Poor Cooling At Idle
If you experience good cooling when driving at higher speeds, but poor cooling when stopped or at low speed, have your cooling fans or fan clutch checked. Good airflow across the condenser and radiator are important for a properly working system. Also, make sure the condenser is clean; free of bugs, debris, large plastic bags that drift up there. Blocked airflow will cause poor cooling.
High High-Side Pressure
A bad compressor DOES NOT pump 200, 300, or 450psi. We occasionally hear from someone with an extremely high high-side reading, stating that their compressor is not working. The compressor is a pump; all it does is pump refrigerant through the system. If your high side reading is high, the compressor is working. Check temperatures at the condenser inlet and outlet (see Temperature Testing below) and gauge readings; likely restricted condenser or bad expansion device.
Temperature Testing
Temperature testing is the most effective method of verifying and diagnosing an A/C system. Gauges measure pressure. Pressure is temperature. Gauges only read at 2 points in the system; where the high and low couplers are. With a good temperature probe one can take temperature readings all over the system, and pinpoint problems. Here's how to temperature test:
AC should be set on MAX, blower on High. Doors and hood should be open. (max heat load)
There should be a minimum 30 degree drop from 1 foot in front of the condenser (front of vehicle) to the center duct in the vehicle.
There should be a 20-40 degree drop from the condenser INLET to the OUTLET. Less than 20 degrees or more than 40 degrees indicates poor airflow, improper refrigerant charge, or INTERNAL RESTRICTION. A very large temperature drop almost always indicates a restriction. The refrigerant is building up at the restriction, causing high pressure (and high heat), while little to no refrigerant is coming from the outlet, causing the big drop in temperature.
The INLET and OUTLET of the evaporator should be within 5 degrees of each other. This test is performed easiest on orifice tube systems, but if you can get to the evaporator side of the expansion valve, the test is still accurate. If the evaporator outlet is WARMER than the inlet, you have a low refrigerant charge, and need to add refrigerant. If the outlet temperature is COOLER than the inlet, there is too much refrigerant in the system. If evaporator inlet and outlet temperatures are the same, or within 5 degrees of each other, this indicates a perfectly charged A/C system.