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

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

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

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Soldered a wire to jumper the burned trace

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

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.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

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.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

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

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

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.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

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

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

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

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Tomorrow.

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Have you sent it? Good news is that the fix is working. With AC on, no more full heat through the vents. Hoses to the monovalve is cold = no flow.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked