Expert: Ernest (Ernie) Kenward - 11/23/2009

Question
QUESTION: Where can i get a diagram on the wiring so I can roll windows up. The switch does not work.  1980 mercedes 300cd 2door.Thankyou

ANSWER: I probably can supply.  I have a 1984 model (turbodiesel 617) of the same car, or actually a European model (Euro headlights) ordered into the US at that time.

In any case, I spent the money on the 123 shop manual CD, and shall look for the appropriate drawing for you over the weekend.  In the event the OEM CD-based manual is not too clear (unfortunately, MB could have scanned the original wiring diagrams using a higher resolution setting in Adobe Acrobat than they did!) I may find a clearer original paper copy, which I can scan as a pdf, and I will set the resolution as necessary.

New switches are still available from MB.  I bought the set, not because mine were non-functional, but because the legends were worn nearly off, and I am in the process of restoring the interior to match a very good exterior.

I shall get back to you.

Cheers ... EGK

(revision 20091122)
Sorry for the delay in getting this to you!  The diagrams on the CD (MB supplied!) were not that good, but I did a wiring diagram from another source.  Note that, while this is sourced from 1984 information, there is no evidence that the 1980 power window system is wired any differently.

Note that the problem you cite may or may not be a switch problem; it is instead possible that the power window drive motor is the problem.  A common problem with DC drive motors such as the series-wound unit (high-torque) used in power window applications is the eventual wearing out of the brushes that make the sliding contact with the rotor (armature) winding.  In the case of a series-wound motor, a simple check for this would be to do a continuity check on the motor, once the switch unit has been disconnected from power window connector in the console.  In the case of the driver's window, the connector for the left-side window switch (identified as the "left master switch" in the drawing) connector provides an easy access by which to check the motor, along with the wires supplying it, for continuity.  Connect an ohm-meter between the BLK and GRN wires on that connector - if no continuity is seen, that would indicate an open circuit either in the brushes (most likely) or in the harness to the motor.  As well, check for continuity from either wire to chassis common ("ground") - there should not be any, as the motor is isolated from the chassis so as to facilitate two-directional operation using only two wires, so a low- or zero-resistance on either of of these could indicate a fault within either the motor or the harness feeding the motor.

You probably have solved the problem by now, I hope!  As it happened, I have just finished a large midterm exam marking load, during which I had to put other things (including sleep) on the back burner - business as always!

<<< ?????? for reasons unknown, NO options nor dialogs opened up at any point during my revising this answer to allow me to attach either a PDF or a JPG file of this page! Not clear as to why, but will query Experts help to wee why - it could be that this is a restricted-access function!

Best wishes ... EGK

---------- FOLLOW-UP ----------

QUESTION: Our weather has been bad, husband also sick. Now hes ready to go at it!!  Thank you.  He wants to know Will you be sending Diagram.  best wishes TL

Answer
Reminder:  Again, please refresh your query by asking the queestion once again.  It is only on answering any question the FIRST time around that the AllExperts system software allows access to the "attach image" dialogue.  Unless and until that is done, I canNOT send you the image of the Wiring Diagram I have produced for you in respect to the Power Window circuitry on your 300CD

Experts help has advised me that, for some reason, the software permits image attachment on the first answer only, and not on revised answers.  They are not aware as to why this is so.

My suggestion therefore is, if you wish to get the drawing copy from me, send me a NEW question.  Just make sure that the subject line is clearly recognised by me as being you requesting the drawing.

Once I open your question, and one hopes that I will see the "attach image" dialog, I shall attach the file image and send it along to you.

Rather lengthy path, wasn't this?

Regards ... EGK


I shall try again later ... for some unknown reason, the access by which to send my wiring diagram document as a jpg attachment did not make itself available when I sent my previous answer.  I have queried AllExperts tech support on this ... EGK



It appears that the "attach an image" option IS available from my current workstation (the original one on which I opened this account and hence the one that does not require my logging in) and so I can - finally - send you the image that I dug up over a week ago!

Just as well that I had to hold off last week, however ... the image in question undoubtedly is copyrighted (MB), even if for a car from the eighties.  Accordingly, if you can wait a few days longer, I shall produce my own schematic for that section of the drawing (As an EE, this is part of what I do) and then send that on to you, without copyright problems.  I will get into that as quickly as I can, but am presently finishing off the marking of a major project to get these back to my engineering students by Friday.  (Probably, I will use this task as a filler when I need a "change-is-as-good-as-a-rest" break!)

As previously indicated, what I will draw (capture in my CAD software, actually) will be for the power window portion of the wiring diagram for a 1984 300CD, but it should be very unlikely that there should be any difference between the window circuits for this and your 1980 model, same 123 chassis.

While you await this, do note that, in a previous reply to you, I did address a test approach by which you could isolate a window drive motor problem, of perhaps the connecting harness itself, from the connector for the left side window master switch, once that switch assembly has been removed.  Check the BLK (black) and GRN (green) wires on the connector for continuity once the switch has been removed from that connector.  Continuity of those wires with each other, coupled with NO continuity between either of them and the chassis common ("ground") should be indicative of an "okay" motor.

If it IS a switch problem, I paid about Cdn$56 for a new combined front and rear left plus lockout switch unit.  

Motor drives are more expensive ($400 plus from MB), however, I am going to dismantle mine (left rear window unit) so as to check out the brushes, given that this motor tests as "open".  At the same time, I will also do an insulation resistance test to verify the windings.  There are motor rewind shops that can address winding problems,a and brushes for rotating machines are available in many sizes to industry-standard callouts, as are bearing sets, O-rings and other industry parts that are more economically bought from specialty houses than specially made for any one application.  In any case, in the absence of burned or pitted iron laminations, I am expecting the motor to be repairable.

Best regards ... EGK

20091207

I had an opportunity over the weekend to produce that document for you.  Submitted herewith is a somewhat generic detail wiring diagram for the power window circuitry for the early-eighties vintage Mercedes-Benz 123-chassis 300CD.  I have relied on the colour codes given in my shop documentation to provide the correct identification.

For further information, you would be well advised to get the shop literature for the car.  At present, the choice is to get the "Model 123 Service Manual Library" CD-rom; this is produced from a scan of the printed manuals and is good for most work but, in my opinion, its wiring diagrams could have been scanned at a higher resolution setting than they were.  In any case, this shop CD is available for about Cdn$165 via the parts department of MB dealers.

An alternate source could be via the Mitchell's on-line shop manual service.

Good luck with the project ... EGK