Description
A/C clutch relay is PCM controlled to delay A/C clutch engagement after A/C is turned on. This allows PCM to adjust engine RPM before A/C clutch engages. PCM 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 PCM. When PCM receives this signal, PCM 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 PCM, PCM 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.
Power for cooling fan motors comes from a fusible link at battery junction block. Relays are energized when current flows from A/P FAN fuse (25-amp) through relay coils to ground through Powertrain Control Module (PCM). No. 1 cooling fan relay control circuit is grounded for low speed fan operation. Relay control circuits for all 3 cooling fan relays are grounded for high speed fan operation.
During low speed fan operation, PCM supplies a ground path for No. 1 cooling fan relay. This closes relay contacts, allowing current to flow from battery junction block through relay and to cooling fan motor No. 1 (left side). Ground path for cooling fan motor No. 1 is through No. 3 cooling fan relay and cooling fan motor No. 2 (right side). The result is a series circuit with both fans running at low speed.
During high speed fan operation, PCM supplies a ground path for all 3 cooling fan relays. This closes all 3 fan relays. At high speed, both cooling fan motors are supplied current from battery junction block and each fan motor has its own ground.
See also:
• ADJUSTMENTS - 5.7L