82 300SD - AC system is charged, compressor/clutch runs OK when 12 V put across it (you could kill a penguin with how cold it blows, so that's OK), but the AC clutch refuses to kick on under normal operating conditions. I've tried jumpering the pressure switch and the temp switch (maybe do both at the same time?)

Is there any other thing that might be funky, like a relay?

How about voltage checks on the wires to the switches. What wires should have what voltages? Fuses look OK.

Any other diagnostics to try? It's driving me nuts being out of whack like this, and I want it back in whack ASAP with the hot weather coming on. Even a (safe) workaround would be acceptable.

Okay, you determined that the AC clutch works and system blows cold.

For the work-around, you could theoretically put a simple toggle switch
in the cabin and run 12 volts to the compressor through the switch.
Sort of how the 300D works with the old ACC from the factory.
Ground the other lead off the compressor.

Do that until you solve the problem.

Behind the passenger side headlight is the AC Receiver/Dryer with
pressure switch. Take the two wires off the pressure switch and jumper
across them. If the compressor engages then this pressure switch
is bad. To replace it the system has to be discharged.
So I might recommend leaving it jumpered and remember to replace
it the next time the AC system gets discharged.

There is not much else to this system. I don't remember anyone ever
needing to replace the brain that is up under the dash, but it probably
is responsible for sending the signal to the compressor.

Maybe somebody else here knows for sure - does the AC compressor have its own relay (the ice cube shaped things) ?

__________________
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

Well, jumpering the pressure switch isn't doing a thing. And I show no voltage across the wires at that end, so the problem seems to be elsewhere in the electrics.

Anybody know if the CD version of the manual has a good wiring diagram? I'm willing to buy the thing if it has what I need.

Going "upstream" from the compressor, electrically speaking: low pressure switch on R/D, relay in fuse box (look for #3 or a "J" ice cube relay), ETR switch, then pushbutton control unit. Work upstream . FWIW

__________________
The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

That's the thing I needed, which relay to look at!

I thought the ETR switch just ran the fan and didn't control the AC clutch. Am I mistaken in this? All to happy to jumper it too, if need be.

On the pusbutton control, are you talking about the row with EC and up/down etc? I also have a temp wheel, but that seems to be making things happen OK, although maybe it has a switch on there that isn't working (like the cold detent position?). I can go from heat to cool and flaps open and close under the dash at least.

Guess it's time to dig in. I ordered the manual on CD, but I wish I had a diagram right now!!

One other thing to check, which was a real "mystery fix" for me, is the thermovalve. I had an accidental FWFW repair that got my A/C blowing cold when I needed to replace the thermovalve insert and, instead, just pulled the entire relatively new thermovalve from my 82 and put it in my 86. I was getting nothing but hot air out the vents, replaced the thermovalve and voila! - cold air. Apparently, there is some circuitry in the thermovalve body that doesn't usually go bad with the every 2 years or so monovalve insert replacement, but it certainly helped me. That may not be the problem, but it's certainly worth checking - not obvious.

__________________
1987 300SDL 265,000 mi.
1982 300SD 325,000 mi. (and holding)
1956 Packard Clipper 150,000 mi.

This is the kind of information that is helpful on this forum. He obviously closely read the post, realized that the compressor clutch wasn't engaging with the CCU. Correctly read that when the clutch is jumpered that the AC blows cold, correctly read that the pressure switch had been jumpered.....
Nice job jbaj!

__________________
Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John

Jimmy L.,
Thanks,..... but.....I cheated.....I looked at the manual's electrical diagram.

Jumpin' Je.,
This is the ETR to me (there is some confusion in terms) . Controls compressor only (to prevent icing of evaporator).
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=23P1AJKI123U03S7F1&year=1982&make=MB&model=300-SD-002&category=R&part=A%2FC+Evap+Temp+Sensor

PM me your email address and I'll send you the electrical diagram for A/C clutch control (too big a file, since it is PDF, for posting in this forum's format).

__________________
The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

Aha, the little red one is the AC temp/fan switch. Now I see my confusion. You'll note above that I thought that was for the fan. Got confused and thought ETR and fan temp were one and the same. As Daffy Duck once said "Pronoun trouble!", or in this case, proper noun trouble.

AC now works. THANK YOU for all your help. I feel like such a dope.