I have a 1978 Mercedes 300SD turbo diesel 116 chassis. The only thing that works on my climate control system is my defroster button (which works at max fan speek at max heat) i was told by a mbz mechanic that this is the default when there is a fault in the climate control system. Can anybody help me? California summers are tough without ac. Thank you.
saab_scania@sbcglobal.net

It is a 3 part system. Control panel, Climate control amp, and servo. When you make a selection on the control panel, the selection goes to the climate control amp which sends the signal to the servo under the hood. On a 116 the amp is behind the glovebox. I would start there. Do a search, there are alot of diagnostic tips on this system.

__________________
1959 Gravely LI, 1963 Gravely L8, 1973 Gravely C12
1982 380SL
1978 450 SEL 6.9 euro restoration at 63% and climbing
1987 300 D
2005 CDI European Delivery
2006 CDI Handed down to daughter
2007 GL CDI. Wifes

Could be a frozen servo. Could be a blown servo fuse (inline, not ont he panel). Could be bad amp, or bad control unit or bad vacuum valves. There is a temp sensor in there too, its all a chain that eeds to be debugged. George Murphy has a disgnostic booklet which is a condensed version of the 116 manual (come up on Ebay now and then)

__________________
MB-less

I'm chasing the same gremlin in a 79 300SD.
You have a vacuum system that includes locks, brake booster, trany and HVAC. Diesel Giant has a nice DIY on this but not the HVAC (in the works).
Does the engine shut off with the key?
Do all the locks lock with the driver's door lock?
Will the door locks lock 15 minutes after the engine has been shut off?
Are the brakes OK?
Trany shifting OK?
Are the rubber acordian hoses between the door and car at the hinge OK? If notm, you may have a cracked hose in there.
Get a hand held mini-vacuum pump (manual) and some golf tees. Starting from the brake booster vacuum line (19mm nut) verify vacuum, should be around 25".
Then work your way around the system marking lines and pluging connectors.
Anyhow you have to narrow down what works and what dosn't, till you find the biggest leak and plug it or repair / replace the part.
Watch out for breaking the nipples off the blue/black check valves near the brake boster, they are very easy to break and cost $10 each.
Start hunting!

HVAC trouble is usually not vaccum related, if it is its usually a control valve gone bad, the others will work. He needs to see if the a/c compressor is engaging and the servo is working. The vacuum diagnostic is mainly for door locks. I think there is a big check valve up front on the drivers side that can go bad. I had leaking door pots and it had no effect on my HVAC system at all.

__________________
MB-less

Search forum for "Climate control" and "ACC-II". Also go to top of page and visit the UW Tools alternative site - download their installation manual for good system diagrams which will help you understand the system - and then you also have the alternative to consider if you decide to replace rather than repair this truly nightmarishly complex system. However, the amp and dash control panel have to work properly in order to install the UWT version, as well, so troubleshooting is required in any case.

The factory CD's have a troubleshooting portion and systems diagrams; one can check it out with a normal hand vacuum pump and multimeter, if one is good at logic troubleshooting and following directions.

Basics include checking the fuse to the amp, which in my 78 280CE is hidden in a small inline fuseholder inside the relay box, as well as the A/C and heater solenoid fuses in the normal fusebox.

Have a whole lot of fun, and may it only be your fuses...

Stan

__________________
stanbabu@3bubbas.com
59 300d
65 250SE/Cpe
70 250/8
72 250C
78 280CE