Yesterday I did a thorough cooling system cleaning on my 84 300SD; during a thirty minute idle I noticed that the AC compressor clutch was engaged - with the CCU OFF button pressed in. I then unplugged the compressor coil connector until I could do some testing today.

The AC has been a NO-OP as the PO lost the freon via a failed O-ring at the expansion valve; the R4 compressor fortunately hasn't seized but I've now been wondering just how much oil is left in it and whether it's still capable of compressing refrigerant gas into a liquid.

Today I checked for voltage at the compressor connector and found the following: (CCU still has the OFF button latched.)

1) with key at positions 0 and 1 - no voltage.

2) with key at position 2, I found 12 volts at the connector.

I previously searched here for threads using keyword KLIMA and found much good information. I have looked under the hood for the KLIMA using the P/N 001.545.74.05 but haven't found it. (The box with 001.545.9832 on it is the glow plug controller judging from the wiring that's plugged to it.)

From reading threads I'm thinking that one of the two relays soldered to the KLIMA board has its contact points (or is it solid-state?) for the compressor clutch coil's 12 volt feed seized closed.

Can anyone advise where the KLIMA is located on an 84 300SD? Also, is it in the same location on an 82 300SD?

Thanks for any info on this.

__________________
Respect, protect, and maintain complex systems - whether natural or of German design, to benefit from their full potential.

1982 300SD W126.120 with an OM617.950 from a W116.120
1984 300SD W126.120 with an OM617.951

It may be behind the kick panel, in the footwell on the right front passenger side. Klima should have electromechanical relays and it's possible that one could have its contacts welded together by years of sparking. EPC might also have information on the location of Klima; put in your VIN and it will give you parts lists and diagrams for all of the parts.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

The '84 SD has no "Klima". It first appeared in '85 and was further enhanced in '86.

What's the issue?

Thanks, Brian, for advising that there is no Klima unit on the 84 SD.

My concern is the fact that my 84 SD's AC compressor's clutch coil is getting 12 volts at all times with the key in position 2 - even with the climate control unit's OFF button pressed in and latched.

I want to locate the source of the 12 volt feed to the clutch coil to determine why the voltage doesn't disappear when the CCU is off. Now with the knowledge that there's no Klima to control voltage to the compressor, I'm suspecting that the CCU itself controls this voltage, according to input from the cabin air temperature sensor, and possibly also from the rotary temperature selector.

I'll now do still more searching to try to find a wiring schematic for the CCU's pinouts to external components in the climate system.

__________________
Respect, protect, and maintain complex systems - whether natural or of German design, to benefit from their full potential.

1982 300SD W126.120 with an OM617.950 from a W116.120
1984 300SD W126.120 with an OM617.951

The CCU provides the signal to close the relay for the compressor clutch. It's possible that the relay is NG, but they usually won't fail closed.

You can check to see if the relay has power on pin 86 (socket #4). If it does, the CCU is the culprit.

I'd suggest that your symptom is a relatively common failure of the CCU............clutch engaged all the time.

Thanks for the additional information about the CCU's function for compressor control.

Overnight I thought about the group of relays at the end of the fuse compartment toward the windshield. I was thinking that the CCU probably sent a compressor control signal voltage to one of these relays, which then switched a relatively stronger current of 12 volts to the clutch coil. This would have helped to keep the amperage values present on the CCU circuit boards to safe levels to guard against heating. But then we know what can happen in the CCU if the auxiliary water pump seizes...

I just realized that the terminal identification numbers on several of the relays in the fuse compartment include the number "86". I'll try to relate the location of "socket 4" that Brian mentioned to one of these relays in the fuse box area, hoping that a relay in here and not in the CCU is the one that's bad with a set of contacts stuck closed. I'm hoping that the control relay is still seeing a control signal from the CCU the presence of which varies according to temperature changes in the cabin and that the heavier controlled voltage is always sent to the clutch coil because of stuck contacts.

__________________
Respect, protect, and maintain complex systems - whether natural or of German design, to benefit from their full potential.

1982 300SD W126.120 with an OM617.950 from a W116.120
1984 300SD W126.120 with an OM617.951

If you stand on the left side of the vehicle and look into the fuse box, the compressor clutch relay will be in the first row (closest to you)..........second relay from the right side (windshield). You'll find a pin "86" on that relay and it will be engaged in socket 4. You can pull the relay and troubleshoot socket 4 as to when it receives voltage.

you could pull the climate control out, and see if the compressor is still engaged too.
then pop the cover off the climate control, and see what the condition of the board looks.

Brian and others, I found something interesting here in the 84 SD after pulling out the compressor clutch relay...

Someone had pushed in a piece of bare stranded copper wire into socket holes 3 and 4 to make relay pins 86 and 30 common. I'm assuming that someone had found no control signal ever present at relay pin 85, so that the relay coil never pulled in to supply pin 87 with 12 volts for the clutch coil.

I think the intent was to keep the compressor clutch engaged at all times with the absence of a control signal, which might have been OK (but not textbook-correct) if all driving was done at low engine RPM, but at highway speeds? Then the AC system high side pressure would tend to increase greatly, until (hopefully) the high temperature refrigerant cutout switch at the receiver-drier would have brought things to a halt. If this never happened, then maybe that's why the O-ring at the expansion valve eventually failed and dumped the refrigerant.

With the CCU temperature dial at full-stop on the cooling side and with the key on at position 2, I didn't see any control voltage at the socket hole for pin 85 - which I am assuming is where control voltage would come in. I may try this test again on a really hot day when the car is not in the garage, to see if the clutch engages.

The clutch relay is OK, tested it on the bench with the battery charger, and heard and felt the relay operate. So the next step will probably be to look at the CCU in the dash area and then learn what a rebuilt unit will cost.

__________________
Respect, protect, and maintain complex systems - whether natural or of German design, to benefit from their full potential.

1982 300SD W126.120 with an OM617.950 from a W116.120
1984 300SD W126.120 with an OM617.951

Agreed. The CCU needs to be replaced.