Hummer H2. Manual - part 1960

The HVAC control module controls the air intake through the recirculation actuator. The recirculation switch 
closes the recirculation door in order to circulate the air within the vehicle. The outside air switch opens the 
recirculation door in order to route outside air into the vehicle. Regardless of the blower motor switch position, 
recirculation is available only in the panel and bi-level mode switch positions. The mode switch must be placed 
in either the panel or bi-level position and the recirculation switch pressed before the blower motor switch is 
placed in the OFF position to achieve recirculation with the blower in the OFF position. In order to reduce 
windshield fogging, outside air is circulated when the mode switch is in the defrost or defog positions. If the 
recirculation switch is pressed into the ON position when the mode switch is in an unavailable mode position, 
then the recirculation switch LED will flash 3 times. If the HVAC control module detects a fault with the 
recirculation door the HVAC control module will try to drive the actuator for a predetermined amount of time, 
to outside air, which is the defaulted position for the recirculation actuator. 

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: 

z

The Auto switch must be activated  

z

The air temperature switch must be in any other position other than full hot or full cold position  

Once the desired temperature is reached, the blower motor, mode, recirculation and temperature actuators will 
automatically be adjusted to maintain the temperature selected. The HVAC control module performs the 
following functions to maintain the desired air temperature: 

z

Monitor the following sensors: 

{

Inside air temperature sensor  

{

Ambient air temperature sensor  

{

Lower left air temperature sensor  

{

Lower right air temperature sensor  

{

Upper left air temperature sensor  

{

Upper right air temperature sensor  

z

Regulate blower motor speed  

z

Position the air temperature actuator  

z

Position the mode actuator  

z

Position the recirculation actuator  

z

Request A/C operation  

AIR TEMPERATURE DESCRIPTION AND OPERATION 

The air temperature controls are divided into 5 areas:

 

2004 Hummer H2 

2004 HVAC HVAC Systems - Automatic - Hummer H2

  

z

HVAC Control Components  

z

Heating and A/C Operation  

z

Automatic Operation  

z

Engine Coolant  

z

A/C Cycle  

HVAC Control Components 

HVAC Control Module 

The HVAC control module is a class 2 device that interfaces between the vehicle operator and the HVAC 
System to maintain air temperature and distribution settings. 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, 
all HVAC DTCs and settings will be erased from KAM. The body control module (BCM), which is the vehicle 
mode master, provides a device on signal. The control module supports the following features: 

Air Temperature Description and Operation 

Air Temperature Actuator 

The air temperature actuators are 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 potentiometers 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. 

Air Temperature Sensors 

The air temperature sensors are a 2-wire negative temperature co-efficient thermistor. The vehicle uses the 
following air temperature sensors: 

z

Ambient air temperature sensor  

z

Inside air temperature sensor assembly 

Feature

Availability

Afterblow

No

Purge

No

Personalization

Yes

Actuator Calibration

Yes

 

2004 Hummer H2 

2004 HVAC HVAC Systems - Automatic - Hummer H2

  

z

Upper left air temperature sensor  

z

Upper right air temperature sensor  

z

Lower left air temperature sensor  

z

Lower right air temperature sensor  

A signal and low reference circuit enables the sensor to operate. As the air temperature surrounding the sensor 
increases, the sensor resistance decreases. The sensor signal voltage decreases as the resistance decreases. The 
sensor operates within a temperature range between -40° C (-40° F) to +101° C (+215° F). The sensor signal 
varies between 0-5 volts. 

The input of the duct air temperature sensors are different from the ambient and inside sensors. The HVAC 
control module converts the signal to a range between 0-255 counts. As the air temperature increases the count 
value will decrease. 

If the HVAC control module detects a malfunctioning sensor, then the control module software will use a 
defaulted air temperature value. The default value for the ambient and inside air temperature sensors will be 
displayed on the scan tool. The default value for the duct air temperature sensors will not be displayed on the 
scan tool. The scan tool parameter for the duct air temperature sensors are the actual state of the signal circuit. 
The default action ensures that the HVAC system can adjust the inside air temperature near the desired 
temperature until the condition is corrected. 

Sunload Sensor Assembly 

The sunload sensor is a 2-wire photo diode. The vehicle uses left and right sunload sensors. The two sensors are 
integrated into the sunload sensor assembly. Low reference and signal circuits enable the sensor to operate. As 
the light shining upon the sensor gets brighter, the sensor resistance increases. The sensor signal decreases as 
the resistance increases. The sensor operates within an intensity range between completely dark and bright. The 
sensor signal varies between 0-5 volts. The BCM converts the signal to a range between 0-255 counts. The 
BCM sends a class 2 serial data message to the HVAC control module for automatic temperature calculations. 

The sunload sensor provides the BCM a measurement of the amount of light shining on the vehicle. Bright, or 
high intensity, light causes the vehicles inside temperature to increase. The HVAC system compensates for the 
increased temperature by diverting additional cool air into the vehicle. 

If the BCM detects a malfunctioning sensor, then the control module software will use a defaulted sunload 
value. This value will not be displayed on the scan tool. The default action ensures that the HVAC system can 
adjust the inside air temperature near the desired temperature until the condition is fixed. The scan tool 
parameter for the sunload sensor is the actual state of the signal circuit. 

A/C Refrigerant Pressure Sensor 

The A/C refrigerant pressure sensor is a 3 wire piezoelectric pressure transducer. A 5 volt reference, low 
reference, and signal circuits enable the sensor to operate. The A/C pressure signal can be between 0-5 volts. 
When the A/C refrigerant pressure is low, the signal value is near 0 volts. When the A/C refrigerant pressure is 
high, the signal value is near 5 volts. The PCM converts the voltage signal to a pressure value. 

 

2004 Hummer H2 

2004 HVAC HVAC Systems - Automatic - Hummer H2

  

The A/C refrigerant pressure sensor protects the A/C system from operating when an excessively high pressure 
condition exists. The PCM disables the compressor clutch if the A/C pressure is more than 2957 kPa (429 psi). 
The clutch will be enabled after the pressure decreases to less than 1578 kPa (229 psi). 

A/C Low Pressure Switch 

The A/C low pressure switch protects the A/C System from a low pressure condition that could damage the A/C 
compressor or cause evaporator icing. The HVAC control module applies 5 volts to the A/C low pressure 
switch signal circuit. The switch will open when the A/C low side pressure reaches 138-172 kPa (20-25 psi) as 
measured at the switch/accumulator. This prevents the A/C compressor from operating. The switch will then 
close when A/C low pressure side reaches 275-317 kPa (40-46 psi) as measured at the switch/accumulator. This 
enables the A/C compressor to turn back ON. 

The low pressure switch uses refrigerant pressure to open and close a set of electrical contacts. When A/C 
request is authorized the switch is closed and shows normal status. During this state the switch will show 0 volts 
on the A/C low pressure sensor signal circuit. When A/C request is denied due to a low pressure condition the 
switch will be open. During this state the switch will show 5 volts on the A/C low pressure sensor signal circuit.

Heating and A/C Operation 

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 effect the rate that the HVAC System can achieve the desired temperature: 

z

Recirculation actuator setting  

z

Difference between inside and desired temperature  

z

Difference between ambient and desired temperature  

z

Blower motor speed setting  

z

Mode setting  

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

z

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.  

z

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.  

z

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  

 

2004 Hummer H2 

2004 HVAC HVAC Systems - Automatic - Hummer H2