I have a 98 E300 I have had for a couple of years and every since I have had it, it will cool great and blow ice cubes but at random will just stop cooling. AC compressor clutch is still engaged but it just stops producing cold air. It seems to do it most often when the set temp is reached or close to being reached and while driving at a steady speed. If you push the EC button to turn compressor off, then back on it will start blowing cold again. If you vary the engine RPM quite a bit such as during a downshift, it will start blowing cold again as well. I have also noticed when its hot outside such as around 90 or above, when accelerating hard, you can feel the car sort of jerk or surge, and can often hear a chirp which I figured out was actually the compressor clutch slipping, not the belt.



In the last couple of days I have started driving it with the sensor data showing. For the pressure reading, static pressure seems to be about 4 bar. When it is first cooling, it will get up to about 15 bar, as is starts cooling the car off the pressure seems to drop to about 7-8 bar but still cools (it was about 75 today). At seemingly random times the pressure will drop to 4-5 bars and blows ambient air. If you either downshift it, or press ec on and off pressure will jump back up and it starts cooling again.


Last year I replaced the expansion valve thinking maybe it was getting stuck, it made no difference. I discovered the aux fan was dead, so I replaced it and it now runs and I thought it fixed it, but it did not.


I am thinking that the valves in the compressor are sticking open or something and it stops building pressure until the compressor is either shut off then back on, or rpm changes and it knocks them loose. Does this make sense?


I also checked the error codes are there are none. Everything with the ACC seems to check out. I even tried swapping the control unit in the dash, still the same issue.

The system varies system pressure for efficiency.

The problem could be as simple as a flaky evaporator temp sensor.

Sixto
98 E320s sedan and wagon
02 C320 wagon

What you are facing is a sticking regulator valve in the compressor, its the piece that you see going in sideways into the compressor on its tailpiece.

The usual reason for stalling is clogged up system or wrong servicing procedure. e.g. lots of ac shops when they replace this compressor dont fill the crankcase of the compressor with oil just like its required on chevy harrison V5 compressors.

The GM cars with those face the same problem too - specially on low refrigerant charge also - the valve sticks on one end too long and takes a while to get back to work.

All the above is only correct if your HVAC manifold set proves that the system is throttling down - if the system is stable and you get warm air, the heater control valves are being activated - this is a climate control problem, the sensor for cabin temperature is right next to the dome lamp - its the little grille you see there, get a duster can to blow it out - it may have accumulated lint or dirt there.

__________________
2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

This sticking regulator valve sounds like it could be the problem to me. While watching the heater core and evap temps on the display the all seem to read about the same temp, and today its about 78 or so out and the pressure would drop to about 6 bar while cruising and the evap/heater core temps would show about 67-68, so it appears it is doing a little bit of cooling, but just not much. As soon as I would downshift it to pass or something like that, pressure would rise up to 12-14 bar and evap temp drop to 37 or so.

Is the fix for this a replacement compressor or is the valve a separate part that can be replaced?

Best would be to first put a manifold set on the lines and see the system running pressures - then the temperatures would make sense.

do you have equal temperatures on all 4 vents on the dashboard? - 2 warm and 2 cold means you are seriously low on refrigerant charge in this car (the evaporator shroud is split right in the middle for left and right, a partial working evap works half way only)

the fix for the valve is to clean the system out and start fresh - but in experience if your original compressor is a denso 7SB16C with the big throttling valve, the compressor is a trash design and needs to be thrown into scrap. The corrected design is numbered as 7SBU16C - the bad one has unsupported reed valves on one quadrant of the plate which cause them to fracture. The updated one doesnt have this issue.

All the replacement compressors you will buy (new) are the updated design, in rebuilt you may get an old style - I have used the UAC and 4 seasons ones on these and on BMWs (similar) and all are working perfectly.

__________________
2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

The cooling then not cooling air is consistent across all the vents. I have put a gauge set on it in the past and when it is doing the not cooling things, the pressures drop to a static or close to static pressure across the high and low sides even though the compressor is turning.

This statement proves that the variable valve is throttling down and getting stuck there.

__________________
2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

I am planning on swapping it out. I have several compressors laying around I am thinking I will just pull one out and swap it.

I pulled a valve out of one of the compressors I had on the shelf, recovered the refrigerant, swapped the valve out real quick, pulled a vacuum for 30 minutes, charged it back up and it would not build pressure at all with this valve. So, I recovered it again, swapped it out with another one I had etc, initially it did not seem to be working either, but then it "kicked in" and pressures started looking like they should and it started producing ice cubes. I let it run for about 20-30 minutes and it never seemed to cut out. The true test will be next week when I drive the car again. If it still has the pressure drop and lack of cooling I will order a new one. I would have normally just ordered a new one but I just wanted to give it a shot and see if I could cure it with what I had on hand.