Chevrolet Silverado / GMC Sierra. Service manual — part 2358

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:

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

The throttle position is 100%.

The A/C low pressure switch is open.

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

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

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

The engine speed is more than 5500 RPM.

Transmission shift

The PCM detects excessive torque load.

The PCM detects insufficient idle quality.

The PCM detects a hard launch condition.

Engine Coolant

Engine coolant is the essential element of the heating system. The thermostat controls the normal
engine operating coolant temperature. The thermostat also creates a restriction for the cooling
system that promotes a positive coolant flow and helps prevent cavitation.

Coolant enters the heater core through the inlet heater hose, in a pressurized state. The heater core
is located inside the HVAC module. The ambient air drawn through the HVAC module absorbs
the heat of the coolant flowing through the heater core. Heated air is distributed to the passenger
compartment, through the HVAC module, for passenger comfort. Opening or closing the air
temperature door controls the amount of heat delivered to the passenger compartment. The
coolant exits the heater core through the return heater hose and recirculated back through the
engine cooling system.

Auxiliary Coolant Pump (w/HP2)

The auxiliary coolant pump circulates coolant through the engine and heater core when HVAC
requires heating and the engine is HOT and OFF. The HCM will turn on the auxiliary coolant
pump when the HVAC control module commands it to do so by sending a class 2 signal to the
HCM.

2008 Chevrolet Silverado 1500

2008 HVAC HVAC - Manual - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

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.

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

2008 Chevrolet Silverado 1500

2008 HVAC HVAC - Manual - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

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

Illustration

Tool Number/ Description

J 43600

ACR 2000 Air Conditioning Service Center

2008 Chevrolet Silverado 1500

2008 HVAC HVAC - Manual - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

2008 BRAKES

Hydraulic Brakes - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

SPECIFICATIONS

FASTENER TIGHTENING SPECIFICATIONS

BRAKE COMPONENT SPECIFICATIONS

Application

Specification

Metric

English

Brake Booster Nuts (Hydraulic)

33 N.m

24 lb ft

Brake Booster Nuts (Vacuum)

33 N.m

24 lb ft

Brake Booster Pushrod Retainer Bolt

10 N.m

89 lb in

Brake Hose Fitting Bolt - Front

40 N.m

30 lb ft

Brake Master Cylinder Pressure Sensor

26 N.m

19 lb ft

Brake Hose Fitting Bolt - Rear

40 N.m

30 lb ft

Brake Pedal Bracket to Dash Panel Bolt

36 N.m

27 lb ft

Brake Pedal Pivot Nut

35 N.m

26 lb ft

Brake Pipe Fittings

25 N.m

18 lb ft

Hose Bracket to Control Arm Bolt

9 N.m

80 lb in

Hose Bracket to Frame Bolt - Front

9 N.m

80 lb in

Hose Bracket to Frame Bolt - Rear

25 N.m

18 lb ft

Hose Bracket to Frame Nut - Rear (JH6, JH7)

25 N.m

18 lb ft

Hose Bracket to Rear Axle Bolt

9 N.m

80 lb in

Hose Bracket to Steering Knuckle Bolt

9 N.m

80 lb in

Hose to Rear Axle Bolt

25 N.m

18 lb ft

Master Cylinder Mounting Nuts

33 N.m

24 lb ft

Power Brake Booster Hose Fitting - Inlet

33 N.m

24 lb ft

Power Brake Booster Hose Fitting - Outlet

33 N.m

24 lb ft

Application

Specification

Metric

English

Brake Caliper Bleeder Valve - Front

13 N.m

115 lb in

Brake Caliper Bleeder Valve - Rear

11 N.m

97 lb in

2008 Chevrolet Silverado 1500

2008 BRAKES Hydraulic Brakes - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

2008 Chevrolet Silverado 1500

2008 BRAKES Hydraulic Brakes - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado