Road Test GM/Workhorse Chassis with Chassis Supplied Low Pressure Switch Set Vacuum Mode Switch to "Max A/C", and Blower Switch at High Speed. Rotate temperature control dial to the coldest setting. Drive vehicle according to your normal road test procedure. Cool air should be discharged from the dash louvers. Depending on conditions, the discharge air should typically be 20-30 degrees cooler than recirc. (cabin) air entering HVAC unit. If outside ambient temperature is 80 degrees (F) or lower, some clutch cycling can be expected, especially at lower humidity levels. At these conditions, intermittent and sometimes rapid cycling may be caused by the GM Low Pressure Switch. If A/C system does not pass step #2, and/or rapid clutch cycling is observed at ambient temperatures greater than 80 degrees (F): Verify that the suction 3-way service valve (at evaporator outlet) is fully open. The stem should be turned all the way out (counter-clockwise). Also verify that the suction hose is not tightly bent or kinked. Verify that heater water valve is fully closed, and heater coil tubes are cool or neutral to the touch (no hot water flow to heater coil). Proceed according to the Field Test Procedure titled "A/C System Operational Check--Evans Midway and High Performance Systems." This test procedure provides a quick and easy method to determine if an operating air conditioning system is adequately charged. If A/C system passes the Field Test, no further service is necessary. At some conditions, GM Low Pressure Switch Cycling is expected. LOW PRESSURE SWITCH CYCLING DOES NOT NECESSARILY INDICATE A LOW REFRIGERANT CHARGE!! Do not recharge or "top-off" a system because the low pressure switch is cycling. Recharging is only performed after a system fails the Field Test, AND all other potential causes are eliminated (water valve cable adjustment, water valve installation, etc.). Go Back to Technical Support