61N91D13MR&year=1987&make=MB&model=300-SDL-001&category=O&part=Window+Regulator+Kit

Pretty simple fix.

My 300SE had that prob and it turned out to be both the window regulator teeth and the sheet metal onto which the regulator bolts onto was bending. The sheet metal fatigue seems to be common on the Se's.

Same car's rear driver side window would not lower with motor, That turned out to be just the plastic bushing referred to by the previous poster.
Anyway, symptoms have diff incarnations.
Take the panel off and snoop.

Den

My sheetmetal actually cracked. JB weld and a piece of scrap metal to the rescue. Stiffening the sheet metal was enough to stop the gear teeth from jumping although it was worn quite a bit.

Would think sheet metal "cracking" is symptom and not the disease.

Disease is probaby misadjustment or something else causing hard running, which then stresses the mounts.

There are no adjustments on an MB sedan window except the lower stop and that should be easy to figure out even by the dumbest window putter inner. The regulator mounting holes can enlarge and the metal can crack if the bolts come loose and the window is continuously operated in that condition. The biggest cause of power window mechanism failures on MBs is the regulator. The teeth at the end get ground down and or the regulator gets twisted way out of shape. Some regulators just suffer from fatigue, but most fail, because idiots keep their finger on the switch in hopes the window will go a couple of feet higher or lower. Those motors are very powerful, and if you keep your finger on the button after the window has reached the stops, it will continue to apply force to that regulator. Since the window can't go anywhere, the force grinds on the teeth and causes the regulator to twist sideways. When you hold your finger on the button in the other direction, the regulator twists in the other direction. Soon it binds up and won't move at all. Having witnessed these failures on hundreds of regulators I have replaced for clients, I take my finger off the button as soon as the window has reached its destination and don't need to replace any regulators in my MBs. My 83 wagon has 355,000 on the clock with same driver's window. Hope this sheads some light.

Problem found and resolved
 

It was the gear teeth at the end of the regulator quadrant. As the first picture shows, 4 of them had become distorted and lost metal from the main drive gear grinding on them at the end of the windows travel. The 'clunk' I was hearing would have been the last revolution of the drive gear skipping over these teeth.
Since I'm a stubborn person when it comes to replacing things, I decided to mig-weld-fill the bad teeth and cut new ones. First step was to hammer everything flat on an anvil, then wire wheel off the cadmium plating in the area. I welded the section until it was filled, then ground both sides flat again.
A 3/4" wide half round file proved a perfect fit in the good teeth. I sprayed the area with machinists blue dye, made a cardboard pattern from the good teeth, then scribed the outline for filing. Cutting was done with a thin grinder cut-off disc and the 1/2 round file, finishing with a small needle file to trim the detail. Picture 2 shows the re-worked area.
I have no idea why Mercedes made this piece out of soft, 1/8" thick steel. I'm sure my welds are tougher than this metal was.
Also, I'm not sure why this happened, as I'm careful to not keep my finger on the button when the window is up (wife may have done this, but I doubt it, since the window stays up most of the time for the A/C). It could be that 20 years of use have just taken their toll and it just plain wore out from metal fatigue.
It now works good, not perfect, as it kind of judders halfway up, as if sticking on the sides. I sprayed silicone into the side tracks, but it didn't help. However, it goes up without any 'clunk', and goes down smooth as, well, glass.
My thanks to all again for the input. All the mounting bolts were tight, and for some reason, my model doesn't appear to have the down-stop adjustment pictured in the MB manual. That wasn't my issue anyway, but I thought I'd mention this, since someone might suggest adjusting to stop the juddering.