Automotive Air Conditioning Information Forum

I have a 1991 Ford F-150 lariat, with the 4.9l inline six and automatic transmission. I bought the truck not running and have been getting it back in good order. The HVAC system is giving me fits and I need advice. I had originally replaced the compressor with a remanufactured one, new dryer and red ford orifice tube, the entire system was solvent flushed, vac'ed down and charged with r-134a. It never worked well and I tried adjusting the charge from lower pressures (35 low, 200 high) up to (45 low, 250 high). I was never able to get good cold air at the vents. I have made sure that the vacuum system and all blend doors are working properly. The system was filled with 10 ounces of ester oil. I began to get curious about other refrigerants so I pulled the system down and flushed it again, I found a little debris in the orifice tube so I replaced the compressor, dryer and orifice tube with new. I flushed the system, vac'ed it down and then charged with 10 ounces of PAG 46 and industrial enviro safe which is a hydrocarbon refrigerant. It blows coolish at idle and the pressures are about what the company recommends at 35 low and 175-200 high. I feel like the charge may be wrong but I'm hoping someone has dealt with this before and may have some advice. The truck has dual electric pusher fans in addition to the engine driven fan. I also note that the compressor cycles fairly frequently at interstate speed, which I would think indicates a low charge, but adding more refrigerant would push the pressures up too high. I'm about at my wits end and am considering paying to have it charged with r12.

How cold is the evaporator outlet line getting? You need an accurate reading with a thermocouple, not an IR gun. It could be that the refrigeration is working OK but the heater core is reheating the air. Bypass the heater core for a test is another option.

The ambient temperature needs to be mentioned to know if the high side is too high. I don't know about HC but it is not that abnormal for a converted system on 134a to be over 300 on hot days.

The compressor is going to cycle, it's a CCOT. If you have proper charge by weight and the compressor cycling off on the highway, the air should be cold. That leads back to the reheating theory.

I was concerned about the heater core issue, running on vent the air doesn't get heated, though bypassing it could make certain. Charging was done at 95* with a box fan blowing on the front of the truck.

The issue must be resolved till now.