Climate Control Systems

CLIMATE CONTROL SYSTEMS

Automatic Temperature Control (ATC)
The body control module (BCM) provides automatic temperature control support both through PCI bus communications and through processing inputs to determine control for actuators and the blower motor. The BCM interfaces with the ATC control head over the PCI bus. Information on button activation, blower setting, and temperature setting are supplied to the BCM. In turn, the BCM supplies the ATC control head with mode information and indication lamp status. The BCM controls blower motor speed by providing a varying duty cycle of a fixed frequency output (pulse width modulation). There are 14 distinct blower speeds, plus a blower off signal. The body control module also controls the HVAC mode, temperature (blend air), and recirculation doors. This system is either fully automatic or fully manual control (no semiautomatic functions); however, the temperature blend door control is always automatically controlled based on sensor reading and comfort setting. The in-car sensor and the aspirator motor are now housed in the ATC control head.

Manual Temperature Control
The body control module interfaces with the a/c-heater control head via hardwired circuits. Information for the temperature setting is supplied to the body control module via a potentiometer. The rear defogger switch, A/C switch and the mode setting is supplied via a resistive multiplexed circuit. The system on/off status is supplied by a dedicated circuit. The BCM drives the A/C and rear defogger indicator lamps via hardwire to the A/C-heater control head.

HVAC Blend, Mode, And Recirculation Door Operation
The trouble codes that indicate a stall failure or a feedback signal failure work together to show what is wrong with the HVAC system. The stall trouble codes mean that the body control module cannot force an actuator to the end of the HVAC unit door. Internal problems in the body control module will set other short-related codes. Having only stall trouble codes indicates that there is a problem with an open or short circuit in the wiring, a bad actuator, a bad HVAC unit door or connecting linkage. It is important that the operation of the HVAC door be checked if an actuator is removed. This can be checked by rotating each unit door shaft to confirm that the door will stop at both ends of travel. The actuator itself has no stops. It must have the HVAC unit door to stop travel to pass a stall test.

A feedback signal failure can occur on the blend or mode door. The body control module monitors the feedback signal to check the position of the actuator. The body control module not only checks the level of the signal but also how much the voltage changes.

A feedback failure can occur if there is a short or open circuit in the wiring, a bad actuator, a bad body control module, a bad HVAC unit door or connecting linkage. The easiest way to diagnose this is to use the DRB to actuate the blend or mode door. Note that the feedback voltage of the actuator should smoothly change as the actuator is moved. A sudden change in the feedback voltage to a 5.0 volt or a 0.0 volt level indicates the actuator is bad. A fixed feedback voltage that is less than 5.0 volts or greater than 0.0 volts without a stall failure, or a short failure indicates that the actuator, the HVAC unit door, or a connecting mechanical linkage is jammed thereby preventing movement. A feedback signal voltage that stays on 5.0 volts or 0.0 volts indicates a wiring or body control module problem. The feedback voltage should always be less than 5.0 volts and greater than 0.0 volts.

The feedback trouble code can also occur from lack of actuator travel. This can be checked by confirming that the feedback signal smoothly changes when the actuator is moved with the DRB. If the signal is OK, the door travel is not correct. The actuator must be removed and the HVAC unit door mechanically checked for proper operation. Typical problems that prevent door movement include screws dropped in the HVAC unit or warped doors. Replace any part that is found defective.

Rear Window Defogger
When the BCM receives a rear defogger request from the climate control head (either ATC or manual) the BCM will activate the rear window defogger relay for 15 minutes. If the BCM receives another rear defogger request while the relay is still activated, the relay will be de-activated. If the relay is allowed to time out (full 15 minutes), further requests during the same ignition on period will cause the BCM to activate the relay for 7.5 minutes.