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jjalex 07-04-2003 05:37 PM

W123 sunroof Closes, won't Open
My '84 300Ds sunroof no longer opens. I started taking it apart and realized that the motor doesn't make noise when I try to open it. However, when I open it manually, the motor will close it fine.

I pulled and tested the switch, but I have very little knowledge of electricity, so my test could have been bogus.

I suppose I should test for current at the plug for the motor. If I get none in the Open position on the switch, and some it the Close position, should I just start with a new switch?

I have to clean and lube the rails anyway since it doesn't move easily by hand. I cleaned and lubed the cable. I suppose if I'm going to be a good boy, I should clean the tube too....


ATLD 07-05-2003 02:26 AM

The switch is your problem.

I took a brief look at the schematics for the SR switch and motor and discovered that there are only two wires that go from the switch to the SR motor. If the roof works in one direction, the motor is good, and the wires too (the ground is sent back through the second wire/this should reverse when the switch is run the opposite direction). Therefore the switch is the culprit as the inversion of the wire polarity is done there. The SR motor is not chassis grounded for operation.

To test this, you can hook up your multimeter to each of the two terminals on the connector that attaches to the SR motor. When the switch is put in either to open or close the roof you should read 12V+ across the terminals, if you do only for one, it's the switch. Both terminals should also read ground when the switch is in the neutral position.


jjalex 07-05-2003 10:14 AM


Thanks a bunch. I will check the voltage later today, after I clean all the old gunk out of the tube and get the SR sliding freely.

I understand the first test, but not the second. How do I tell if they each read ground when the switch is not engaged?

Is that simply the lack of any current?

Sorry about the dumb question, but I am willing to learn...

jjalex 07-05-2003 11:45 AM

The switch is fine. After I tested it I pulgged the motor back in. Once again it worked in one direction and not the other. Once it ran vary slowly in the other direction. Then I adjusted the clutch nut, tighter and looser, with no affect. It did not run in that direction again.....

I will take the motor and transmission out today and clean up the gears to make sure nothing is binding, but I think the motor may be toast....

Does that make sense?


ATLD 07-05-2003 04:37 PM

The motor is a brushed DC motor so that it can run in two directions. There are two wires that run from the switch to the motor. Current (should) run through both wires whenever the SR motor is engaged, positive in one and ground via the other. All the switch does is reverse the direction of the current when the SR direction is reversed. If the motor works in one direction, there is a 95% probability that the motor is fine. The wiring also obviously must also be fine for the roof to work in any direction. Please test that you have the same voltage across both connectors (on the wire that attaches to the SR motor) first before you replace the motor ($$$), if the voltage is not exactly the same, this can indicate a bad contact! There are other ways to test this mroe accurately, but it might be a bit harder for you to test. The second test is attaching a wire to a good grounding pt. of the car and seeing if it makes a good circuit to both the connector that would attach to the SR motor.

Since you said that the motor ran slowly for a bit in the other direction, it leads me still to think that the switch contacts are the problem.



jjalex 07-05-2003 10:36 PM


Yes it looks like I have the same voltage on both wires.

What if I connected the wires to the opposite contacts?

Then if its the switch it should open when I press close and if is the motor it should only run in the same direction as before.


ATLD 07-06-2003 02:11 PM

That's right. When you reverse it, if the SR runs in the opposite direction, then it's deffinately the switch. If not, it could be the motor itself, though I've never heard of a DC reversible brushed motor to fail in only one direction.

That's a good test that I didn't think of. I have the equipment here to diffinitively make a judgement about the failure quickly, but in a different and more complicated way that would be very difficult to reproduce with a simple multi-meter.

OK, if you have 12V to each contact, then that's half of the circuit. Now see if you can test for a good ground in that same wire. With the switch engaged in one direction, test the other contact (the opposite one that reads 12V at that setting) and make sure that it produced a good vehicular ground. This can be tested by setting your meter to test for resistance. It should read either 0 ohms, or at the worst, 1 or two. Reverse the switch and check the other contact similarly (again, the other wire that does not read 12V in that setting). At least, they should read the same resistance. If not, then again the switch is the culprit. If you have an analog meter, make sure that it's properly zeroed/calibrated first. Remember, becase this is a closed loop system, you have to have a good positive and ground wire to get the motor to run. The switch changes both the ground and positive between the contacts to make the motor reversible. I'd still put my money on the switch.


Let me know the results.

jjalex 07-13-2003 01:16 PM


You are correct, it is the switch.

Yesterday, with someone to watch the motor spin, I switched the contacts to the motor and it only rotated in the "open" direction. I switched the contacts back and it only moved in the "closed" direction.

Then I ordered a switch from Fastlane.

Thanks for all your help!!!

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