Hi
I bought this car little over a year ago. It has 71k orginal mileage
5-6 Months ago My Problems started. First one was jammed air pump It wasn't big deal replaced and fine now.
But later on started A/C problem
First problem was center outlet was giving cool air side was giving hot. After extensive research I found that Climate Control unit at fault replaced worked fine for a while.
Second it started to give less and less cool air, first added freon it didn't help much. Because of the high/low gauges was not normal first suspect (cheaper) was expansion valve replaced but just before to replaced a/c totally gone
Replaced to expansion valve anyway.
After replacing expansion valve Problem at the time clutch wasn't engaging, again extensive search find out it was Compressor speed sensor (one in the back of compresor). Replaced speed sensor system starts to work but not cold enough and gauges still same (instead of high on high low on low it was closer value) Second and expensive suspect compressor. Replaced the compressor but now there is no clutch activity. First suspect was speed sensor replaced again but still not working.
12 pole compressor control module (Relay) Behind the battery (N6)
Check voltage supply terminal 1(-) and 5(+) voltage close battery voltage

Check Pressure switch connection 5(+) and 10 (-) voltage close battery voltage

Check Electromagnetic coupling jump terminal 5 and 7 Compressor engages nice cool air everything working.

Check Speed Sensor terminal 9 and 11 resistance about 545 ohm But when you run the system while 5 and 7 jumped. I supposed get AC 0.3 volt but instead shows 0.003 volt. So change the speed sensor again and same think

Might be a compressor control module (one behind the battery) tested with different one same thing no clutch activity.

So I am running out of ideas. Climate control unit, expansion valve, drier, compressor, speed sensor (2 times) and compressor control relay has been replaced. 7-8 times system charged and recharged due to repairs.
Before I torch down this piece of junk anyone please give some ideas to what to do or look or test
Thanks

If you could post your actual high and low side readings and the ambient temp when you're taking the readings there are some smart ppl here that could help you.

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Daily Driver: 02 E430 4MATIC
In the family: '03 E500 // '04 ML500 // 64 220SE

The only time I have ever had a speed sensor problem the reason was that it was a rebuilt (POS) compressor and the crankshaft was too far away from the sensor to read properly.

If you have good air when jumped then you definitely need to finish your compressor control diagnosis. What are the voltages again on the pressure switch?

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Can you use standart multimeter to read speed sensor or Do you supposed to use ossiloscop. And yes it has rebuild compressor thats what I thought.
I do not have acces to low and high value at the time but problem looks like electrical.
I have alldata information to check diagnostics if you have any other informations please send it to me.
Also other think I realized last night I do not know if it's normal or not but
Multimeter negative in battery negative when you measure pressure switch you read only 0.2 volt range. But if you disconnect to cable and read it it's battery voltage.
Let me clarify ehat I said little bit Pressure switch has 2 cable if you disconnect the cables 1 has battery voltage other nothing as it should. Then you connect the cable with battery voltage to pressure switch and you can read 13 volt from other side of the pressure switch which means I guess switch working. Bu when you connect to cable which is going to N6 compresor controle module to pressure switch. and try to read voltage drops to 0.2-3 voltage.