W210 A/C not cooling (1996 E300)
 

Air conditioning not working this year--no cooling whatsoever. Worked okay last summer, and heat worked fine over the winter. I was hoping it was just low on refrigerant, but it won't accept any more through the low-side port. Compressor is original as far as I know.

CCU readings today (temps in deg C), with ambient temp 82F/27C, humidity 71%, CCU set to Auto/72F:
1 In-car temp – 33 (91F; this may be accurate, since I was driving the car in the sun for a few minutes)
2 Outside temp – 20 (obviously wrong)
3 Heater core L – 31
4 Heater core R – 31
5 Evapor temp – 30
6 Coolant temp – 83
7 Refrig press – 05
8 Refrig temp – 29

Error codes: CCU displays three error codes, which reappear after clearing and engine restart:
- B1459 (series interface (K2) connection to instrument cluster)
- B1422 (series interface (K1) connection to instrument cluster)
- B1226 (in-car temp sensor)

Other observations:
- Compressor clutch engages and disengages quietly (too quietly?) when EC button pressed
- No physical signs of compressor failure (leaks, burns, bad electrical connections, hissing)
- EC LED does not illuminate except when button pressed
- Rule out duovalve (no change in symptoms with hose clamped)
- Rule out serpentine belt, tensioner, fan clutch (all replaced last year)
- Rule out aux fan, blower motor controller (both recently replaced)

Sensors:
- CCU reading #2 is almost always 20C regardless of actual temp, but occasionally it flashes a value consistent with the LED temp display in the instrument cluster, which is accurate. This suggests the ambient temp sensor (in front bumper) works, but there's a signal problem between it and the CCU.
- CCU reading #6 is almost always 105C, but occasionally it flashes a reading that matches the coolant temp gauge in the instrument cluster.
- CCU #21 (engine speed sensor) always reads 25 (2500 rpm).
- CCU #22 (vehicle speed sensor) always reads 50 (km/h).
- In-car temp sensor appears okay physically (fan spins, wiring undamaged, receiving power), but perhaps it's generating a faulty signal

I'm inclined to rule out the evaporator temperature sensor since it generally gives a reading consistent with the cores.

Refrigerant pressure seems low and does not change by more than 1 bar (if at all) when EC button is pressed. I'm hoping that doesn't mean the compressor is shot. Maybe there's a blockage somewhere (expansion valve?)

What are my next diagnostic steps?

Thanks.

Paul
1996 E300D (W210), 185K mi

I don't know much about A/C, but I'd always start with an evacuation followed by the correct fill. These cars are a bit touchy in terms of having the correct charge, I know our 1998 certainly is.