A/C saga - '78 300D (still NOT very cold) - PeachParts Mercedes-Benz Forum

rgnprof · #1 04-28-2006, 08:34 AM

I am going to try and summarize what I have done up to this point. When I got the car - no A/C, no gas at all. I have replaced the suction hose (compressor to evaporator) and the hose from the compressor to condensor, new drier, new belt (and tightened), new expansion valve, new pressure switch and new temp switch (both located on the drier) and new aux fan. This has been done in stages (some of you will recall my previous posts). Yesterday, I had my mechanic evac and vacuum the system as we decided to replace the expansion valve as the old one was leaking. Recharged with R12 - the best he could get the low pressure was about 20 and the high pressure was up to 225 - with car running at approx 2000 rpm and ambient temperature in the upper 70's, maybe 80 degrees.

This car has the York compressor and the ACC servo.

Vent temps are still around 60 - dropping to mid 50's (which would be less than a 30 degree difference from ambient to AC air). The auxiliary fan is now working properly since I have installed a new 52 degree C temp switch in the drier (fan runs when I ground the wire leading to the thermostat housing sensor and when I ground the temp switch wires together at the drier). Yesterday, the fan did not come on at all, even as the car sat at idle. At idle, the compressor cycled on for approx 45 seconds, then off for 12-14 seconds - with ambient temps in the upper 70's.

The car cools better at idle than it did, but I am still disappointed in the performance. Any other TS suggestions? I have noticed than when I turn the system to Auto HI, some air is blowing out the defrost vents, so I'm wondering if I could have a vacuum related problem and that I'm still mixing too much warm outside air with cooled air??

Or could I just have a tired compressor - I have noticed that the clutch doesn't seem to rotate as fast as it should at higher rpms??

I'm just at a loss...

Ryan

Diesel Giant · #2 04-28-2006, 10:25 AM

If you have such a low psi on the low side there is a blockage in the system. The expansion valve and the compressor are the dividing points for the low/high side. The compressor may be tired and have an internal problem.

dannym · #3 04-28-2006, 12:27 PM

I don't see how you could have a "tired" compressor if you pumping out 225lbs of pressure.

You never mentioned that you flushed the evap and condenser. Was that done?

Danny

rgnprof · #4 04-28-2006, 12:44 PM

No, and I am thinking now that's part of the problem! If I can have my mechanic recover the gas, can I flush it myself? What's the easiest, most effective way to flush the system?

ryan

vstech · #5 04-28-2006, 12:53 PM

the 225psi head is the result of packing in more 12 than required. pressures are the result of two forces, compression and evaporation/condensing. flow restrictions from the pressure line can cause low pressures either from a strainer plugged, or expansion valve not adjusted properly. more likely is a VERY dirty evaporator coil.
enjoy cleaning that sucker.
John

rgnprof · #6 04-28-2006, 12:55 PM

Any way to clean the evap coil WITHOUT removing from the car?

ryan

dannym · #7 04-28-2006, 01:10 PM

Quote:

Originally Posted by vstech

the 225psi head is the result of packing in more 12 than required. pressures are the result of two forces, compression and evaporation/condensing. flow restrictions from the pressure line can cause low pressures either from a strainer plugged, or expansion valve not adjusted properly. more likely is a VERY dirty evaporator coil.
enjoy cleaning that sucker.
John

You could also have a blockage in the condenser or a dirty condenser. Or too much oil in the system.
Too much oil is very likely since you didn't flush. Did your mechanic add oil? BTW get a new mechanic.

You can flush the condenser by disconnecting at the compressor. I flushed the evaporator by removing the expansion valve and putting clear hose on the lines to and from it.
You should also remove the compressor and drain as much oil out as you can and at least blow pressurized air through your new hoses.
I just used mineral spirits to flush.

that way you will have a fresh clean system to start with. Now you can add the proper amount of oil and refrigerant.

Also if you open it up again to flush you will need a new receiver/dryer.

Danny