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here is the R12 specs for your truck. not the same ofc if r134a but heres some more:
The high side pressure will definitely be affected by the ambient or outside air temperature. A system that is operating normally will indicate a high side gauge reading between
150 to170 psi with an 80°F ambient temperature. The same system will register
210 to 230 psi with an ambient temperature of
100°F. No two systems will register exactly the same, which requires that allowance for variations in head pressures must be considered. Following are the most important normal readings likely to be encountered during the season.
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ELATIVE TEMPERATURE OF HIGH & LOW SIDES
The high side of the system should be uniformly hot to the touch throughout. A difference in temperature will indicate a partial blockage of liquid or gas at this point.
The low side of the system should be uniformly cool to the touch with no excessive sweating of the suction line or low side service valve. Excessive sweating or frosting of the low side service valve usually indicates an expansion valve is allowing an excessive amount of refrigerant into the evaporator.