The Chevrolet truck uses two air conditioning pressure switches: a high- and low-pressure switch. The high-pressure switch is designed to shut down the compressor if the line pressure gets dangerously high. This will usually happen when a condenser fan fails. The low-pressure switch is designed to shut the compressor down if the pressure becomes to low or the evaporator freezes. The oil for the compressor is suspended in the Freon, and if a leak in the system occurs, the oil, being heavier than the Freon, will be the first thing to leak out. Low pressure will destroy a compressor through a lack of oil, and must therefore be repaired as soon as possible.
Locate the high- and the low-side switches. The high-side switch is located on the smaller of the two air conditioning lines close to the radiator on the passenger side of the vehicle. The low-side switch will be located on the largest diameter air conditioning line or on the side of the accumulator (round container) on the passenger side firewall.