I have a problem with my ACII system (imagine that) that I hope you folks can help me out with.
Over the past 5 years I’ve replaced all the vacuum pots, vacuum harness, push button control unit, electric/vacuum switches, Air Cond (compressor) switch, the AC amplifier and the servo. None of the major components are more than 5 years old.
Here’s the deal.
When I push the Auto Hi or Auto Lo button (with the Air Cond switch “on” of course. Temp dial set to <75) I don’t get the compressor to come on. Auto HI doesn’t go to the top fan speeds either. If I push DEF for the defrosters, then the compressor kicks on and the fan goes to high. For a while, I could trick the system by pushing the DEF on for just a few seconds and then hit Auto Hi, but that trick doesn’t work now either. If I sit and play with the buttons, sometimes it will kick on (or stay on from DEF) but more often not. If it is on, it wont be if I shut down and start the engine again.
I unplugged the green and yellow vac line under the hood and put the Mity-Vac on it, it seems to hold (is not wide open type leak down anyway). Door locks work fine. Engine shut-off is fine.
Any ideas? Bad servo? (I can hear it wind down (Park) when I shut the engine off)
Bad amp?
Any help would be greatly appreciated. It’s getting warm here.

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1980 300D - Veggie Burner !

If I get this right you are saying that the fan is not operating on Auto Lo
or Auto Hi (at least not consistently).

This problem is related to blower motor brushes (which are quite easily replaced).

You did not mention the blower motor in your list of things done.

Specifically, when putting in Defrost mode the thing kicks in, this is one of the clues.

Plenty of threads on brushes for the fan.
A place called **************.com (I think has them with instructions or brushes only).
I got mine from a place in Fountain Valley California (2 sets with shipping and tax for less than $20).

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80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250

Sorry.
I did replace the blower (fan) unit about 6 years ago. Brushes were gone, and so were the bearings.
The fan will go to high speed when in DEF mode.
It's running faster in HI than in LO, but still not the upper speeds. The big issue is that it's tied in with the AC compressor as well.
The compressor wont come on in either HI or LO. When I get luckey (about 5%of the time) and it does kick on, then the fan operates as it should, reaching the higher speeds.

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1980 300D - Veggie Burner !

Sounds like the ACC control unit is not doing its job correctly.

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1981 300D 147k
1998 VW Jetta Tdi 320k
2001 Dodge Ram 2500 141k
1979 300D 234k (sold)
1984 300D "Astor" 262k(sold)
Mercedes How-To and Repair Pictorials
I love the smell of diesel smoke in my hair

As in - the servo?

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1980 300D - Veggie Burner !