I've got a 2005 M6800 cab with the AC unit in the roof. I've done a few AC systems but I'm at still DIY level and I've gotten over my head...again. Cooling was poor so I checked the pressures against the service manual specs. The system was cycling but the pressures were low. I opened the roof unit to make sure everything looked okay and the evap was clean. Next I vacuumed it down, cold filled 1.1lb on the high side per the manual then finished with vapor fill on the low side to the manuals specified 2.2lb. The compressor cycled normally and the pressures were IAW the manual. Yea. I buttoned up the top unit and double checked since the gauges were still on. The pressure were high on both sides with the high side climbing over 350 before I could shut it down. To make a long story short, I've repeated this whole process several times and it works fine every time until I put the lid on the unit. All that's up there is the thermostst, a pressure switch and the expansion valve. I've played with wiring. pushed on the valve, etc and I can't duplicate the problem without putting the lid on. It happens right away and I can open the lid quickly to verify there's no icing. I'm stuck fellas - what am I missing???
M6800 Air Conditioning Mystery
- Thread starter camobob
- Start date
Not fully understanding what is going on but have you looked at the condenser yet? Poorly operating one will create high preasures.
Maybe I wasn't clear - I don't understand it either. The AC unit on the roof is two pieces. It's basically a tub with a lid. In the tub is the fan and the evap. With the lid off the entire system operates normally as far as I can tell. When I put the lid on the compressor engages and the pressures go up and up and up. It's really weird. The low side begins to pump down but at 35-40psi it starts climbing along with the high side. With the lid off the low side pumps down to about 25 and the high side gets to 190'ish before the compressor cycles off.
kubotafreak
Well-known member
Equipment
GRAND l6060, L3560, B6100, gr2100, tg 1860, g1800, g1900, g2160