What does it mean?
OBD II fault code P0645 is a generic code that is defined as, “A/C Clutch Relay Control Circuit”, and is set when the PCM (Powertrain Control Module) detects an undefined failure, defect, or malfunction in the low-amperage circuit that controls the relay that activates the A/C compressor’s clutch mechanism. On older A/C systems in which the compressor clutch is not controlled by the PCM with pulse width modulated signals, the compressors’ clutch control system is fairly simple.
In its simplest form, the clutch incorporates an electromagnet that is activated when a relay closes to supply the clutch coil with battery voltage. However, like all other relays, the clutch relay is controlled by a switchable low-amperage current. When this current is switched “ON”, the current energizes a small coil in the relay, which then closes a pair of contacts to complete the circuit that feeds battery power to the A/C compressor’s clutch mechanism.
This relay represents a sort of electrical “crossroads” where a low amperage and high amperage current meet without actually coming into contact with each other. As a result, the low amperage current can be seen as the control current that activates and deactivates the high-amperage current that controls the A/C compressor’s clutch mechanism.
In practice, though, the low-amperage control circuit is a fairly complex one that can only be completed when several “switches” are all in the “ON” position. For instance, if the gas pressure in the A/C system falls below a specified threshold, the pressure switches that monitor the system pressure will be “OPEN”, thus preventing the control circuit from activating the clutch relay.
Similarly, if the thermostat that controls the cabin temperature is defective, or registers a preset temperature, it will also prevent the control circuit from activating the clutch relay. Older A/C systems were largely self-sufficient, in the sense that if all components were supplied with power, the system could operate indefinitely in a sort of closed-loop fashion without inputs from a driver, passenger, or for that matter, from a control module.
Nonetheless, to prevent damage to the system, the PCM or other dedicated control module(s) monitor the overall status and health of the A/C system and when a fault or failure occurs in any part of the system, the PCM or dedicated control module will set one or more appropriate fault codes. Note that depending on the vehicle and the nature of the problem, the A/C system might be deactivated to protect both the A/C system and some engine parts against damage.
As a practical matter, though, some faults in older A/C systems are undefined, in the sense that the PCM or dedicated control module cannot relate the fault to a specific part or component in the overall system with any degree of accuracy. In the case of code P0645, the PCM or dedicated control module “knows” the fault is in the clutch relay’s low-amperage control system, but it cannot identify the exact cause or nature of the problem.