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Air Delivery



AIR DELIVERY DESCRIPTION AND OPERATION

The air delivery controls are divided into 4 areas:
- HVAC control components
- Air speed
- Air distribution
- Recirculation operation
- Automatic operation

HVAC Control Components

HVAC Control Module




The HVAC control module is a GMLAN device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. The control module communicates on the low-speed data bus. The battery positive voltage circuit provides power that the control module uses for keep alive memory (KAM). If the battery positive voltage circuit loses power, then all HVAC DTCs and settings will be erased from KAM. The temperature display reading at the control head can be converted from °C to °F by pressing the Auto button for a minimum of 3 seconds. The ignition 3 voltage circuit provides a device on signal. The control module supports the features.

Mode/Recirculation Actuator
The air temperature 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 12-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 0-12 volts. The HVAC control module uses a range of 0-255 counts to index the actuator position. 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 Valve Assembly
The film valve system has several operating characteristics:
- The cycle time for a mode change will vary based on the starting point of the film valve relative to the requested mode position.
- The maximum time to achieve a mode change occur when the film valve travels between vent and defrost.
- Mode change times may very slightly based on variations in system airflow, temperature and voltage.
- The amount of air rush noise heard when a mode change is requested that causes the film valve to move through 3 or more primary modes is dependent on blower speed.
- Recalibration will typically occur when the engine is OFF. The actuator noise heard during recalibration may be more noticeable than during normal operation.

Blower Motor Control Processor
The blower motor control processor is an interface between the HVAC control module and the blower motor. The blower motor speed control, battery positive voltage and ground circuits enable the control processor to operate. The HVAC control module provides a pulse width modulation (PWM) signal to the control processor in order to command the blower motor speed. The processor supplies 12 volts to the blower motor through the blower motor voltage supply circuit. The control processor uses the blower motor ground as a low side control to adjust the blower motor speed.

Air Speed
The blower motor forces air to circulate within the vehicles interior. The vehicle operator determines the blower motors speed by placing the blower motor switch in a desired speed position. The blower motor will only operate if the blower motor switch is in any position other than OFF, and the ignition switch is in the RUN position.

Once a blower speed is selected, the blower speed remains constant until a new speed is selected.

As the requested blower speed increases, the following conditions occur:
- The HVAC control module increases the amount of time that the blower motor speed control circuit is modulated to ground.
- The voltage and duty cycle, measured between the blower motor speed control circuit and ground, decrease.

As the requested blower speed decreases, the following conditions occur:
- The HVAC control module decreases the amount of time that the blower motor speed control circuit is modulated to ground.
- The voltage and duty cycle, measured between the blower motor speed control circuit and ground, increase.

Afterblow
The after blow feature is a dealer turn on option. It is inactive in a new vehicle. The afterblow relay resides in the body control module (BCM). The BCM will directly turn on the hi blower fan relay to start the after blow process. The afterblow is a method where by the HVAC blower motor is run for a few minutes after the vehicle ignition is turned off in order to physically blow off excess condensation and eliminate odors associated with A/C usage.

The following conditions must be met for afterblow to operate:
- The A/C compressor operated for more than 10 seconds.
- The A/C compressor, after operating for more than 10 seconds has not been deactivated for more than 10 minutes.
- The system voltage is at least 11 volts to start and 10 volts to continue to run.
- The ignition has been in the OFF position for at least 20 minutes.

Once the above conditions have been met the following sequence of events will occur:
- The blower will run for 4 minutes at 59 percent blower speed.
- The recirculation door moves to outside air position.
- The mode valve moves to the floor position.

Air Distribution
The HVAC control module controls the mode actuator in order to distribute airflow to a desired outlet. The mode switch provides the vehicle operator with the ability to override the automatic setting. When the mode switch is moved to the bi-level through defrost positions, the A/C compressor clutch engages and the recirculation actuator will be moved to the outside air position. When VENT is pressed, the following will occur:
- The mode valve will be moved to the panel position.
- The recirculation actuator will follow user request if blower is on and power mode is in run.
- The A/C compressor will follow user request if blower is on and power mode is in run.

Recirculation Operation
The HVAC control module controls the air intake through the recirculation actuator. Recirculation is not available when the mode is in bi-level through defrost. The operator must activate the blower for Recirculation operation.

Automatic Operation
In automatic operation, the HVAC control module will maintain the comfort level inside of the vehicle by controlling the A/C compressor clutch, the blower motor, the air temperature actuators, mode actuator and recirculation.

To place the HVAC system in automatic mode, the following is required:
- The blower motor switch must be in the AUTO position.
- The air temperature switch must be in any other position other than 60 or 90 degrees.
- The mode switch must be in the AUTO position.

Once the desired temperature is reached, the blower motor, mode, recirculation and temperature actuators will automatically adjust to maintain the temperature selected. The HVAC control module performs the following functions to maintain the desired air temperature:
- Regulate blower motor speed
- Position the air temperature actuator
- Position the mode actuator
- Position the recirculation actuator
- Request A/C operation

When the warmest position is selected in automatic operation the blower speed will increase gradually until the vehicle reaches normal operating temperature. When normal operating temperature is reached the blower will stay on high speed and the air temperature actuators will stay in the full heat position. When the coldest position is selected in automatic operation the blower will stay on high and the air temperature actuators will stay in the full cold position.

In cold temperatures, the automatic HVAC system will provide heat in the most efficient manner. The vehicle operator can select an extreme temperature setting but the system will not warm the vehicle any faster. In warm temperatures, the automatic HVAC system will also provide air conditioning in the most efficient manner. Selecting an extreme cool temperature will not cool the vehicle any faster.

Remote Vehicle Start HVAC Operation
The HVAC preset RVS settings are as follows:
- Inside air temperature input below 22°C (72°F) the HVAC system will set the blower motor speed to high speed, set the mode door to the defrost position, set the temperature door to the full hot position and set the recirculation door to the outside air position.
- Inside air temperature input above 26°C (79°F) the HVAC system will set the blower motor speed to high speed, set the mode door to the panel position, set the temperature door to the full cold position, request A/C compressor operation and set the recirculation door to the recirculate position.
- Inside air temperature input between 22°C (72°F) and 26°C (79°F) the HVAC system will set the blower motor speed to a medium speed, set the mode door to the panel position, set the temperature door to the full cold position, request A/C compressor operation and set the recirculation door to the outside air position.

When you transition from remote start to normal operation (by entering car, inserting key and turning on) the C68 control head reverts to the prior remote start function settings.
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