Weak power to sunroof motor
 

I am having trouble with the sunroof in my '82 300SD. I have unplugged the motor from the harness and checked it with my multi-meter and am only getting 6 volts at the plug. I have tested the switch, verified that I have 12 volts to the switch, and cleaned the fuse holder and replaced the fuse. I have also verified that the motor is good by jumping it to a battery, the sunroof then opened and closed just fine. I also tested the voltage at the plug with the motor plugged in and show 0 volts when the switch is pressed.

What am I missing? Is there a relay in the system somewhere?

Thanks

A short or a bad connection? Have you tried measuring the voltage when the plug is under load? Is there by chance a ice cube relay in control here? Replacing those things solved all my electrical problems in my sd

I had some of the same issues on my 85 300CD, I ended up removing the switches, roof motor,windows,anntena,and rebuilt them all. Once you pop the switches out, the switch cover pops off at the pivit point of the switch, just be careful there are small springs and ball bearings in there. The old grease from the factory gets nasty and so do the contacts. Clean the grease out with contact cleaner or some alcohal based cleaner and clean the contacts with some scotch brite or what have you. After I did this things got better.

There is no voltage across the plug when it is under load. I tested the switch for continuity and showed 0 K ohms, but maybe it has enough internal resistance to cause the problems. That motor must draw a lot of current judging by the size if the wires that come out of the switch! I'll pick up a new switch at the yard this weekend. The picture on the switch is worn off anyway.

Bump

No ideas? :o

OK, I'm bumping this again now that it's starting to get nice around here. I have verified that it is not the switch by swapping in a known working switch. The problem seems to be between the switch and the motor.

Any help would be appreciated.

There has to be a relay.I wish we all new what ones are for what.Maybe need technical wiring manual.

this is from a w123 manual, but it's probably the same
http://mb.thatchermathias.com/sunroof.pdf

Check resistance from switch to motor. Mine buggered out this winter too, I assume burnt motor from never cleaning the tube.

Assuming the setup is like the w123. Then there is no relay between the switch and sun roof. It would be the switch. Although you stated you have good 12 volt power at the switch and the switch is a known to be good, it could be still the switch. They sometimes get full of grease and dirt and act finicky. I would remove the switch and see if you can get the motor to run by jumping the switch's power and ground leads to the two leads to the motor. If it opens and closes as you swap lead connection, then it is your switch. If not then try using another ground source and do the test again. Hopefully is one of those issues.

The wiring seems to be the same as the W123. I have tried swapping the switch with a known working one from another car and jumpering the plug. I need to find a better ohm meter, mine only works in Kohms which is not sensitive enough for something like this. I wonder if the ground from the switch could be bad...

Yeah, I would check your grounds.

corrison, bad grounds, loose connections?

would it be very difficult to use an electricians "fish tape" to run a new wire from the switch back to the motor?

Once I get all the engine issues sorted out and swap out the springs and shocks - this summer I'd like to get the roof working in mine too....