Code 66, Low A/C Refrigerant Charge, (W/O Digital Display): Notes

ECM monitors A/C request and completes ground for A/C relay when an A/C mode is selected at control head and refrigerant pressure is sufficient to close pressure cycling switch. If A/C pressure is low and clutch cycles too often, ECM will protect compressor by disabling A/C relay and setting Code 66. Relay will be disabled until next ignition cycle. If Code 66 is set during 3 consecutive ignition cycles, A/C relay will be disabled until Code 66 is cleared from memory. Code 66 does not illuminate SERVICE ENGINE SOON light. Code 66 will set if A/C request signal lasts less than 1.5 seconds for 10 or more consecutive compressor "on" cycles within a 15 minute period.

1. A stored Code 66 may not allow compressor to engage if it has been present during last 3 ignition cycles. Ensure code has been cleared before attempting diagnosis.
2. Pressure varies greatly depending upon temperature.

Fig 1: Code 66, Schematic, Low A/C Refrigerant Charge (W/O Digital Display)