Problem: Engine warm, but heater Blowing cold air only.

I have reason to believe there IS hot water reaching the heater core.

Fan blower works fine..
CCU functionality is fine...I believe
Hint: There is only about five seconds worth of hot air coming out of the vents at the beginning upon starting the engine (with a previously warmed up engine) but then quickly will continue to blow cold air only.
My next place to look, and this is my question: where is the vacuum actuator that might open the hot air vent? How to troubleshoot THAT, and Is this a reasonable place to look next or am I barking up the wrong tree here?

I have replaced mono valve w/new unit, and checked functionality.
The Auxiliary water pump was checked for functionality.
The Climate Control Unit IS new and seem to function normally.

1987 300SDL

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1987 300SDL

What model and year?

1987 300 sdl

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1987 300SDL

If I unplug the electric connector to the monovalve, it should be full on heat right?
Update: I did this, drove for 10 miles and the air blowing was still cold. Still, when driving,
if I turn the push button control to the off position, let it rest for a moment, then turn it back on
again, I will get a blast of hot air for a few seconds, then gradually a few seconds later back to cold air.

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1987 300SDL

I've been playing around with my SDL heating inconsistencies similar to yours and found that unplugging the auxiliary water pump returns the hot air until I'm around 2500 rpms. so I'm thinking I need to pull the aux. water pump (clean or replace). With the aux. pump plugged in I get heat only for a minute or two after start up.

One theory is that a poorly running aux. pump motor can trip the CCU box related fusing that resets when the ignition is turned off. Prior year CCU boxes apparently did not have this built in overload protection.

I've read where the aux pump is to support low rpm heat function however I get full heat with the aux pump unplugged. Counter intuitive it seems.

Re: box & vent doors you can put a vacuum pump on lines going to each vacuum pod to test function. Google W126 vacuum pod (hvac) testing for lots of guides.

Keep me posted on your progress & I'll do the same.

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1986 300SDL 440,xxx

Update #2
I swapped the monovalve working parts (valve and coil) from my other SDL (known functional) and now there is plenty of heat in this car. When I have time I will like to narrow the problem down: was the faulty part the new monovalve that I purchased or was it a bad coil. They both bench tested fine for actuation. I did not check resistance in the coils even for comparison purposes and I have no standard to compare to anyway. Maybe it's in the FSM...
Also, I'm curious if the screen element in the monovalve is something that needs to be cleaned periodically? Seems the screen is so small they get filled with rust very quickly. Is there some purpose the screen needs to be cleaned, what if the screen was missing? maybe it's just to keep debris out of the working part of the valve.

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1987 300SDL

Was the new monovalve aftermarket? There was a 300SD on the forum a little while ago that would only produce heat at a stand still but not when moving. It turned out tot be the aftermarket monovalve assembly.

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2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

They do have a problem with sticking especially after summer. While some here think I joke when I say adjusting the monovalve with a rubber hammer, I am not. That usually frees things up.

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00 E420
98 E300 DT
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97 S500
97 E300D
86 Holden Jackaroo Turbo D
86 300SDL
(o\|/o)

yes it was an aftermarket rebuild kit (the plunger with new seals) it was an MTC I'm thinking about just putting the old O.E. unit back in. the rubber looks in good condition

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1987 300SDL