Hello all
New to this site
I have a heating problem problem on my 1987 300SDL. So far done the following : Change and flush coolant, new thermostat , new monovalve (checked if working properlybefore installing it). Car heats on idle as soon as I hit 65/70 on hwy heat dies and only cold air blows.The O button on the climate control unit makes a clicking noise as if would turn on/off . I have a vacuum leak in the trunk .Could this affect the ACU unit ?
All opinions welcome
Thank you

I don't know much about this subject but there was some talk on it.

Ok, but which Mono-valve repair kit?
Hunter; one of the Moderators has organized our do-it-yourself stuff:
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If your Blower Fan is also going off at the higher speeds along with no heat; on My Year and Model there is a minimum Coolant temp that has to be reached before the Blower and the Blower will not turn on until that min temp is reached.

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84 300D, 82 Volvo 244Gl Diesel

Is your engine temperature rising normally to ~80 *C and holding on the highway?

The mono valve is supposed to fail open (i.e. allowing flow) so control failures cause the mono valve to pass all the time. I believe however if you have a torn diaphragm on the mono valve it causes it to block under high flow conditions.

Also check your aux pump, it is supposed to help with low flow conditions (i.e. at idle), however if the impeller is jammed I could see how that could cause blockage under high flow conditions.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Loss of heat at highway speed is the classic symptom of a torn monovalve diaphragm.

Exactly! The past 3 MTC monovalve kits I have purchased have failed like this, and this is the symptom I get. The first 2 I could actually see the little tear. On the 3rd, I could not find it, but when I removed the rubber gasket, coolant poured out of it. I am installing an OEM on Monday.

Jon

'85 300D 102k