DavidY

So, i have finally noticed a pattern with my "no shut off" condition on my 1984 300SD. It seems to be related to the climate control system vacuum pods which control the vent dampers

I live in a hot climate so I always drive with the A/C on. I notice that the dampers randomly switch back and forth between blowing out the center vents and blowing out the defrost vents at the windshield.

The car usually will not shut off with the key, but recently I have noticed that every time that the car actually does shut off, its when the air is coming out the center vents (the correct vents the air should be coming out). And everytime the car does not shut off is when the air is coming out the defrost vents. I've observed this correlation many many times.

This leads me to believe that i have a leak in vacuum pressure in one or more of the vacuum actuator pods. I would like to verify this theory by isolating the climate control system vacuum lines from the rest of the vacuum system, but I cannot for the life of me figure out which vacuum line it is that goes from the engine bay to the climate control system. I'd like to disconnect that line and plug the rubber fitting to see if my engine shut off issue disappears.

Can someone please tell me which color vacuum tube in the engine compartment goes to the climate control system? Does anyone have a vacuum diagram for this car? I've looked for hours on the online forums and only seem to find diagrams for other models and years.

PelicanParts.com

I hope this helps you.

Here is a link to the free W126 on-line factory service manual supplied by Mercedes Benz.
https://www.startekinfo.com/StarTek/...tedDocId=12253




roy@pelicanparts.com

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DavidY

Thank you very much for the link. Its funny, when I opened the manual and went to the vacuum system troubleshooting pages the very first note says "excluding model 126.120" which is my car!! just my damn luck. BUT, there were enough similarities that it helped me get on the right track and I eventually found the leak.

Some interesting observations and things I learned:

1. My no shut off condition / erratic center vent damper position was an intermittent problem which really makes troubleshooting difficult

2. Its always best to start with step one. Test the vacuum pump and then start working on individual vacuum circuits

3. finding a vacuum system schematic for a 126.120 online is impossible!

4. The culprit in my system was a rubber elbow fitting which connects a clear/blue stripe vacuum line to a small solenoid by the coolant reservoir. The leaky fitting caused a lack of vacuum pressure going to the climate control system, fuel shutoff valve, and turbo wastegate.

5. Thank god by HVAC vacuum pots in my dash are all OK.

I'd be happy help anyone troubleshoot their vacuum system on the same model. It took me a long time to gain an understanding of the system without having schematics, but I think I've got it now!

kboonzaayer

Hey i know this is an old thread but I have a 300sd that I am trying to get the HVAC straightened out on. Currently I have a good pushbutton module and can turn the heat on and off but I only get airflow out of the defrost vents.

I am going to replace the compressor which I have been told is bad and am hoping I will get cold air if I do that. If I can't get air out of the center vents is that a vacuum pod issue? How bad is it to fix those?

Thanks,
Karl