Hummer H2. Manual - part 1961

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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 class 2 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: 

z

The ambient air temperature above 4° C (40° F)  

z

The A/C low pressure switch signal circuit is grounded  

z

The A/C refrigerant pressure sensor parameter is less than 2957 kPa (429 psi)  

z

The PCM receives an A/C request from the HVAC control module  

z

The engine coolant temperature (ECT) is less than 121° C (250° F)  

z

The engine rpm is more than 550 rpm  

z

The throttle position is less than 100%  

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: 

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The A/C high side pressure  

z

An A/C system load on the engine  

z

An excessive A/C high side pressure  

z

The heat load at the A/C condenser  

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

z

The ambient air temperature is less than 4° C (40° F)  

z

The throttle position (TP) is 100 percent  

z

The A/C low pressure switch is open  

z

The A/C high side pressure is more than 2957 kPa (429 psi) 

 

2004 Hummer H2 

2004 HVAC HVAC Systems - Automatic - Hummer H2

  

z

The A/C low side pressure is less than 151 kPa (22 psi)  

z

The engine coolant temperature (ECT) is more than 121° C (250° F)  

z

The engine speed is more than 5,500 rpm  

z

The transmission shift  

z

The PCM detects excessive torque load  

z

The PCM detects insufficient idle quality  

z

The PCM detects a hard launch condition  

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, the mode actuator and 
the recirculation actuator. 

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: 

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Monitors the following sensors: 

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The inside air temperature sensor  

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The ambient air temperature sensor  

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The lower left air temperature sensor  

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The lower right air temperature sensor  

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The upper left air temperature sensor  

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The upper right air temperature sensor  

z

Regulates blower motor speed  

z

Positions the air temperature actuators  

z

Positions the mode actuator  

z

Positions the recirculation actuator  

z

Requests the A/C operation  

A/C Cycle 

Refrigerant is the key element in an air conditioning system. R-134a is presently the only EPA approved 
refrigerant for automotive use. R-134a is an very low temperature gas that can transfer the undesirable heat and 
moisture from the passenger compartment to the outside air.

 

2004 Hummer H2 

2004 HVAC HVAC Systems - Automatic - Hummer H2

  

The A/C compressor is belt driven and operates when the magnetic clutch is engaged. The compressor builds 
pressure on the vapor refrigerant. Compressing the refrigerant also adds heat to the refrigerant. The refrigerant 
is discharged from the compressor, through the discharge hose, and forced to flow to the condenser and then 
through the balance of the A/C system. The A/C system is mechanically protected with the use of a high 
pressure relief valve. If the A/C refrigerant pressure sensor were to fail or if the refrigerant system becomes 
restricted and refrigerant pressure continued to rise, the high pressure relief will pop open and release 
refrigerant from the system. 

Compressed refrigerant enters the condenser in a high temperature, high pressure vapor state. As the refrigerant 
flows through the condenser, the heat of the refrigerant is transferred to the ambient air passing through the 
condenser. Cooling the refrigerant causes the refrigerant to condense and change from a vapor to a liquid state. 

The condenser is located in front of the radiator for maximum heat transfer. The condenser is made of 
aluminum tubing and aluminum cooling fins, which allows rapid heat transfer for the refrigerant. The semi-
cooled liquid refrigerant exits the condenser and flows through the liquid line, to the orifice tube. 

The orifice tube is located in the liquid line between the condenser and the evaporator. The orifice tube is the 
dividing point for the high and the low pressure sides of the A/C system. As the refrigerant passes through the 
orifice tube, the pressure on the refrigerant is lowered. Due to the pressure differential on the liquid refrigerant, 
the refrigerant will begin to vaporize at the orifice tube. The orifice tube also meters the amount of liquid 
refrigerant that can flow into the evaporator. 

Refrigerant exiting the orifice tube flows into the evaporator core in a low pressure, liquid state. Ambient air is 
drawn through the HVAC module and passes through the evaporator core. Warm and moist air will cause the 
liquid refrigerant boil inside of the evaporator core. The boiling refrigerant absorbs heat from the ambient air 
and draws moisture onto the evaporator. The refrigerant exits the evaporator through the suction line and back 
to the compressor, in a vapor state, and completing the A/C cycle of heat removal. At the compressor, the 
refrigerant is compressed again and the cycle of heat removal is repeated. 

The conditioned air is distributed through the HVAC module for passenger comfort. The heat and moisture 
removed from the passenger compartment will also change form, or condense, and is discharged from the 
HVAC module as water. 

SPECIAL TOOLS AND EQUIPMENT 

SPECIAL TOOLS 

Special Tools 

Illustration

Tool Number/Description

 

2004 Hummer H2 

2004 HVAC HVAC Systems - Automatic - Hummer H2

  

Scan Tool

J 43600 

ACR 2000 Refrigerant Station

 

2004 Hummer H2 

2004 HVAC HVAC Systems - Automatic - Hummer H2