I think I have this issue under control, but just wanted to doublecheck I'm taking the right course of action. My monovalve has entirely quit at this point, in that I get 12v to it but still have constant heat. I tried supplying 12v to the pins myself and get no sound. I have a spare used monovalve on my shelf, I think also from am 87 300D, and when I put 12v to that, I get a substantial click. Can I assume that the coil in my monovalve is kaput? So the whole unit needs to be replaced? Or is the lack of any noise possibly due to the valve being stuck ... in which case I could rebuild it I suppose (I noticed via a search that a lot of people rebuilt them).
In either case, is there any reason not to use the used one I have? It seems to work electronically and I thought I could just swap it in and see if it works.

__________________
1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

You can go a step further and put an ohm meter across the leads on the "dead" one and verify that it has an open circuit to the coil.

It may be possible to take it apart and find the break in the wire and solder (your favorite thing) it back together. Bring it to Chads and we can have a look. I need some more stuff to fill up the day...

__________________
-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250

Well, all my soldering equipment is still in a pile on the floor from the last project, so I might as well use it again before I put it away. Do you think it'd be OK to use the spare used one while I fiddle with the other? Even with the break in the weather, the constant blazing heat trickling out of my vents is a bit oppressive.

__________________
1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

It can't hurt to swap it out and then try to repair the one from the car. Then, you can keep that one for a spare if you manage to fix it.

__________________
-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250

Before swapping in the used mono valve, applied 12 volts then pour some water into it to test for a torn diaphragm.

__________________
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

Good idea! It holds the water with 12v supplied. Will swap it in today.

__________________
1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

New-used monovalve installed ... I think it's working now, though I will have to try a test drive. I got it up to operating temp in the driveway and the air coming out is not blazing hot like it was before (though it's still warm, probably because it's warm out ... no A/C in this car).
Stupid question: Is it normal that the metal cylindrical part of the housing gets a bit hot (not too hot to touch, just toasty), even before the coolant got hot? Is that just heat from the electrical current? As usual, I'm nervous it's just going to burst into flames.

__________________
1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

The coil indeed gets hot if it's energized (when turning off the heat). When supplying full heat to the car it gets hot from the hot coolant ("hottant?").

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