Hummer H2. Manual - part 1481

TAC system is self-diagnosing and provides diagnostic information to the powertrain control module (PCM) 
through a dedicated serial data line. The TAC achieves throttle positioning by providing a pulse width 
modulated voltage to the TAC, as directed by the PCM. 

Powertrain Control Module 

The powertrain control module (PCM) determines the driver's intent, then calculates the appropriate throttle 
response. This information is sent to the throttle actuator control (TAC) module through a dedicated serial data 
line. 

Modes of Operation 

Normal Mode 

During the operation of the throttle actuator control (TAC) system, several modes or functions are considered 
normal. The following modes may be entered during normal operation: 

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Minimum pedal value-At key-up the powertrain control module (PCM) updates the learned minimum 
pedal value.  

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Minimum throttle position (TP) values-At key-up the PCM updates the learned minimum TP value. In 
order to learn the minimum TP value, the throttle blade is moved to the closed position.  

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Ice break mode-If the throttle is not able to reach a predetermined minimum throttle position, the ice 
break mode is entered. During the ice break mode, the control module commands the maximum pulse 
width several times to the throttle actuator motor in the closing direction.  

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Battery saver mode-After a predetermined time without engine RPM, the control module commands the 
battery saver mode. During the battery saver mode, the TAC module removes the voltage from the motor 
control circuits, which removes the current draw used to maintain the idle position and allows the throttle 
to return to the spring loaded default position.  

Reduced Engine Power Mode 

When the PCM detects a condition with the TAC system, the PCM may enter a reduced engine power mode. 
Reduced engine power may cause one or more of the following conditions: 

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Acceleration limiting-The control module will continue to use the accelerator pedal for throttle control; 
however, the vehicle acceleration is limited.  

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Limited throttle mode-The control module will continue to use the accelerator pedal for throttle control; 
however, the maximum throttle opening is limited.  

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Throttle default mode-The control module will turn off the throttle actuator motor and the throttle will 
return to the spring loaded default position.  

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Forced idle mode-The control module will perform the following actions: 

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Limit engine speed to idle by positioning throttle position, or by controlling fuel and spark if 
throttle is turned off.  

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Ignore accelerator pedal input.  

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Engine shutdown mode-The control module will disable fuel and de-energize the throttle actuator. 

 

2004 Hummer H2 

2004 ENGINE PERFORMANCE Engine Controls (Introduction) - 4.8L, 5.3L, and 6.0L - Hummer H2

  

FUEL SYSTEM DESCRIPTION 

Fuel System Overview 

The Fuel System is a returnless on-demand design. The fuel pressure regulator is a part of the fuel sender 
assembly, eliminating the need for a return pipe from the engine. A returnless fuel system reduces the internal 
temperature of the fuel tank by not returning hot fuel from the engine to the fuel tank. Reducing the internal 
temperature of the fuel tank results in lower evaporative emissions. 

An electric turbine style fuel pump attaches to the fuel sender assembly inside the fuel tank. The fuel pump 
supplies high pressure fuel through the fuel filter and the fuel feed pipe to the fuel injection system. The fuel 
pump provides fuel at a higher rate of flow than is needed by the fuel injection system. The fuel pump also 
supplies fuel to a venturi pump located on the bottom of the fuel sender assembly. The function of the venturi 
pump is to fill the fuel sender assembly reservoir. The fuel pressure regulator, a part of the fuel sender 
assembly, maintains the correct fuel pressure to the fuel injection system. The fuel pump and sender assembly 
contains a reverse flow check valve. The check valve and the fuel pressure regulator maintain fuel pressure in 
the fuel feed pipe and the fuel rail in order to prevent long cranking times. 

Fuel Tanks 

The fuel tanks store the fuel supply. The fuel tank is held in place by 2 metal straps that attach to the frame. The 
fuel tank is molded from high density polyethylene. 

Fuel Fill Pipe 

The fuel fill pipe has a built-in restrictor in order to prevent refueling with leaded fuel. Once the fill vent is 
obstructed, fuel backs up the fill pipe and trips the dispensing nozzle. The fuel tank vent valves are connected 
and route to the canister to collect hydrocarbon emissions during operation of the vehicle. 

Fuel Filler Cap 

Fig. 194: Fuel Filler Cap 
Courtesy of GENERAL MOTORS CORP.

 

2004 Hummer H2 

2004 ENGINE PERFORMANCE Engine Controls (Introduction) - 4.8L, 5.3L, and 6.0L - Hummer H2

  

Fig. 195: Fuel Filler Door View 
Courtesy of GENERAL MOTORS CORP. 

The fuel fill pipe has a tethered fuel filler cap. A torque-limiting device prevents the cap from being over 
tightened. To install the cap, turn the cap clockwise until you hear clicks. This indicates that the cap is correctly 
torqued and fully seated. A built-in device indicates that the fuel filler cap is fully seated. A fuel filler cap that is 
not fully seated may cause a malfunction in the emission system. 

Fuel Sender Assembly 

 

2004 Hummer H2 

2004 ENGINE PERFORMANCE Engine Controls (Introduction) - 4.8L, 5.3L, and 6.0L - Hummer H2

  

Fig. 196: Fuel Sender Assembly 
Courtesy of GENERAL MOTORS CORP. 

The fuel sender assembly on single tank applications consists of the following major components: 

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The fuel level sensor  

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The fuel tank pressure (FTP) sensor  

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The fuel tank fuel pump module  

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The fuel strainer  

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The fuel filter  

Fuel Level Sensor 

The fuel level sensor consists of a float, a wire float arm, and a ceramic resistor cord. The position of the float 

 

2004 Hummer H2 

2004 ENGINE PERFORMANCE Engine Controls (Introduction) - 4.8L, 5.3L, and 6.0L - Hummer H2