I just purchased a 1983 300D Turbo Diesel and I'm having problems with the climate control - even with the temperature dial set all the way to cold/blue and the fan off, all of the dashboard vents leak a steady flow of hot air into the passenger compartment. Discovered the problem on last week's family drive up Hwy 5 through 105 degree heat in Redding, CA - even at midnight we were sweltering inside. The sunroof is great but...
Is there a valve or control unit somewhere that might be the culprit here? On our old 1982 240D there was no A/C but we could still keep the temp dial at cold and the master vent lever pulled all the way left to open to get clean cool air from the outside into the passenger compartment via the dash vents. But there is no master vent open/close lever on this dashboard and the temp dial isn't doing us any good. Any and all suggestions would be greatly appreciated.

It sounds like you need a new monovalve cartridge or the monovalve itself is bad. I don't know where the monovalve is in a 123 but follow the heater line from water pump to a small auxiliary pump to the monovalve all under the hood. The top of the monovalve has a cover with 4 screws. Remove the screws, you won't lose but a few teaspoons of coolant, and extract the cartridge. You'll probably find a tear in the screen or diaphragm. A new cartridge is about $20 the last time I looked (a minute ago on FastLane ). With the ACC set to MIN, the monovalve should get a solid 12V. Actually, it gets a solid 12V anytime the engine is running and ACC is not set to defrost. ACC manipulates the ground signal to control cabin heat. Regardless, if you check the voltage at monovalve connector, you should see 0V, solid 12V or pulsed 12V.

Sixto
87 300D

Very common problem. The system "fail-safes" to max heat and defrost if it fails, due to safety reasons. (They reasoned it is ok to cook the passengers but not ok if they can not see out a frosted windshield.)

Do a search on the forum for ACC, there is a wealth of stuff here. You have the Type 3 (NOT the Chrysler system) in an '83.

There is an electrically actuated valve, known as the Monovalve, that regulates the flow of hot coolant into the heat core. It is arranged to fail in such a way that you get full hot flow if there is no control signal, so you can't unplug it to kill it. You will have to stop or redirect the flow of coolant until it is fixed. If you follow the heater hoses at the firewall, it is pretty straightforward what is going on and what needs to be done to disable the heat.

__________________
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

thanks for the information. i did find an aftermarket monovalve repair kit for about $15 at some discount parts outfits online (regular list was $135??). will this do the trick?

The OEM Bosch part doesn't seem to be available right now. The alternative that most people now use is branded MTC. I think the consensus is that the MTC may not be as durable as the Bosch, long term, but gets the job done fine.

If the one you are looking at is branded URO, the answer is NO (which also goes for anything else under the URO brand).

__________________


Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Yes, MTC was the brand I found for $15. I guess for that price I won't worry that it isn't as durable.

A new cartridge usually does it. Ironically, the symptom of a failed cartridge is insufficient cabin heat at speed. Check the voltage anyway. A new cartridge won't help if there's no power to the monovalve. Default is wide open flow to coolant to the heater core as jay_bob said.

Sixto
87 300D

Sometimes the problem is a poor electrical connection at the monovalve. Before I started buying parts, I would at least verify that I had battery voltage at one monovalve terminal and continuity to ground at the other. If you don't have that, the repair kit isn't going to solve your problem.

Yes the OP is going to have to troubleshoot the system using the resources available on this site. There are also service manuals posted online for the ACCIII.

My suggestion was only intended for him to get it to stop cooking the passengers until he can fix it for real.

__________________
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