Unbelievable A/C pressure readings - need help - 1984 300SD - Page 2 - PeachParts Mercedes-Benz Forum

ROLLGUY · #16 09-07-2020, 01:28 PM

Be aware that there are vacuum water valves that work either way- vacuum applied=no flow, or vacuum applied=flow. Tailor your system to work accordingly. I like to have the valves closed all the time, so the A/C does not have to play catch-up from the hot coolant getting into the core when the engine is shut down, and then started up minutes later (running errands and going many places).
You could also use your compressor signal to actuate the vacuum valve to close the water valve/s when the compressor is running.
Many months ago I bought a water valve, meaning to do this mod to one of my cars. I now just install a ball valve in the hose above the oil filter canister, and open it during the winter when I need heat.
Even with a good monovalve, heat rises in the hoses and heats up the core, even with the monovalve closed. This taxes the A/C in the hot weather. The other idea I had was to re-route the coolant hoses so the hose coming off the head goes directly to the monovalve inlet, and then the core receives the hot coolant flowing out of the monovalve. This would be quite a lot of work compared to installing a vacuum valve (or valves), and would accomplish the same thing......Rich

engatwork · #17 09-08-2020, 06:37 PM

Make sure the valve you use to shut it off is NC or no vacuum = valve closed.

JHZR2 · #18 09-23-2020, 02:46 PM

Quote:

Originally Posted by Mercedes Man

Charging with R134a.

I decided to change the TXV. Some bubbly black oil came out of the bottom of the TXV. Is this the black death?

I blew the evaporator out. A few drops of oil came out.

I pulled the vacuum and charged the system to 70psi static and started the engine.

The readings were:

At idle:
High side: 15psi
Low side: 15psi

Engine off:
High: 5psi
Low: 65psi

Looks the compressor is not working. I am going to ask for a new compressor.

Opening a can of R134 and waiting for a static pressure of 70 psi means nothing, IMO. The static pressure is indicative of the pressure in the “headspace” in a system that has liquid in it. If all you did was open a valve under vacuum, and let the tank re-flash, you just got the vapor to equilibrate, you didn’t get the full charge necessary.

You need to subsequently open the fill valve, with the engine and compressor running at 15 psi, which will introduce more refrigerant into the system (since it wants to be at roughly 70 psi in the vapor phase.

I don’t know that these results you reported are indicative of a bad compressor. Maybe they are... but you’re undercharged. The 65psi on the low side with engine off shows that. It is curious that high side says 5 psi... but I don’t know that this is related to the compressor versus elsewhere in the system...