Monovalve short damaged PBU - PeachParts Mercedes-Benz Forum

funola · #1 07-24-2013, 05:37 PM

My 85 was getting intermittent heat with the AC on max. In troubleshooting whether the monovalve was at fault I may have shorted the 2 pins in the monovalve connector, and the intermittent heat became permanent heat.

I determined that the monovalve was not getting a ground signal. Fuse 14 is ok so the monovalve is getting power, just not being grounded (by the PBU). Took the PBU out and traced the ground wire (brown) to pin 4 of the connector on the right in this pic

funola · #2 07-24-2013, 05:39 PM

Took apart the PBU and found a burned trace from pin 4

funola · #3 07-24-2013, 05:41 PM

Soldered a wire to jumper the burned trace

funola · #4 07-24-2013, 05:45 PM

Put the unit back together and test. Light bulb connected across the monovalve is lighting up so it is getting a ground again. By turning the temp control dial to coldest detent click, monovalve is on constant (light on= no heat). Temp dial on hottest detent click monovalve is off (light off = full heat).

Still need to road test with AC on to make sure it works.

funola · #5 07-24-2013, 05:49 PM

I am surprised a short on the monovalve does not blow the fuse but blow the PBU instead. I think I will be adding a fuse to the monovalve to prevent this from happening again.

Jeremy5848 · #6 07-24-2013, 07:30 PM

This has long been an aux pump problem with the W123's ACC but I've never heard of one involving the monovalve. You did a good job diagnosing and repairing. A fuse would be a good idea. The later ACC versions (W124) were built to be short-circuit resistant so MB must have gotten a lot of complaints.

Jeremy

funola · #7 07-24-2013, 11:11 PM

The fault was shorting the 2 monovalve wires together. Easy to do after taking the connector housing apart to squeeze the contacts to make them tighter forgetting the key is on.

Do you have a more legible copy of this diag.? Where did you find it?

http://i323.photobucket.com/albums/nn447/Jeremy5848/Mercedes%20Stuff/Schematic.jpg

I think the same thing happened on my 83 300D a few years ago. I posted about it but didn't document or take pics of the fix and had to figure it out all over again.

Jeremy5848 · #8 07-25-2013, 02:07 AM

The diagram probably came from the W123 climate control manual, which I no longer own. Clicking on the link brings you a small copy of the diagram but my PC gives me a (+) magnifying glass symbol for the cursor; clicking that enlarges the diagram and it is very legible in the large size. I still have my copy of the file that I uploaded; it is 4446 X 4415 pixels and 1.88 MB in size so there should be no problems seeing all of the detail.

Jeremy

funola · #9 07-25-2013, 02:30 AM

Quote:

Originally Posted by Jeremy5848

The diagram probably came from the W123 climate control manual, which I no longer own. Clicking on the link brings you a small copy of the diagram but my PC gives me a (+) magnifying glass symbol for the cursor; clicking that enlarges the diagram and it is very legible in the large size. I still have my copy of the file that I uploaded; it is 4446 X 4415 pixels and 1.88 MB in size so there should be no problems seeing all of the detail.

Jeremy

The jpg I downloaded from the link is only 700kb. Can you email me (in my profile) the orig hi rez file? Thanks