hello, i have 1988 260e with 115,000 miles and have had a/c problems since last summer. it's hot this time of year in charlotte and i am trying to get it fixed. a local mb mechanic put dye in system to check for leaks and identified a leak at the expansion valve. i ordered a new one online and he installed and charged the system with refrigerant, which he felt would solve my problem. now he says that an internal switch inside the compressor is cutting off which only allow the compressor to run for 10-15 seconds before shutting off. he says that a switch or relay used to be available to override the internal switch, but the supplier he used to be able to get it from no longer carries it. should i purchase a rebuilt compressor with or without clutch or is there such an override switch or relay available? i want to get the system working and cooling at it's maximum capability, but want to get it fixed in one shot, not by trial and error. prior to replacing the expansion valve, the freshly charged system would work fine and cool for a month or so and then would only blow warm or hot air. please help!

thanks, hoss260e

I'll throw in my 2 cents. Other opinions will follow.

There's a sensor on the back of the compressor that can detect a loose serpentine belt. In that situation, the compressor is turned off, but will reactivate if you turn the car off and back on again. If that's happening, consider that sensor.

I'm not sure about an "internal" sensor that shuts off the compressor after 10-15 secs. There is a pressure switch inline with the receiver-dryer that will cycle off the compressor if an overcharge or excessively high pressure condition is detected. On some models there's both a low/high pressure sensor and the compressor will also cyle off if the charge is too low. Maybe that's what me meant?

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

thanks mike for the info, i'll check that out.

hoss260e

There is a speed sensor at the back of the compressor. But this is a renewable part according to MB manuals. Check with MB dealer. How does the mechanic know this is bad ? The resistance across the sensor should be about 550 to 650 ohms. And when the car is running, there should atleast be 0.3V ac voltage across it.

I have posted a systematic way to check the compressor cut-off, check this:

http://www.peachparts.com/shopforum/showthread.php?t=158591&page=5

I am not sure with a bad sensor, how does the compressor remain on after about 6 seconds. And as Mike said, this test is only performed once after the car is started, so if you stop the car once the compressor has cut off and start it again quickly, the compressor should come on again for atleast 6 seconds.

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Saumil S. Patel

many thanks saumil. i printed out all the info you directed me to and will take it to another mechanic and let him look at it. if i do have to replace the compressor, should i put new clutch, speed sensor and even dryer to take care of things for the long haul? i was told that the evaporator was not leaking, but should it be cleaned? i was told that these evaporators are prone to leak and they were surprised that mine did not. i only have 115,000 miles on this car.

thanks, hoss260e

You should buy a compressor with a new clutch and speed sensor already installed.

And definitely replace the receiver/dryer.

And thoroughly evacuate the system!

If you have good airflow out the vents, no need to clean the evaporator.

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2012 E350
2006 Callaway SC560

Yes as mentioned above, if the sensor cant be replaced or is unavailable (which is hard to believe because these compressors can be rebuilt), then you are left with little choice. Whenever you open the system for extended period of time, you have to replace the drier. Whenever you replace the compressor, for the sake of the new compressor, you should replace the drier. If you have to replace the compressor and the drier, you may as well replace the sensors on the drier if they have not been replaced ever. If the system is flushed, then the system can be filled accurately with oil otherwise there is some guessing involved. Otherwise, your failure does not sound like an internal compressor failure, so flushing may not be necessary.

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Saumil S. Patel

Also, most compressor warranties require that the expansion valve and the dryer be replaced.

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2012 E350
2006 Callaway SC560

thanks for the info, for some strange reason my a/c worked yesterday (good and cold) and did not cut off. i am wondering if a temperature sensor could have been damaged a while back when the car overheated due to the top radiator hose that came loose (clamp not tightened down). this happened after installing a new water pump and the engine got really hot.i'm not a mechanic, but this was just a thought. i will monitor my a/c today and am scheduled to let a mechanic look at things for me on 7/12.

Test the cut-off systematically using the post I mentioned before coming to any conclusion. There seems to be some intermittency, in such cases you almost wish that it stops again so you can find out, but I know you would prefer it runs during the summer, even if it is doing it magically. There are a lot of sensors that can cut-off the compressor. A coolant temperature sensor on the engine (second last from the windshield side), refrigerant pressure sensor on the drier, speed sensor from the compressor (this is what you are suspecting, but can be easily tested), engine speed sensor, temperature sensors (ambient, evaporator) inside the car (mainly controlled via the push button unit) etc etc.

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Saumil S. Patel

many thanks, i will let you know of my findings after testing.

thanks, hoss260e