It may simply be 'frozen' to the pulley.
Kinda rare, but CAN happen if it has been slipping a bit too much.


EDIT: Finished reading post... So it releases when the clutch is disconnected?
Would have to look at wiring and advise... if I have it I will peruse and return.

 

Thanks for that.
The clutch does disengage when the electrical connector is removed from the compressor and also when ignition is OFF. As soon as ignition is ON the clutch engages again irrespective of whether the aircon button is ON or OFF. It even stays engaged when I remove the Klima relay, which I assume is the aircon relay behind the battery.

 

I would swap out your KLIMA relay with another to see if that solves the problem.

 

With the symptoms you describe the problem almost has to be a short in the wiring harness somewhere because the AC clutch is directly connected to the klima relay with no other control circuitry in between. What you describe could be a short between the AC clutch solenoid lead and another circuit that carries +12 volts.

I don't think your car is part of the infamous self-destructing wiring era, but that part of the harness runs through the lower engine bay and could have rubbed against something or contacted something hot and melted. It's a tough environment down there.

A visual inspection of the wiring in the engine bay may be in order. If you're brave, you can try removing some of the fuses (one at a time) that protect circuits that run through the engine bay to see which one is actually supplying the short circuit power to the clutch. Knowing where the errant supply voltage is coming from may help track the location of the problem.

 

Thankyou.
Will get underneath and have a good look. I don't really know where to look but will try with a good light. It has been hard locating any wiring diagrams for this car, especially for the A.C. system. Since there is a lot of electrics in it wiring diagrams are essential to keeping it on the road.
Regards

 

Thanks. I will have good look.

 

Mystery Solved!

Thanks again one and all for your much appreciated comments.
After some investigation, rolled the temperature dials on the dash console up and down a few times between stops.
This seems to have freed something which may have been shorting there.
Now the A.C. compressor seems to be functioning normally.
Fingers crossed!!!
Next time I'll get in there with some WD40.
Regards.
Ed

 

I would not recommend spraying anything into the wheel.

 

Woops! Spoke too soon.
The AC compressor is back to its old ways and will not dissengage unless unplugged at the compressor.
Spoke to an MB mechanic who suggested it may be the temparature control wheels.
Just had a good look at the heater control unit and cleaned up any untidy solder connections.
Tried disconnecting the connector plug at rear of the unit as well as pulling the Klima relay: the compressor stays on.
My thought is that the compressor may be wired like the heater valve and only turn off when any of the relevant circuitry is interrupted or fails. This seems nonsensical but it is how the heater valve is wired. In that case, anything that faults in the AC compressor electricals will cause it to default to ON.
Still need to have a closer look at the wiring and harnesses for this system to check nothing is shorting. Wierdly the thing worked OK for an hour or so the other day.
Any input appreciated.

 

Ed -

These are the closest I could come to a schematic for the A/C in your car. They are for a 1992 300D with automatic climate control, not a manual unit like yours. The main reason I posted them is to illustrate the direct connection from the Klima relay to the compressor clutch, so if the clutch stays engaged with the Klima relay removed, there has to be a short somewhere in the wiring. One thing I noted while posting them is that the Klima wire bundle also includes the +12 vdc power feed wire from the Fuse 7 circuit that powers the A/C (BLK/RED, pin 5). If your car is the same, there could be a short in or near the Klima relay socket that is feeding +12 directly to the compressor clutch wire (BLU/GRN), pin 5), even when the Klima relay is removed. In a typical MB connector, pins 5 and 7 would be right next to each other. Does the clutch stay engaged when Fuse 7 is removed?

- FD

 

Thanks heaps for that.
I did go through and pull all the fuses to see which one affected power to the AC. Only fuse #7 did that.
After replacing the blue/green wire between ac compressor and #7 pin at the Klima relay it all seems to be working.
The short circuit has to have come from somewhere so there is still an exposed 12V wire somewhere which is next on my list of electrical issues to resolve.
I bought the car second hand and am unsure of its previous maintenance history. The operating environment in Australia is genarally hot. This car would have seen some heat. Much effort has been made to keep it warm for european conditions. I am trying to keep it as cool as possible. It runs quite a lot hotter with the AC on. The electric fan in front of the radiator does not run at all while compressor is on but does when car reaches high temp. I will wire it so that the fan runs at slow speed whenever AC compressor is on.

 

Another reprieve!

Replaced the blue and green wire between the compressor and no.7 pin (87) of the Klima relay and presto...the whole thing started working again.
A burnt and shorting wire, I guess. Need to pull the whole harness out and have a good look.
Still can't find a decent wiring diagram for this car and the A.C. system.
Cheers all.

 

Ed -

I'm pretty sure you don't need to rewire anything as the fun should run at low speed when the pressure in the A/C system reaches a high enough value. I posted previously on another forum about the A/C fan control circuit here: View Single Post - Overheating due to aux fan flaky-ness .

On the receiver dryer, there should be two pressure switches - one is the low pressure cut-out (protects the system) and the other is the pressure switch that activates the fan. This is sometimes called a superheat switch per my other post. To test the A/C fan operation, find the one with the BRN/BLU / BRN wires and jumper it. With the car running, the fan should run with the jumper in place and shut off when removed.

If the fan does not run, the problem is likely the fuse that's part of the fan relays. These are located under the cover formed by the back portion of the fuse box. To access them, you will find 4 phillips (cross-point) head screws that need to be removed to get the cover off. Pull the relays and you will see the fuses on them. I lost the A/C fan operation on my car because one of these fuses was blown.

BTW, if your car is indeed designed to run the fan via the pressure switch, running the fan when the compressor is running will actually reduce the efficiency of your A/C system - you will lose the superheat effect.

Again, the big caveat - I'm only presuming that the A/C control circuit design is the same or similar for your car (earlier year, diesel, manual CC, RHD), but I think it's not a bad assumption because all the W124 A/C system feature the Klima relay and it's control points

Good luck

 

Ed -

I'm pretty sure you don't need to rewire anything as the fun should run at low speed when the pressure in the A/C system reaches a high enough value. I posted previously on another forum about the A/C fan control circuit here: View Single Post - Overheating due to aux fan flaky-ness .

On the receiver dryer, there should be two pressure switches - one is the low pressure cut-out (protects the system) and the other is the pressure switch that activates the fan. This is sometimes called a superheat switch per my other post. To test the A/C fan operation, find the one with the BRN/BLU / BRN wires and jumper it. With the car running, the fan should run with the jumper in place and shut off when removed.

If the fan does not run, the problem is likely the fuse that's part of the fan relays. These are located under the cover formed by the back portion of the fuse box. To access them, you will find 4 phillips (cross-point) head screws that need to be removed to get the cover off. Pull the relays and you will see the fuses on them. I lost the A/C fan operation on my car because one of these fuses was blown.

BTW, if your car is indeed designed to run the fan via the pressure switch, running the fan when the compressor is running will actually reduce the efficiency of your A/C system - you will lose the superheat effect.

Again, the big caveat - I'm only presuming that the A/C control circuit design is the same or similar for your car (earlier year, diesel, manual CC, RHD), but I think it's not a bad assumption because all the W124 A/C system feature the Klima relay and it's control points

Good luck