I have an 87 300TD and there is no heat in the cabin. When the car was at idle it would only blow cold air, but as soon as you started moving, the heat kicked in. Now it is just blowing cold air! Help! It was 27° this morning!

__________________
I am not who you think I am

Lots of places to look—clogged aux pump? clogged monovalve? broken monovalve not opening? clogged heater core (you better hope not!)? failure of the control unit?

How much do you know about the car?

__________________

"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

I know I replaced the controls in the dasboard about 9 months ago because none of the buttons were working. Other than that, not much. I have never messed around with AC or heat before.

__________________
I am not who you think I am

Probably the first step then is to trace the coolant lines from the engine to the heater core and back, looking for anything that could be blocking the flow of hot coolant.

__________________

"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

There is an excellent troubleshooting guide in the factory manual.

The manual can be found at startekinfo.com, select MB Workshop Resources on the left, then go about 2-3 pages of results to "124 Service Info".

Or try w124-zone.com. You have to register but it's free.

Locate chapter 83 for the climate system.

The tests of the sensors call for a fancy breakout box, but if you have a copy of the schematic (also available as part of the FSM) then you can follow the process to ring out the sensors from the CCU plugs using a multimeter. You'll have to find the sensor in the diagram and see which pins on which connector belong the sensor you are being asked to test. If they had been smart the breakout box pins would have matched the actual connectors but no, they are randomly numbered.

You'll need the following:
Multimeter - a cheap Harbor Freight one will work for this
MightyVac or equivalent
Thermometer - the kind you use in a restaurant to test food - try bed bath & beyond. The sensor readings are temperature dependent.

The system is a wonder of analog control engineering and if correctly sorted it works better than a lot of the modern cars out there.

__________________
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