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G
3.6
Climate Control Systems
E
3.6.1
Automatic Temperature Control (ATC)
N
E
The body control module (BCM) provides automatic temperature control support both through PCI
R
bus communications and through processing inputs to determine control for actuators and the
A
blower motor. The BCM interfaces with the ATC control head over the PCI bus. Information on
L
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
I
controls blower motor speed by providing a varying duty cycle of a fixed frequency output (pulse
N
width modulation). There are 14 distinct blower speeds, plus a blower off signal. The body control
F
module also controls the HVAC mode, temperature (blend air), and recirculation doors. This
O
system is either fully automatic or fully manual control (no semi-automatic functions); however, the
R
temperature blend door control is always automatically controlled based on sensor reading and
M
comfort setting. The in-car sensor and the aspirator motor are now housed in the ATC control
A
head.
T
I
3.6.2
Manual Temperature Control
O
N
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.
3.6.3
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.
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Table of Contents
Troubleshooting
