OK, so I put in a new servo (aluminum body, $250+tax from Texas European Motors, Austin, TX) and now I only get blower motor and compressor when switch is on "DEF" --no blower in any other switch position. And STILL NO HEAT! Aux water pump is new but only seems to spin when I jump it to the battery. I tested the resistance circuit, and the three components (in-car
temp sensor, the ambient temp sensor, and the rheostat temp selector) all check out.

Sounds like the amplifier, right? Well, I have substituted three different ACC amps, all with the same result. Could they all be fried? Is there some other relay or something that would fry the amps? How can I bench test the amp? I don't want tojust buy a new one and plug it in only to fry it, too...

__________________
1978 300SD 149K miles silver-blue "Maxine" (mine!)
1979 300SD 318K miles red "Helga" (SOLD)
1980 300SD 321K miles yellow "Linda" (younger daughter's)
1980 300SD 148K miles blue (SOLD)
1980 300SD 153K miles green(parts car, SOLD)
Yes, I have a W116 fetish :-)

Let's try this again, shall we? Here is a copy of my prior response - I will add clarifications in bold:

There is a main vacuum switch located next to all the other vacuum switches that controls the on-off functioning of the entire system. It is supplied with vacuum from the intake manifold via a thermoswitch located on the bottom of the servo (or is it the servo bracket – memory’s going…)The line is black to one of the ports on the thermoswitch, and is of a larger diameter than the other CC vacuum lines. . If the same previously stated main switch does not receive vacuum (from the manifold - via the aformentioned thermoswitch) , the system blower will not turn on. This is likely to be your problem. However, whenever you select the defrost button, the blower & AC compressor will be supplied voltage directly, without the need for vacuum to operate ANY of the vacuum switches that *normally* control those functions. OR you could have a large enough vacuum leak from elsewhere in the system from a leaking diaphragm, switch, or connector which will not allow enough vacuum to develop to operate some/any of the vacuum switches. Whip out the vacuum pump and test every diaphragm in the system. If they are all good, check the vacuum switches directly – current should flow (continuity) when vacuum is applied.. The fact that some of your vents don't open is yet another symptom of no vacuum to the vacuum switches - certain vents are controlled via one or more of those vacuum switches - I'm working from memory here, so I can't be more specific without risk of disemenating erroneous information

SO... Did you check the main vacuum source thermoswitch, all the diapragms, & the vacuum switches? When you find the actual problem, (which WILL be the cause of) a lack of vacuum within the system, let me know if you want to unload that new servo of yours - I could use one for one of my 107s.

RTH

One other thing that I just thought of...

There is a coil of green wire (under the dash) that is plugged into the vacuum system via a rubber connecter. It looks like a stupid mistake of some sort, (like maybe Hans at the factory had a little too much beer at lunch) but it is actually the way that the factory "engineered" a controlled leak point into the system. Do not remove this high tech engineered vaccum bleed-off.

Thanks, RTH, I remember your previous post and, well, sort of disregarded it because of your mention of the intake manifold as source of vacuum...this is a diesel discussion, remember?

My servo was definitely bad--cracked body and frozen motor. Yes, I know about the vacuum switch on the bottom of the servo and I have vacuum there, just not very strong. I am in the process of deciphering the vacuum test procedures from the CD manual.

I also learned that there is an in-line fuse for the ACC amp, located in the fuse box. I wondered what that fuse was for!

Thanks again for your insight and I'll let you know what I discover.

-=-=-=Eddie

__________________
1978 300SD 149K miles silver-blue "Maxine" (mine!)
1979 300SD 318K miles red "Helga" (SOLD)
1980 300SD 321K miles yellow "Linda" (younger daughter's)
1980 300SD 148K miles blue (SOLD)
1980 300SD 153K miles green(parts car, SOLD)
Yes, I have a W116 fetish :-)

My bad. I wish I had an excuse for the brain fart, but I'll have to own up to this one. I forgot that you're working on a diesel -- this plus the fact that I was pulling the information out of my memory (or perhaps some bodily orifice) didn't help. I have owned several cars with the ACCII type system but they were all gasoline engine cars, thus the correlation.

Another reason to disregard my previous post would be the comment about Chrysler based systems. That statement was NOT due to brain-fade. That (expletive deleted) Rube Goldberg designed servo was originally used on Chrysler vehicles. Mercedes-Benz, on occasion, will not try to reinvent the wheel itself and will purchase "well engineered" components from the best available source; this explains the use of Delco A-6 compressors on the early V-8's. They probably took one look at the servo, wiring & vacuum diagrams and were suitably impressed by its complexity. I can just imagine the conversation…. Look ut zis Hans - Zose Americans vinally got sumzing right – vee vill uze zeez…

Good luck

RTH