Heating and Air Conditioning: Description and Operation

Circuit Operation

Electronic Thermostat

The electronic thermostat senses the temperature of cool air from the evaporator by means of the evaporator temperature sensor. This information determines whether the thermostat will provide ground to the thermo switch relay. When the temperature becomes too low, ground is not provided.

OFF: Below 1.0 ± 0.5 °C (33.8 ± 0.9 °F)
ON: Above 3.5 ± 0.3 °C (38.3 ± 0.9 °F)

A/C Compressor Clutch (3.2L)
With the engine running, battery voltage is applied to the A/C switch, the A/C triple pressure switch (A/T) or A/C dual pressure switch (M/T), the coil of the A/C compressor relay and the air selector knob. When you press the A/C switch, battery voltage is applied to the electronic thermostat. The solid-state switch in the electronic thermostat closes, providing a path to ground for the thermo switch relay, with the fan control switch in any position except OFF. Voltage is also applied through the A/C triple pressure switch (A/T) or the A/C dual pressure switch (M/T) to the coil of the thermo switch relay. This closes the thermo switch relay contacts, applying battery voltage to the A/C ON input of the engine control module (ECM). The ECM then provides ground for the A/C compressor relay. The A/C compressor relay contacts close, applying battery voltage to the compressor clutch. If the A/C triple pressure switch (A/T)/dual pressure switch (M/T) opens, the compressor clutch disengages.

A/C Compressor Clutch (3.5L DI)
With the engine running, battery voltage is applied to the A/C switch, the:coil of the A/C compressor relay and the air selector knob. When you press the A/C switch, battery voltage is applied to the electronic thermostat. The solid-state switch in the electronic thermostat closes, providing an A/C on input to the Engine Control Module (ECM) with the fan control switch in any position except OFF. The ECM then provides ground for the A/C compressor relay. The A/C compressor relay contacts close, applying battery voltage to the compressor clutch. The A/C pressure sensor monitors system pressure and will turn off the compressor clutch by removing ground from the A/C compressor relay.

A/C Dual Pressure Switch (3.2L M/T)
To prevent compressor damage, the dual pressure switch will open if system pressure is too low or too high: below 176.5 ± 19.6 kPa (25.6 ± 2.8 psi) or above 2943 ± 196.2 kPa (426.7 ± 28.4 psi).

A/C Triple Pressure Switch (3.2L A/T)
Low and high switch operation is the same as the dual pressure switch. The medium pressure switch will close when system pressure exceeds 1471.5 ± 98.1 kPa (213.3 ± 14.2 psi), turning on the condenser fan.

Condenser Fan
With the engine running, battery voltage is applied to the A/C switch. When the A/C switch is pressed, battery voltage is then applied through the A/C triple pressure switch (A/T only) to the coil side of the condenser fan relay, which is continually grounded. Once the relay is energized, the contacts close, allowing battery voltage from the (COND. FAN) fuse EB-20 to be applied to the condenser fan. The motor, being grounded continually, is energized, and the fan is activated.

Condenser Fan (3.5L DI)
To prevent compressor damage, the A/C pressure sensor will signal the ECM to turn off the A/C compressor if the system pressure is too low or too high: below 284.4 kPa (41.2 psi) or above 2922.4 kPa (423.8 psi).