I just picked up a nice '95 Roadmaster wagon for a parts chaser and future tow vehicle. The problem is when bringing it home after driving it 50 miles the AC stopped working. When I first started driving I was turning it lower it was so cold. Then as it started warming I tried to turn the fan on higher to no avail. With the fan on high speed manual you could hear it blowing like the dickens inside the dash but no air was coming out any of the vents. When we popped the hood the dryer was covered in ice. This leads me to believe the evaporator was frozen solid also and that was what was blocking the air in the dash. After it was sitting an hour after I got home I tried it out and the ac was blowing cold so that fits. It was 90 out yesterday and humid so that may have helped it along. The compressor was just changed for the place that I bought the car from. My friend(who works with home ac) thinks it is actually undercharged. I have yet to take some measurements. I was wondering if this system is the type that uses the pressure switch to modulate the compressor and could it be stuck on so it never cycles. I was thinking that may have been the demise of the original compressor. The car has just under 100K miles. I have the manifold gauges from working on my '76 Century(converted to 134a) and the service manual for it but I have yet to get one for the '95. Any ideas on what kind of pressures I should be looking for? Does this type of system use the pressure switch to cycle the compressor often? I never figured that I would need a manual this soon.
While evap icing due to a slight undercharge is possible under some conditions, the problem is more likely a cycling switch or compressor relay that is stuck closed. If the low side pressure is going below 28-29 degrees F, then it's probably one of those components. Pull the connector on the cycling switch. If the compressor turns off, it's the switch. If not, look toward the relay.
I actually did pull the connector off the pressure switch by the dryer and the clutch disengaged. So the clutch should disengage at on a fairly regular cycle after the car has been running for awhile? Do those switches usually have a schr. valve behind it so I can change it without any fuss, right?
The main purpose of the switch is to keep the evaporator pressure from dropping below 28-29psi (cycles the compressor off). This prevents the evaporator from freezing. Yes, there is a schrader valve on the switch fitting. Unscrew the old one and screw on the new one. Simple as that.
Well I tried the switch and it didn't help. As usual I should done some more diagnostics first. I installed the gauges and the at idle(there's an aux fan that runs when the compressor is on plus the mechanical fan) the pressure started at 47 psi/345 psi. It was around 90 degrees/med humidty and the gauge kept going up slowly to 60/370. That pretty much keeps the switch from working since it is a low pressure switch. From gleaming a '95 Riviera and a '95 Century(I have yet to get the Roadmaster) service manual that was indicating either a blockage on the HP side or an overcharge. With the compressor being changed either one is a possibility. I'm wondering how bad it looks in the dryer. Well I couldn't do anything about blockage quickly so I decided to(EPA avert eyes) let out some 134. I opened the low side valve and let it on for about 15-20 sec with the car running and watched the pressure. The pressure dropped to about 44/255 and was holding steady at idle. Temp at the outlets seemed good. The next day I got a digital temp gauge and went for a ride. 85-88 outside and the center vent showed low 40's coming out. When I pulled in I actually heard the compressor and electric helper fan had turned off. I checked pressure and it was down to 32/250 but the compressor had kicked back on. Something the service manuals said to check was the temp at the orifice tube by feeling it. Before the pressure release the area before and after the orifice tube was very cold and now there is a noticable difference between the sides. No ice now on the dryer. Well my problem is mostly solved but maybe later this fall I'll have the system evacuated and I'll do some checking of the dryer and orifice tube for debris.
I agree with the temp is good. I'm just not sure about what the guys serviced/cleaned up when they replaced the compressor. Mainly for peace of mind.
