Circuit Operation

CIRCUIT OPERATION

BLOWER CONTROLS

In all blower speeds except HI, the AC HEATER FUSE supplies current to the Blower Motor. In high speed operation, current from the Blower Switch energizes the High Blower Relay, permitting current to flow from the Battery Junction Block to the Blower Motor.


BLOWER LOW SPEED OPERATION

The Blower Motor operates in Low Speed only through the AC-Heater Selector Switch. With the AC-Heater Selector Switch in MAX AC, NORM AC, BI-LEVEL, VENT, HEAT, or DEFROST, power through the AC-Heater Selector Switch is available to the Blower Resistors through the 64 (BRN/WHT) wire. The current then flows through all three resistors and the normally closed contacts of the High Blower Relay to the Blower Motor and the Blower Motor runs at low speed.


BLOWER MEDIUM LOW SPEED OPERATION

With the Blower Switch in M1, current flows from the AC HEATER FUSE, through the AC-Heater Selector Switch and the Blower Switch to the 63 (TAN) wire at the Blower Resistors. Current then flows through two of the resistors and the normally closed contacts of the High Blower Relay to the Blower Motor. The Blower Motor then runs at Medium Low speed.


BLOWER MEDIUM HIGH SPEED OPERATION

With the Blower Switch in M2, the blower operates the same as in Medium Low Speed, except that the current is directed to the Blower Resistors through the 72 (LT BLU) wire. Current passes through only one resistor. The Blower Motor then runs at Medium High Speed.


BLOWER HIGH SPEED OPERATION

With the Blower Switch in HI, current bypasses the Blower Resistors and flows to the High Blower Relay. This current energizes the relay and closes the normally open contacts of the relay. Current then flows from the Battery Junction Block, through the relay to the Blower Motor, and the Blower Motor runs at High Speed.



COMPRESSOR CONTROLS

The Compressor for the Air Conditioning System is driven by the engine by means of a belt drive and the AC Compressor Clutch. The Clutch allows the Compressor to be disengaged when Air Conditioning is not required. Power to the AC Compressor Clutch is supplied by the HTR-AC Fuse.

AC Compressor Clutch Diode
The AC Compressor Clutch Diode is connected across the terminals of the AC Compressor Clutch. Whenever the clutch is de-energized, the magnetic field around it collapses, generating an induced voltage in the clutch coil. The Diode provides a path for the current resulting from the induced voltage so that other circuit components are not damaged.

2.5L VIN E (LN8)
AC-Heater Selector Switch
With the AC-Heater Selector Switch in any mode except VENT or HEAT, current flows from the HTR-AC Fuse, through the Low Pressure Switch, to the AC Control Relay terminal B and the Electronic Control Module (ECM).

AC Request signal
The AC Request signal to the ECM closes the AC Cutout Switch in the ECM providing a ground at terminal C of the AC Control Relay. This allows current to flow from the Gages Fuse to terminal A of the AC Control Relay, energizing the relay. Current from the HTR-AC Fuse then flows through the relay to energize the AC Compressor Clutch. The AC Request signal to the ECM also increases engine idle speed.

The AC Compressor can be turned off in one of three ways:
^ Moving the Function Lever to the VENT or HEAT position on the AC-Heater Selector Switch stops the current flow to the system from the HTR-AC fuse.

^ If the AC system pressure drops too low, the Low Pressure Switch will open stopping the current flow.

^ If, while turning the vehicle, power steering system pressure rises above 3792 kPa (550 psi), the Power Steering Pressure Switch opens. This interrupts the ground signal to the ECM causing the AC Cutout switch in the ECM to open de-energizing the AC Control Relay. When the power steering system pressure drops below 3792 kPa (550 psi), the Power Steering Pressure Switch closes and the AC Compressor again starts operating.

2.8L VIN R (LL2) AND 4.3L VIN Z (LL4)
Evaporator Pressure Control Switch
The AC Compressor for these engines is controlled the same way as the 2.5L VIN E, except that an Evaporator Pressure Control Switch is used in place of the Low Pressure Switch. Also, these engines do not use a Power Steering Pressure Switch. On these engines, if the AC system pressure drops below 172 kPa (25 psi), the Evaporator Pressure Control Switch opens stopping current flow to the AC Compressor. When the AC system pressure rises to 310 kPa (45 psi), the switch closes and current again flows to the AC Compressor.