What is the most common issue with the 123 sunroof, the teleflex cable getting stripped or ruined or the teeth of the gear on the motor getting broken? My sunroof will open with some help, and I can hear the motor working but can't tell whether it's a problem with the gear or the cable.

Most common issue is lubrication. Lack of lubrication, or wrong lube.

Only use mercedes benz sunroof lube......Thats all mine needed.

__________________
1992 300TE 4matic - Just obtained 12/22/06
1983 300D - SOLD
1997 Dodge 2500 - CTD 400+HP, 1100ft/lbs torque

If the teeth were bad the sunroof would move until you hit the bad spot on the gear and would likely make a grinding noise. You would then have to move it manually to get past the flat spot.
Lube first then if the sunroof motor turns, but the does not easily move the roof tighten the nut for the gear clutch mechanism on the sunroof gear housing. Do not overtighten of you could break the teeth of the gear. - th

Tried the lubrication, but need to get some MB grease. Does Fastlane sell the gear for the teleflex cable? I think that I read somewhere that they also sell the grease.

You must clean the rails first with an acid brush and carb cleaner or something to get the caked former lube off. Clean it real good, then apply only MB sunroof grease.
I had to crank on the emergency mechanism with a ratchet to break the sunroof open when I first got the car.

__________________
'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)

When I first got my 83 300CD the sunroof wouldn't move at all...After thorough examination I found the teleflex cable had all the teeth down at one end all worn off hence no movement at all..Why did this happen? I found out when I tryed to remove the cable which was very difficult to pull out of the tube. Before installing the new cable I sprayed brake clean down the tube all the while moving the old cable back and forth and had a bucket under the tube in the trunk...It was amazing the amount of old lubrication that was released...I kept at it till the brake clean ran out fairly clean...Originally was black and turned to a light brown. When I installed the new cable I coated the cable with lubriplate or light white grease. After the new cable was installed I had to adjust the sunroof but it works well now however the whole operation took about 4 hours. The cable was only $100 so it was worth it as I took a 4 hour trip today with the sunroof open in the nice sunshine...

__________________
83 300CDTurbo 307,000
2005 Honda Element 266,000
56 Nash Ambassador Country Club Special 34,000(under restoration presently)

http://www.peachparts.com/shopforum/car-audio-multimedia/125099-123-install-extreme-dynamat.html

I never looked at the W123 drive motor but the W116 assy has a rubber link between the motor adn gearbox that could die.

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche