1989 Mercedes 300E A/C Speed Sensor

Does anyone know the procedure for replacing an A/C speed sensor on a 1989 300E? I know it's located on the back of the comrpessor and that's it. I have a new manual coming in the mail but it's not here yet.
I conducted a check they way "Wan-tan ing" (spelling ?) decribed as follows, and determined the cut-off was my problem. "The easiest way to check if it is the speed cut-off, turn off the engine and then restart. The speed comparison logic is reset by engine off. If the A/C runs again after restarting engine, it is definitely the speed cut-off."
Thanks,
Gene
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However, the "speed cut-off" does not always mean the speed sensor is the culprit. It may be loose wire, incorrect belt tension, or a slippery clutch.
Based on your past postings, it seems you have already replaced the belt and tensioner. How about the the compressor clutch itself? Is the gap within spec? Is the clutch surface clean without any trace of oil/fluid? I think the power steering pump is on top of it, right? Is it leaking?
I would leave the compressor speed sensor as the last suspect. First, if the sensor is completely bad, the A/C won't even run a second, but yours may run 20 minutes. I doubt the sensor will gradually fail over time since all speed sensors I know (rpm, wheel speed in ABS) are simple magnet/teeth and there is nothing to break except for dirt. Second, I don't recall the sensor part is individually available from dealer. They only sell the entire compressor. Third, IIRC, replacing the sensor requires taking out the compressor and some disassembling. It may not worth that much effort to revive an old compressor.
Another thought. Have you checked the low/high side pressures when A/C runs? If the expansive valve is partially stuck, the pressure build-up may shut down the system, although you should be able to get A/C back after a while, without the need to stop the engine.
By the way, if you do have to take out the compressor for any repair, convert to R-134a now so you can easily work on A/C yourself. Remember to drain the mineral oil from the compressor and replace with either ester or PAG. If the dryer is not recent, replace it too. Replace all O-rings that you encounter but leave others if you don't have to touch. Get a vacuum pump (check tool rental places) to suck the system for 30 minutes then fill the R-134a up to 85% of the R-12 capacity.
Gene via CarKB.com wrote:

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Thanks for the detailed response Wan-ning Tan,
I have replaced the belt, tensioner, and adjustment. I also replaced the water pump while I was at it. All new systems seem to be working fine now.
I have a Klima relay and AC speed sensor coming in the mail tomorrow. The speed sensor was $65 and has tow wire prongs on the end.
My plan is to swap out klima relays first to see if that changes anything. The place where I bought the parts (parts wharehouse) said it was okay to return electronic components, which I thought was great. The pressure was tested on both sides while running. You can see the refrigerant running through the little "eye" then the compressor just turns off after about 20 minutes. By the way, the auxillary fan does not run while the AC is on. It does, however, turn on when the engine temp reaches 105 C and cools the engine down considerably within a matter of minutes.
I cleaned everything on the compressor and clutch. I also cleaned up the power steering pump and checked for leaks. I tigthented the bolts on the PS pump and have not noticed any leakage since putting it back together. There isn't any noise coming from the comrpessor, PS, or anything while the AC is running.
If the klima relay doesn't work, maybe I'll just take the copmpressor down (4 bolts right?) and go through it like you described. Do I have to remove anything to get to the compressor? I'm willing to buy a rebuilt compressor at a reasonable price. Does it come with a new cut off switch/sensor?
Thanks,
Gene
Wan-ning Tan wrote:

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You are on the right track and pretty much have done everything that is possible.
The discussion below are from my experience working on W201 and W126. I have not yet worked on W124 but I do have such car and I remember seeing similar configuration.
Since the aux fan is not running, the refrigerant may not cool down fast enough. This definitely affects the performance but I am not sure if it will cause too much pressure. Remember that there are two pressure switches in the system. One is to turn on the aux fan circuit. The other measures the pressure so it won't be too low (after freon leaking out) or too high (risk of burst, I guess). I don't have the manual at hand but I think the high pressure cut-out is around 30 bars. So what was the high side pressure shortly before the cut out?
The aux fan is controlled by two separate parallel circuits: A/C and engine temp. Apparently the fan itself and the engine temp circuit work. The A/C fan circuit works like this: pressure switch (near dryer) closes when the system pressure reaches the threshold (again, I don't recall the value. If you do need it I can look at the manual tomorrow). This usually happens within 10-20 seconds after A/C is turned on. This closed switch turns on the relay which feeds current via a resistor to the aux fan. The fan runs on low speed due to the resistor (which is white ceramic mounted near dryer). This resistor usually fails more frequently than the rest. If it does fail and you don't (can't/won't) find the right part, just jump the two wires. The fan will then run on high speed, noisier but no hurt.
The compressor is mounted with 4 long bolts (13mm hex head) from the side. Of course you need to loosen the belt and the under-carriage shield. Nothing else SHOULD be removed but it does help if the fan shroud can be turned away. The space there is tight so more available space makes the work easier. I drop the compressor from the bottom. Note that there are one or two notches on the mounting surface so the compressor can stay there even without bolts (making mounting easier). Be careful not to lay under it since it is heavy and you don't want it to drop on you.
IIRC, the official MB manual does mention how to replace the speed sensor though I don't remember the detail. Send me an email if you do need that info.
The rebuilt compressor does come with the speed sensor, though a few may not come with the clutch/pulley. I would recommend finding one with the clutch/pulley.
Good luck
Tan
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I think the resistor for the fan circuit is the problem. I checked it out today and noticed it had a few deep cracks in it. The fan should come on when the AC is on, like you and the AC shop said. Since it's not, I'll run that route.
I was getting ready to jump the resistor when I noticed that the water pump I installed 10 days ago was leaking coolant down the side of the block. I bought the pump new (Geba) and installed new o rings and ensured contact surfaces were smooth. I can't see exactly where it's leaking because it's so tight in that area. Will I have to take everything off just to get a look at it? Man, this is not making my day!
I'll be looking forward to any replies you may have before I dig into this first thing tomorrow.
Thanks,
Gene
Wan-ning Tan wrote:

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