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Old 08-18-2001, 10:44 AM
stevebfl stevebfl is offline
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Well, I tend to disagree with everyone but Donnie, although I think David is right about the reed valves causings low high sides along with high low sides. I do not believe that a low charge could cause these conditions in any senario.

I believe overfilling, over oiling, residual flush could cause this or a STUCK OPEN expansion valve. The pressures are not OK. They totally reflect the temperature problem that is the symptom.

Low side pressures on a R12 system should drop below 30psi and the system should cycle when achieving its goal. The temp of the evap will be almost a direct conversion from the pressure into deg F. In other words something like this (I do not have the chart handy) 30psi = 35deg F, 40psi = 45deg F, 50psi = 55 deg F.

I would replace the exp valve and drier and evaluate the amount of oil that is released on evacuation. I would also see how much oil was in the drier and/or cut it open for a view of the inside of the system.

Absolute temperatures or pressures are hard to specify as this is a dynamic system very much dependant on the ambient temperature. If the day was 100 degrees I would accept a duct temp of 60 degrees and the resulting pressures at 50mph and hiblow. At idle they would go to 70 deg and probably 60psi.

Actually I would accept those numbers for a 134 conversion but it would be very borderline for a R12 system. A real good system would knock 50 degrees off ambient at 50mph hiblow. Most new 134 systems easily do this.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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