'87 300E A/C problem - PeachParts Mercedes-Benz Forum

guitarman · #1 08-03-2004, 11:57 AM

Hi all! My A/C on my '87 300E quit working. Evacuated system to -25 inch @ ambient temp of about 93F with 70% humidity. Held for 30 minutes. I charged it to 35psi with R12. Clutch engaged. Pressure readings: low side 35psi, high side 35psi. It's obviously not pumping. Is it "Black Death" or could something be clogged enough to cause this? There is a noticable leak behind the expansion valve (lines are wet with oil). I am sure the evaporator needs to be changed. I took the dryer off and blew compressed air through it with a white cloth to catch anything that may come out. Just a little oil, nothing that looked like teflon flakes (unless they look like oil or powdered graphite). Where should I look next? Thanks in advance.

stevebfl · #2 08-03-2004, 12:54 PM

R-12 has a pressure temp relationship that will have it at over 100psi on a summer day. If your system has only 35psi on the high side, you have almost no refrigerant in the system.

As to your procedure for evacuation. Unless you are on the top of Everest on a 100deg day, you are not doing the job with only 25in of vacuum. The concept you are trying to achieve is that you are wishing to boil the water at ambient temp. This gets easier at altitude but at sea level and a 90 degree day thevacuum that must be pulled to make the boiling point drop below 90 degrees is closer to zero psi absolute than standard gauges will tell you. one needs a micron gauge to view the actual level necessary.

The concept is to lower the pressure to the point moisure is boiled at ambient temp and then time is needed to finish the boiling.

guitarman · #3 08-03-2004, 01:38 PM

Thanks for the reply. I realize that at 90F I would need to pull about 28.5 inches of vacuum (about 35,000 microns) to boil. With the leak in the system, it's not possible. My main question is when the clutch engages, shouldn't the high side go up and the low side go down? I have no difference in the two sides with the compressor running. Even with an undercharge, shouldn't it move refrigerant?

stevebfl · #4 08-03-2004, 02:05 PM

Yes, I'm guessing you aren't getting the schrader valve open to your gauge. Put more refrigerant in and try again. Be sure the center of the clutch hub is spinning. The hub is rubber and they do tear allowing the otherside to spin with the actual compressor not.