Heating and A/C Operation

WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2008 GMC Yukon XL, 2008 GMC Yukon, 2008 Chevrolet Tahoe, 2008 Chevrolet Suburban, and 2008 Chevrolet Avalanche. However, it is still accessible from the selected car via links, so may be relevant.

The purpose of the heating and A/C system is to provide heated and cooled air to the interior of the vehicle. The A/C system will also remove humidity from the interior and reduce windshield fogging. The vehicle operator can determine the passenger compartment temperature by adjusting the air temperature switch. The vehicle passenger can offset the passenger temperature as much as 16.7°C (30°F). Regardless of the temperature setting, the following can affect the rate that the HVAC system can achieve the desired temperature:

Recirculation actuator setting
Difference between inside and desired temperature
Difference between ambient and desired temperature
Blower motor speed setting
Mode setting
Auxiliary HVAC settings

The control module makes the following actions when automatic operation is not selected, and an air temperature setting is selected:

When the air temperature switch is placed in the warmest position, the control module commands the air temperature door to divert maximum air past the heater core.
When the air temperature switch is placed in the coldest position, the control module commands the air temperature door to direct air to bypass the heater core.

When the air temperature switch is placed between the warmest and coldest positions, the control module monitors the following sensor inputs to determine the air temperature door position that diverts the appropriate amount of air past the heater core in order to achieve the desired temperature:

Sunload
Duct temperatures
Ambient temperature
Inside temperature

The A/C system can be engaged by either pressing the A/C switch or during automatic operation. The HVAC control module sends a GMLAN message to the powertrain control module (PCM) for A/C compressor engagement. The PCM will provide a ground for the A/C compressor relay enabling it to close its internal contacts to send battery voltage to the A/C compressor clutch coil. The A/C compressor diode will prevent a voltage spike, resulting from the collapse of the magnetic field of the coil, from entering the vehicle electrical system when the compressor is disengaged.

The following conditions must be met in order for the A/C compressor clutch to turn ON if they are not all met and an A/C request is made the LED on the A/C switch will flash:

Ambient air temperature above 7°C (45°F)
A/C low pressure switch signal circuit is grounded
A/C refrigerant pressure sensor parameter is less than 2957 kPa (429 psi)
PCM receives an A/C request from the HVAC control module
Engine coolant temperature (ECT) is less than 121°C (250°F)
The engine rpm is more than 550 RPM
The throttle position is less than 100 percent

The HVAC control module monitors the A/C low pressure switch signal circuit. If the voltage signal on this circuit has no voltage drop the module will interpret this condition as a low pressure, disabling the A/C request. The A/C low pressure switch will open its internal contacts at 151 kPa (22 psi). Then close the contacts at 275 kPa (40 psi) to resume A/C operation. This switch assists in cycling the A/C compressor and prevents A/C compressor operation if system has a low refrigerant level.

The PCM monitors the A/C refrigerant pressure sensor signal circuit. The voltage signal on this circuit is proportional to the refrigerant pressure inside the A/C high side pressure line. As the pressure inside the line increases, so does the voltage signal. If the pressure is above 2957 kPa (429 psi), the A/C compressor output is disabled. When the pressure lowers to 1578 kPa (229 psi), the PCM enables the compressor to operate.

The sensor information is used by the PCM to determine the following:

The A/C high side pressure
An A/C system load on the engine
An excessive A/C high side pressure
The heat load at the A/C condenser

Once engaged, the compressor clutch will be disengaged for the following conditions:

Ambient air temperature is less than 7°C (45°F)
Throttle position is 100 percent
The A/C low pressure switch is open
A/C high side pressure is more than 2957 kPa (429 psi)
A/C low side pressure is less than 151 kPa (22 psi)
ECT is more than 121°C (250°F)
Engine speed is more than 5500 RPM
Transmission shift
PCM detects excessive torque load
PCM detects insufficient idle quality
PCM detects a hard launch condition