Ran into an issue with the heater recently on my 1980 300td. It used to function sporadically with me occasionally getting heat, and after I replaced the thermostat it ceased to work, which is troublesome with my state inspection deadline getting closer. From what I've heard/read, my car has the push button climate control and the heater servo that operates on hopes and dreams, which I suspect to be the problem. Any help is appreciated.

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Current Collection:
1990 Ford Ranger 5-spd ~168xxx miles (now totaled )
1980 Mercedes 300TD ~189xxxmiles

If your climate control buttons look like this, you have the servo:



In which case, odds are good that the servo is stuck shut i.e. it cannot open to provide the proper flow of coolant to the heater core.

When this happens, the climate control amplifier device often fails in concert.

Here is a fun link to read through (Mercedes borrowed this system from Chrysler since they didn't yet have a system of their own design).

1971 Imperial & Chrysler Auto-Temp II Service Book (Session 281) from Master Technician Service Conference

It will help you wrap your head a bit better around the operation of it.

There are some options -

Source and install a good used servo (hard to find) and a new amplifier.

Source and install a rebuilt servo Performance Analysis | Climate Control | and a new amp

Source the digital upgrade for the servo https://unwiredtools.com/accii.asp

Or you can install a manually operated water valve in place of all this. If you get really tricky you could even install a cable in the cabin attached to a slider or knob to open and close the valve from the driver's seat!

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1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

http://i.imgur.com/LslW733.jpg

The Baja Arizona Oil Burners Send a message if you'd like to join the fun
Left to Right - UberWasser, Iridium, Stuttgart-->Seattle,, mannys9130

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Wow, thank you. Thanks for all the information, now I just need the time to fix it, haha. Really do appreciate your help.

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Current Collection:
1990 Ford Ranger 5-spd ~168xxx miles (now totaled )
1980 Mercedes 300TD ~189xxxmiles

You're welcome. Best of luck. Please follow up on your diagnosis, repair, and results if you get a chance. I know I'd like to know how it turns out.

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1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

http://i.imgur.com/LslW733.jpg

The Baja Arizona Oil Burners Send a message if you'd like to join the fun
Left to Right - UberWasser, Iridium, Stuttgart-->Seattle,, mannys9130

Visit the W123 page on iFixit for over 70 helpful DIY guides!

What is your heating system doing?

The chain of events that lead to the death of the system..

Vacuum leaks

The aux water pump freezes, killing the amp and then the servo.

If your not getting heat on Deforst, you can connect the two red lines that run to the bottom of the servo together....there is a thermostat on the bottom of the servo, controlled by vacuum....it opens and allows heat in when the temp gets to that special point.

You also have a foam sensor tube that is behind the glove box, this turns to dust and thus the auto system cannot sense the cabin temp...

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If I had one of these cars and the servo system turned out to be a basket case I think I would go the manual route and swap in the lever and knob system from a 240D. The elegance of simplicity.

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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

Not much, sometimes it randomly gets warm-ish, never hot though, and the vents have a mind of their own.

Now that I think about it, I don't think it really ever worked properly since I had it. (I've only had it for about a year at this point.) I have replaced the vacuum pump and the thermostat, however the gauge inside the car never gets past about 60 degrees. That in particular is kind of odd, and maybe this is a subject for a separate thread, but I have a suspicion that either my fan clutch is stuck on, or the temp sender is bad. The rad hoses are still hot to the touch at operating temp though.

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Current Collection:
1990 Ford Ranger 5-spd ~168xxx miles (now totaled )
1980 Mercedes 300TD ~189xxxmiles

Definitely will. I'm on limited funding right now, and some of the replacement servos are blindingly expensive so it might be a while, but I won't leave you guys in the dark.

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Current Collection:
1990 Ford Ranger 5-spd ~168xxx miles (now totaled )
1980 Mercedes 300TD ~189xxxmiles