A/C RPM sensor

[Maddog]

I think I have diagnosed my A/C problems down to my RPM sensor in the compressor, but I would like to make sure before I spend $100. My main symptom is, the compressor comes on and off intermittently. I checked the resistance of the sensor and it was supposed to be 500-600 ohms but i got something up in the thousands. I am pretty sure it is bad but I just want to make sure before I spend the money. Thanks for anyones help!

[Mike Richards]

George:

I suppose this sensor could cause short cycling - can't say for sure. I've read here in numerous posts that if it's bad, the compressor will shut-off and will not come back on until the engine has been shut off and restarted.

[Benzwood]

I just had my A/C sensor (incorrectly) diagnosed as bad by a Mercedes dealer. The part cost is not bad, it's the labor that's nasty -- the compressor must be removed. Labor to replace the sensor was about $300.

My problem eventually turned out to be the MAS control unit in my '92 300E, which was especially frustrating since I suggested that as a potential problem (I was having a fuel-pump problem, and the MAS unit in my car is in place of both the fuel-pump relay and the KLIMA relay used for the A/C).

Anyway, the dealer that correctly diagnosed it said that the compressor can be spun manually and a reading taken from the speed sensor while the compressor is moving.

If you decide you need the speed sensor, I've got a used one for sale cheap.

[Maddog]

Could you tell me the proper way to diagnose it? Hold that part for me. How much do you want for it? New they are like $100 so Id love to get one used. Email me: no_end87@hotmail.com
Thankyou!

[Benzwood]

I don't know how to diagnose it... but I believe Mike is correct in the RPM sensor not causing cycling.

My understanding is this (which could easily be wrong):

If the sensor indicates the compressor is not turning (i.e. it's locked up for some reason), then the A/C clutch is released to allow the A/C pulley to spin freely to prevent damage to the belt. The system does not attempt to re-engage the clutch until it is reset by switching the ignition off and on.


Re: the used part, I'm trying to get the first dealer that misdiagnosed it to compensate me for my expenses, and he may want me to send him the old part. If not, you're welcome to it for the cost of postage. I'll let you know.

[Joreto]

Hi Mike,
that's exactly how my A/C behaves. At times I can drive all day with the A/C on and it works fine but at other times it'll just shut down and I have to switch the car off and then on again in order to get it working. Does this mean that the sensor is bad ? Where is the sensor located on the 190e 16v, and how can I test it ?


later

[Mike Richards]

It's my understanding the a/c speed sensor is on the back side of the compressor. I seem to recall a fairly recent post where a photo was attached.

The following may be of benefit to the person who started this thread:

SPEED SENSOR ON A/C COMP.

[jeffsr]

To rule out (or rule in) the speed sensor, engage the system, set to max cool, pull the spade connectors off the binary pressure switch (on the receiver dryer), short the leads together (the wires, not the switch). If the compressor comes on and stays on, it's not your speed sensor. If it comes on and the releases, guess what??

where do I look next. I have a '88 300E that is acting up.
In warm weather the A/C seems fine, and the car can get quite cold. However, when it gets into the high 90's and higher, the vents will blow cold air for about the first 10 minutes and then hot-humid air for 2 minutes then back to cool air. It seems like the compressor is turning it's self on and off. The hotter it is out side, the shorter the time it blows cold. It may cycle back and forth or just give up and blow hot air. This may have nothing to do with it, but sometimes the air bag light comes on and then goes out.

newbie- Mark Potter

[stevebfl]

I have gotten so tired of chasing these intermittants that I have used the relay bypass on a number of cars. It can't be easily done on cars with MAS or BM though, so proper diagnostics are still necessary.

If you are interested in diagnosing it look at the A/C article in DIY. There are scope patterns showing what the sensor signal looks like.

Every bad sensor I have found has really been a compressor problem. All were rebuilts and the problem was that the sensor was too far from the crank shaft which it senses.

[Benzwood]

I got the dealer who misdiagnosed my problem to pay for the unneccessary repairs (yay!) so I don't need my old AC speed sensor any more.

So if someone can use it, send me a PM and I'll send it out with my compliments. Maddog gets first dibs if he still needs it.

[Piotr]

Germanstar.net has the sensors for $60. I have a dis-similar problem. The shop (non mercedes) installing my compressor did not know they have to put the switch from the old to new (rebuilt) compressor. Since I supplied the part, they said I was supposed to tell'em to do it. now they have to discharge the system put the switch on, and I will connect it. By the way, does anybody knows if, in fact, changing the speed sensor will discharge the system? also, it does not appear that we will have to take the compressor out (a 3 hour job).

[Benzwood]

From what I can tell, the speed sensor (on my compressor anyway) does not require discharging the system. It appears to be some sort of electromagnetic sensor that just slips in the axis of the compressor, i.e. no contact with the refrigerant.

That's in theory, anyway.

However, in my car, it is virtually impossible to get at the sensor and the wiring, so to change the sensor requires removing the compressor. Which of course requires discharging the system.

If your car doesn't require removing the compressor, you should be ok.