I'm hoping there is a 126 HVAC guru amongst the diesel elite here.

The problem:

No heat on any setting EXCEPT defrost. Also, blower motor will change speed, but not shut off.

What I have done thus far:

Replaced the CCU with one tested in a known operational '91 126. Replaced the aux. water pump, monovalve and Klima relay. I also have a replacement porcupine resistor, which I expect will solve the blower motor issue.

The only other thing I am aware of that might have an impact on heat production might be the temp sensor that is in the HVAC loop. Which sensor is this on my OM603.970, and is there anything else I should be looking at?

I hate MB's hvac as it ages!

Thanks

__________________
Chief

1991 350SD

The FSM has a diagnostic procedure for the ACC. It only requires a multimeter.

Yours should be new enough to have self diagnosis. It gives blink codes to tell you what's wrong. For that you will need to have an impulse counter. It is an LED, push button, and resistor with some test leads.

The procedure calls for a special breakout box that plugs into the 2 big plugs behind the PBU but you can read between the lines in the procedure and figure out which pins on the 2 connectors go to whichever sensor or actuator the test is calling for.

If you suspect a sensor, grab the ETM schematic and ohm them out from the PBU plugs.

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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

Go to the Programa website. There is a complete DIY troubleshooting pdf file on your vehicles CCU that allows you to use a multimeter to check all of your sensors. It is an awesome site. I used it to diagnose my 86 300sdl's CCU issues.

Good Luck

__________________
John
1986 300sdl
1985 380se

By the way, default program in the CCU is Defrost. That way you can always have a clear windshield.

__________________
John
1986 300sdl
1985 380se

Thanks guys. I figured the default- a thoughtful bit of engineering!

__________________
Chief

1991 350SD