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1981 300SD climate control -- help me diagnose

701 views 6 replies 6 participants last post by  John350 
#1 ·
Hi folks.

I've been working on the HVAC as long as I've owned the 300SD (nearly 3 years now). It came to me without a compressor, a dead monovalve, and a half-dead CCU.

I installed a new compressor, a remanufactured monovalve, and a CCU on which all buttons actually work and close circuits.

Here's what I've observed about function:

- The whole shebang will only turn on if I start with the defroster button. If I start with it off ("O" button?) and press anything but defroster, there are no clicks, no vacuum noises, nothing. If start with the defroster, it springs to life, then all other buttons make a difference (switching output vents, turning compressor on/off).
- After a few minutes of use, the whole thing stops working. Just shuts off. Won't turn back on until much later in the drive, or the next drive.
- Yesterday when I drove it in 70F+ weather, it only blew hot, hot air. The compressor was running and the fat pipe was cold to the touch.
- This morning, driving to work in ~50F weather, it only blew air that felt like it was as cold as the outdoors. No heat to speak of.
- I have no idea whether the aux fan is kicking on or not while driving. It never ran while idling. I just replaced it because the old one wouldn't budge even when I wired it directly to 12v.

Diagnosis tried so far:
- Because I got nothing but heat yesterday, I checked the monovalve first. It's getting 12v (on my multimeter) when grounded to the body, but when grounded to the other terminal in the plug, I read 0v. This was set to bi-level, MIN on the temp dial, and fan on auto. Unfortunately, I didn't try the circuit with the temp dial set to cool, but not MIN. I believe that's two different circuits, with MIN bypassing the temp reg unit and anything up to MIN going through the temp reg unit.

My first thought is to replace the temp reg unit, because the aux fan may not be kicking on and the monovalve is not getting it's ground closed. But the missus is getting sick of me buying parts and hoping the next one will solve the problem, so I want to know what further diagnosis you folks would suggest to be more sure it's the temp reg unit. Other than checking the voltage to the monovalve circuit when it should be grounded by the temp reg unit, what further diagnosis would you suggest?

Thanks for any and all replies.
 
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#3 ·
Lots of odd behaviors it looks like. If it were me I would re-flow the push button unit, temperature regulator, and blower motor control units. Really sounds like solder/capacitor issue. IMO. If you are interested drop me a PM/email.

IIRC The aux fan is controlled by sensor(s) on the accumulator and not by other higher grade electronics, could be wrong tho.
 
#4 ·
I've had my share of issues on the climate control for the SD as well.

The problems have also been the CCU itself and the temperature control unit.

Fundamentally, any of the used equipment where the seller claims "works perfectly" are suspect. They may work "perfectly" for one minute and fail to work whatsoever the next minute.

Right at the moment, the CCU also doesn't work properly with a unit that was claimed as "new".

Without a full battery of test equipment necessary to simulate the operation of all the attached controls, it is effectively impossible to troubleshoot this system and one must resort to being a parts changer. Sad, but it is what it is.

The rebuilt units from Programma give a product that definitely works properly for the short term. Long term durability is suspect.
 
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