Ac 123

[mdlwolf]

Last year I had the AC repaired in my 83' 300D. The repair included a new compressor, drier, and expansion valve. It performed marginally and after several returns to the shop it still wasn't cooling properly. After the first return I asked the service manager what the problem was and he replied, "Who the hell charged this thing, it was way overcharged. Works fine now." Work fine it did not so I figured for $1000 I can overcharge the thing myself and bought a set of gauges and a vacuum pump and began reading about automotive AC on the internet. Here's what I did to get my AC working. The best I have been able to get is 42 out the vents on full fan speed and 36 at low fan speed.

When I checked the pressures, high idle of 1500 with aux fan jumped on, I was getting a low side pressure around 18-19 that would quickly drop when the rpm increased and compressor seemed to be cutting out at 17 psi. I was getting 46 out the vents. The system seemed undercharged so I added 134 slowly and was able to get the low side to not dip to the cut out point at rpm's between 1200-2000. But I now had a high side pressure of 300 that seemed to be cutting the compressor out when the high side would hit about 310. It was 100 degrees ambient in from of the condenser at this time and I was now getting 44 out the vents but the compressor was cutting out when I revved the engine due to what seemed to be excessive high side pressure. It seemed I either had to settle for too low of a low side or too high of a high side, but I was getting better vent temps with the higher high side. If I could only get the compressor to not cut out with the increased high side pressures. Then I remembered that my Ford truck has an adjustable pressure switch on the accumulator which changed the low side cut out. I wondered if the Merc had one but upon initial inspection it didn't seem so. I cleaned the pressure switch with parts cleaner and in the very center I found a small allen screw. One click counter-clockwise and the compressor was not cutting out. High side pressures were up to 340 at about 2000 rpm. I watched the gauges for awhile and noticed that something else was cycling the compressor. After some research I discovered the evaporator temp switch under the passenger kick panel. In the center of that there is a small flat head screw. By turning it ever so slightly in both directions I could here the compressor cycling. One slight bump clockwise and I was able to get the vent temps to 42 at high idle with full fan speed. Road testing I got cold so turned the fan to low and I got 36 out the vents.

That's the best I think I'll be able to get out of this vehicles AC. I've asked around about the high side pressures and had people tell me that the it's too high. I've had other people tell me that it's fine because once the vehicle is moving the high side will drop as the air cools the condenser. As a test I sprayed water on the condenser and watched the high side drop dramatically.

So what do the AC experts think? Can the high side run this high at idle and be o.k.?