Description
A/C clutch relay is controlled by Vehicle Control Module (VCM) to delay A/C clutch engagement after A/C is turned on. This allows VCM to adjust engine RPM before A/C clutch engages. VCM will engage A/C clutch any time A/C has been requested unless coolant temperature is high, A/C system pressure is low, A/C system pressure is high, wide open throttle, or high engine RPM.
When A/C-heater control panel is placed in A/C mode, a 12-volt signal is sent to VCM. When VCM receives this signal, VCM will ground A/C clutch relay control circuit to energize A/C relay. This is shown on scan tool as A/C REQUEST YES.
When an A/C request has been detected by VCM, VCM will ground A/C clutch relay control circuit. As a result, relay contacts close, allowing current to flow through relay to A/C compressor clutch. When A/C REQUEST has been detected by PCM, cooling fans will be turned on when A/C pressure is greater than a predetermined value.
Auxiliary cooling fan operating voltage is supplied by AUX FAN fuse (30-amp). Cooling fan relay (coil-side) voltage is supplied by A/C COMP fuse (10-amp). Both fuses are located in underhood fuse/relay center. Voltage is provided from fuses directly to auxiliary cooling fan relay. When engine coolant temperature reaches a set point, or A/C operation is requested, Vehicle Control Module (VCM) supplies a ground path for coil-side of cooling fan relay. This allows relay contacts to close resulting in auxiliary cooling fan operation.
Note. Check VCM Diagnostic Trouble Codes (DTCs) that may affect system. If any DTCs are present, perform testing for applicable DTC. Some DTCs may cause auxiliary cooling fan to come on at all times. Repair all DTCs that are set before proceeding with testing. See TESTS W/CODES - 5.7L article.
On models not equipped with Vehicle Control Module (VCM), ENG 1 fuse (20-amp) located in instrument panel fuse block supplies voltage to coil side of auxiliary cooling fan relay with ignition switch in ON position. A ground path for cooling fan relay is supplied by A/C high pressure switch, or auxiliary fan control switch (if equipped). On models equipped with VCM, a ground path for fan control relay is supplied by VCM. As a result, relay contacts close, allowing current to flow to cooling fan motor through relay, resulting in auxiliary cooling fan motor operation.
Note. A fully charged A/C system is required for proper auxiliary cooling fan operation.