Hello,
I'm planning to replace my leaking and worn compressor with a Sanden unit, and replace the TXV which is also leaking. While I'm at it, I think I have vacuum issues in the control unit but not sure where or how to start. Symptoms are slightly different than other threads I've read. The temperature knob functions. In all modes other than defrost, the front upper vents and rear center vent (behind console) will blow cold or hot to maintain whatever temp you select and it seems to maintain a temperature fairly well and keep fan speed to a minimum. In defrost mode, hot air comes out of the defrost vents only regardless of temperature selected (by design?) In all modes, no air ever comes out of the floor vents. Also, it seems to not allow the fan to come on at all (for heat or cool air) until you've driven for 10-15 minutes - long after the temp has passed the 40 degree mark. Again, not sure if this is by design but can't imagine I shouldn't be able to have cool fresh air blowing in immediately after starting.
EDIT: While I have the dash apart anyway, does anyone recommend replacing parts of this system as a matter of preventative maintenance? Vacuum pods, etc? I only want to have to do this once.
Thanks for any advice in advance!

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-1981 300SD 242k, stock - sold @ 250k
-1994 Toyota Pickup 4x4 R150F 5 Speed, 5VZ V6 swap.
-2004 Volvo V70-R 2.5T

On the W126, a majority of the vacuum pods for the Climate Control system are accessible without dash removal. Personally, I wouldn't replace them until needed. Having them on hand wouldn't be a bad idea.

From your description, it sounds as if everything except the delay is functioning normal. There is a temp. sensor near the thermostat that controls this delay. It may be something as simple as a slightly loose/corroded connection at the sensor.

Here is the section of the FSM that covers ACC:
https://www.startekinfo.com/StarTek/outside/12253/program/climate_300SD.htm
It contains an operational check that should help. I went through my ACC a few years ago and it's a nice system, once you understand how MB designed it to work.

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Sam

84 300SD 350K+ miles ( Blue Belle )

You can check the vacuum pods with a vacuum pump. The diaphragms go bad. Some have 1 diaphragm, others 2 diaphragms. They can be replaced. There is some disagreement on how well replacements work and whether the refurbished pods allow full range of motion. The pods can be removed with the dash installed and some in the yards have good diaphragms.

I would do everything possible when the dash is out. On other vehicles trasg accumulates in the box that holds the heater core. Clean it if this is the case on your car. Remove the wood from the dash and re-glue it.

Per destructions in a replacement heater core I had, electrolysis scavenges metal from the core and causes leaks. You can measure electricity in the coolant by putting 1 probe of a volt meter into the coolant and grounding the other. You will need to snake a wire into the coolant because it isn't easily accessible on the SD. Anything .3 v (3/10V) indicates that coolant needs to be changed.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

So, I found the pods attached to the Blue/Red (#3 on the passengers side manifold) and Red/White (#1) to be leaking. Doesn't hold any vacuum. Now to determine where it is...I can hear it but lose the line after it runs up above the CCU.

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-1981 300SD 242k, stock - sold @ 250k
-1994 Toyota Pickup 4x4 R150F 5 Speed, 5VZ V6 swap.
-2004 Volvo V70-R 2.5T

Both of those lines go to the Defrost Pod. It has dual diaphragm elements. The best option is to just replace the dual element pods. To locate it, remove the glove box liner and it is inside to the left, on the side of the air plenum.

https://www.startekinfo.com/StarTek/outside/12253/program/climate_300SD.htm
https://www.startekinfo.com/StarTek/outside/12253/program/Climate/83-604.pdf
https://www.startekinfo.com/StarTek/outside/12253/program/Climate/83-665.pdf

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )