Electronic Climate Control

A/C Vacuum Schematic-With A/C:

GENERAL INFORMATION
Voltage signals are transmitted to the air control module (ACM) when:

- Either the select switch or blower speed switch are operated.
- The rear and front defrost push buttons are operated. This determines the air door position for the new mode. It also transmits serial data to the climate display module so that it will advance the display to the new selection.

If the select switch is held depressed for longer than one second, the mode indication advances again in the same direction. It will continue advancing once every half-second until either the select switch is released, or the last available mode in that direction is reached. The ACM then waits for one second and begins operation in the new mode.

The ACM determines blower speed according to the selection set by the operator.

The relays provide a ground for the blower motor either directly or through part of the blower resistor block. If none of the relays are operated, there is no path to ground and the blower does not run. For high speed operation, the ACM grounds the terminal A3 to operate the HI speed relay. The HI speed relay contacts ground to the blower motor which then receives fill battery voltage and runs at maximum speed. For low speed operation, both the M1 speed and M2 speed relays are operated. Current through the blower motor bypasses the normally closed contacts of the HI speed relay, and passes through:

- The 0.17 ohm resistance in the blower resistor block.
- The closed contacts of the M2 speed relay.
- The remainder of the blower resistor block to ground. The resistor block limits the current so that the blower motor runs at low speed. Intermediate speeds are obtained by operation of either the M2 speed relay or the M1 speed relay. Each relay, when energized, connects a shunt across one of the resistors in the blower resistor block.

Temperature is selected by sliding the temperature selector. The potentiometer output voltage varies with the setting. The voltage is transmitted to the ACM. The ACM then operates the air mixture door control to position the door according to the new setting. The potentiometer in the air mixture door control moves with the door and provides position indication to the program circuits in the ACM.

The ACM can automatically detect the following faults in the system. when it does, the mode indicator flashes Er for about five seconds each time the ignition switch is turned ON. If the fault does not occur again for 60 ignition turn-on cycles:

- The condition is cleared from memory.
- The mode indicator no longer flashes Er. See Troubleshooting for definitions of the fault codes and the procedure for displaying them.

The current temperature setting is displayed by the climate display module. Pressing the top or bottom half of the switch immediately raises or lowers the temperature by one degree. The setting on the English/Metric switch determines whether the step change is 1°F or 1°C. If the temp switch is pressed for longer than one second, the temperature setting is advanced one degree every half-second. This will happen until the switch is released, or the end of the temperature range is reached. The temperature range extends from (18°C) 65°F to (29°C) 85°F. Two additional settings at (10°) 60° and (82°C) 90° are used for maximum cooling or maximum heat when checking the system operation.

The temperature display is advanced by serial data transmitted to the climate display.

The ACM maintains the selected temperature inside by reading the in-car and outside temperature sensors and positioning the air mixture door. The sensor consists of resistors which vary inversely with the temperature. A small current flowing through the sensors generates a voltage drop. The amount of this voltage drop varies according to the resistance of sensor, and provides an input to the ACM. The ACM positions the sir mixture door. Then, the interior car temperature will reach the level set by the operator in the shortest period of time possible.

When one of the A/C modes is engaged, voltage is applied to the A/C compressor clutch when the A/C clutch relay is operated. The A/C refrigerant pressure is monitored by the A/C low pressure cut out switch. When the pressure drops to 25 psi, the switch opens, removing ground from terminal C8 of the ACM. The ACM then de-energizes the A/C clutch relay. The A/C compressor clutch is disengaged until the A/C low pressure cut out switch closes again. This causes the A/C compressor to cycle on and off. This cycling will keep the temperature of the evaporator from dropping low enough to cause it to freeze.

There are three lines between the air control module (ACM) and the electronic control unit (ECU) to provide an interface between the two modules. They are the:

- Predict Output

- Select Output

- Cut-Out Output

1. The A/C (predict output) from the ACM rises to battery voltage about 0.5 seconds before the ACM energizes the A/C clutch relay. The ECU then incresses the engine idle speed before the A/C compressor clutch is engaged. when the A/C compressor clutch is disengaged, the A/C predict output returns to 0 volts. This is done to signal the ECU that engine idle speed can be returned to normal.

2. The A/C (select output) from the ACM is at battery voltage whenever the system is operating in the MAX, NORM, BI-LEV, HEAT/DEF or DEFROST modes. It remains at battery voltage even though the A/C compressor clutch is cycled ON and OFF. During operation in the HEAT or VENT modes, the A/C select output is at ground.

3. The A/C (cut out output) from the ECU is at battery voltage when the engine is under a heavy load condition (full throttle) or the engine is cold. This voltage signals the ACM to disengage the A/C compressor clutch during heavy engine load. It also signals the blower motor to turn off when starting, or, in HEAT or HEAT/DEF modes when the engine has not yet heated up.