TrailBlazer 4WD L6-4.2L VIN S (2002)
Air Door Actuator / Motor: Description and Operation
HVAC System - Manual
Air Distribution Control
The HVAC control module controls the distribution of air by the use of a defrost actuator and a mode actuator. The modes that may be selected are:
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Defrost
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Defog
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Panel
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Bi-Level
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Floor
The mode and defrost actuators are connected to the mode and defrost doors by a cam type linkage system. Depending on the position of the door, air is
directed through the HVAC module and distributed through various ducts leading to the outlets in the dash. If the HVAC control module detects a fault
with the mode or defrost doors the HVAC control module will try to drive the actuator for a predetermined amount of time, to defrost, which is the
defaulted position for the mode and defrost door actuators. When the mode switch is placed in the defrost or defog positions the A/C is commanded on
and the recirculation door is moved to the outside air position to help reduce window fogging. A/C is available in all modes and recirculation is only
available in the panel and bi-level modes.
Mode Actuator
The mode actuator is an electronic stepper motor with feedback potentiometers. The HVAC control module sends signals to the mode door actuator
through the mode door control circuit. Zero volts drives the actuator in one direction while 5-volts moves the actuator in the opposite direction. When
the actuator receives 2.5 volts, the actuator rotation stops. A 5-volt reference signal is sent out over the 5-volt reference circuit to the mode actuator.
When you select a desired mode setting, logic determines the value of the mode actuator signals. The HVAC control module's software uses this
reference voltage in order to determine the position of the mode actuator through the mode door position signal circuit. The motor moves the mode
door to the desired position.
Defrost Actuator
The defrost actuator operates the same as the mode actuator. The HVAC control module sends signals to the mode door actuator through the defrost
door control circuit Zero volts drives the actuator in one direction while 5-volts moves the actuator in the opposite direction. When the actuator
receives 2.5 volts, the actuator rotation stops. A 5-volt reference signal is sent out over the 5-volt reference circuit to the defrost actuator. When you
select a defrost setting, logic determines the value of the defrost actuator signals. The HVAC control module's software uses this reference voltage in
order to determine the position of the mode actuator through the defrost door position signal circuit. The motor moves the defrost door to the desired
position.
Front Defrost
When defrost is selected, the A/C compressor is activated. The A/C compressor clutch will engage when ambient temperatures are above 3°C (38°F).
The blower motor will be activated, regardless of the coolant temperature. The HVAC control module will override the auxiliary HVAC control
module so a high volume of air is delivered to the front defrost vents. The rear window defogger does not affect the HVAC system.
HVAC Control Actuator Descriptions
DEFROST ACTUATOR
The defrost actuator is a 5 wire bi-directional electric motor that incorporates a feedback potentiometer. Ignition 3 voltage, low reference, control, 5
volt reference and position signal circuits enable the actuator to operate. The control circuit uses either a 0, 2.5 or 5 volt signal to command the
actuator movement. When the actuator is at rest, the control circuit value is 2.5 volts. A 0 or 5 volt control signal commands the actuator movement in
opposite directions. When the actuator shaft rotates, the potentiometer's adjustable contact changes the door position signal between 0-5 volts.
The HVAC control module uses a range of 0-255 counts to index the actuator position. The door position signal voltage is converted to a 0-255 count
range. When the module sets a commanded, or targeted, value, the control signal is changed to either 0 or 5 volts depending upon the direction that
the actuator needs to rotate to reach the commanded value. As the actuator shaft rotates the changing position signal is sent to the module. Once the
position signal and the commanded value are the same, the module changes the control signal to 2.5 volts.
MODE ACTUATOR
The mode actuator is a 5 wire bi-directional electric motor that incorporates a feedback potentiometer. Ignition 3 voltage, low reference, control, 5
volt reference and position signal circuits enable the actuator to operate. The control circuit uses either a 0, 2.5 or 5 volt signal to command the
actuator movement. When the actuator is at rest, the control circuit value is 2.5 volts. A 0 or 5 volt control signal commands the actuator movement in
opposite directions. When the actuator shaft rotates, the potentiometer's adjustable contact changes the door position signal between 0-5 volts.
The HVAC control module uses a range of 0-255 counts to index the actuator position. The door position signal voltage is converted to a 0-255 count
range. When the module sets a commanded, or targeted, value, the control signal is changed to either 0 or 5 volts depending upon the direction that
the actuator needs to rotate to reach the commanded value. As the actuator shaft rotates the changing position signal is sent to the module. Once the
position signal and the commanded value are the same, the module changes the control signal to 2.5 volts.
AUXILIARY MODE ACTUATOR
The auxiliary mode actuator is a 5 wire bi-directional electric motor that incorporates a feedback potentiometer. Low reference, 5 volt reference,