This is a condition that causes the A/C system to operate constantly to maintain maximum capacity. On systems with cycling clutches (CCOT) and orifice tubes, a high load or high heat load would mean that the compressor does not cycle on and off as frequently, if at all.

Remember that the compressor cycle is controlled by a pressure cycling switch that monitors refrigerant pressure on the low side of the system, usually at the evaporator outlet. When that pressure gets too low, the compressor is cycled off.

Under high heat load conditions, the compressor is unable to pull the low pressure low enough (usually in the range of 22 psi) to cycle the compressor off. Therefore, the compressor runs constantly.

The quick test diagnosis for the condition is to remove the electrical connector to the blower motor. That will eliminate all the heat load on the A/C system and the compressor should be able to pull the low side of the A/C system down to 22 psi and cycle the compressor off.

The high heat load condition also occurs during some testing operations. With pressure gauges connected, typically the hood is up. When the hood is up, the hood seal is lost and the A/C system is not only pulling hot outside air in, it’s also pulling hot underhood air in. That means the A/C system is trying to cool air temperatures that commonly exceed 200°F. That high heat load will also cause the compressor not to cycle during some diagnosis and testing procedures.