Chevrolet Silverado / GMC Sierra. Manual

The IPC displays LOW COOLANT LEVEL message when the IPC receives a class 2 message
from the PCM requesting illumination of this driver warning.

REDUCED ENGINE POWER

The IPC displays REDUCED ENGINE POWER message when the IPC detects a reduced engine
power condition from the PCM. The IPC receives a class 2 message from the PCM requesting
illumination when the engine temperature reaches 132°C (270°F).

Cooling Fan Control - Two Fan System

The engine cooling fan system consists of 2 electrical cooling fans and 3 fan relays. The relays are
arranged in a series/parallel configuration that allows the powertrain control module (PCM) to
operate both fans together at low or high speeds. The cooling fans and fan relays receive battery
positive voltage from the underhood fuse block.

During low speed operation, the PCM supplies the ground path for the low speed fan relay
through the low speed cooling fan relay control circuit. This energizes the low speed fan relay
coil, closes the relay contacts, and supplies battery positive voltage from the low fan fuse through
the cooling fan motor supply voltage circuit to the left cooling fan. The ground path for the left
cooling fan is through the cooling fan s/p relay and the right cooling fan. The result is a series
circuit with both fans running at low speed.

During high speed operation the PCM supplies the ground path for the low speed fan relay
through the low speed cooling fan relay control circuit. After a 3 second delay, the PCM supplies
a ground path for the high speed fan relay and the cooling fan s/p relay through the high speed
cooling fan relay control circuit. This energizes the cooling fan s/p relay coil, closes the relay
contacts, and provides a ground path for the left cooling fan. At the same time the high speed fan
relay coil is energized closing the relay contacts and provides battery positive voltage from the
high fan fuse on the cooling fan motor supply voltage circuit to the right cooling fan. During high
speed fan operation, both engine cooling fans have there own ground path. The result is a parallel
circuit with both fans running at high speed.

The PCM commands the low speed cooling fans ON under the following conditions:

Engine coolant temperature exceeds approximately 104.25°C (220°F).

A/C refrigerant pressure exceeds 1447 kPa (210 psi).

IMPORTANT: The right and left cooling fan connectors are interchangeable.

When servicing the fans be sure that the connectors are plugged
into the correct fan.

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Cooling - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

After the vehicle is shut OFF if the engine coolant temperature at key-off is greater than
101°C (214°F) the low speed fans will run for a minimum of 60 seconds After 60 seconds, if
the coolant temperature drops below 101°C (214°F) the fans will shut OFF. The fans will
automatically shut OFF after 3 min. regardless of coolant temperature.

The PCM commands the high speed fans ON under the following conditions:

Engine coolant temperature exceeds approximately 107.2°C (220°F).

A/C refrigerant pressure exceeds approximately 1824 kPa (265 psi).

When certain DTCs set.

At idle and very low vehicle speeds the cooling fans are only allowed to increase in speed if
required. This insures idle stability by preventing the fans from cycling between high and low
speed.

Coolant Level Control (If Equipped)

The engine cooling system contains an engine coolant level switch to alert the driver in the event
of a coolant loss. The powertrain control module (PCM) sends out a coolant loss signal over the
coolant level switch signal circuit. When the engine coolant level switch reads a low coolant level
in the fill tank, the switch opens. The message center receives its power from engine wiring
harness junction block on the battery positive voltage circuit. Ground is provided by the ground
circuits via the body wiring harness junction block and the engine wiring harness junction block.
The cluster receives the class 2 message from the PCM indicating Low Coolant and displays the
LOW COOLANT LEVEL message on the driver information center (DIC).

Coolant Heater

The optional engine coolant heater (RPO K05) operates using 110-volt AC external power and is
designed to warm the coolant in the engine block area for improved starting in very cold weather
-18°C (0°F). The coolant heater helps reduce fuel consumption when a cold engine is warming
up. The unit is equipped with a detachable AC power cord. A weather shield on the cord is
provided to protect the plug when not in use.

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 ENGINE Engine Cooling - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

Cooling System

The cooling system's function is to maintain an efficient engine operating temperature during all
engine speeds and operating conditions. The cooling system is designed to remove approximately
one-third of the heat produced by the burning of the air-fuel mixture. When the engine is cold, the
coolant does not flow to the radiator until the thermostat opens. This allows the engine to warm
quickly.

Cooling Cycle

Coolant is drawn from the radiator outlet and into the water pump inlet by the water pump.
Coolant will then be pumped through the water pump outlet and into the engine block. In the
engine block, the coolant circulates through the water pump outlet and into the engine block. In
the engine block, the coolant circulates through the water jackets surrounding the cylinders,
where it absorbs heat.

Some coolant is also pumped from the water pump to the heater core, then back to the water
pump. This provides the passenger compartment with heat and defrost.

The coolant is then forced through the cylinder head gasket openings and into the cylinder heads.
In the cylinder heads, the coolant flows through the water jackets surrounding the combustion
chambers and valve seats, where it absorbs additional heat.

Coolant is also directed to the throttle body. There it circulates through passages in the casting.
During initial start up, the coolant assists in warming the throttle body. During normal operating
temperatures, the coolant assists in regulating the throttle body temperature.

Cooling Cycle (6.6L Diesel Engine)

Coolant is drawn from the radiator outlet and into the water pump inlet by the water pump. The
coolant flows to the heater core while the engine is running. This provides the passenger
compartment with heat and defrost.

Coolant is then pumped through the water pump outlet and through the coolant pipe to the engine
oil cooler. The coolant flows around the oil cooler element and to the rear engine cover. The rear
engine cover distributes the coolant flow to both banks of the engine block. In the engine block,
the coolant circulates through the water jackets surrounding the cylinders where it absorbs heat.

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Cooling - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

Coolant is also directed to the turbocharger. There it circulates through passages in the center
housing. During engine warm-up cycle the bypass valve located in the turbocharger inlet hose at
the outlet pipe prevents coolant flow. During normal operating temperatures, the coolant assists in
keeping the turbocharger cool.

From the cylinder heads, the coolant flows to the thermostats. The coolant flows from the
thermostat housing to the water pump through the bypass pipe until the enginereaches 85°C (185°
F).

Operation of the cooling system requires proper functioning of all cooling system components.
The cooling system consists of the following components:

Coolant

The engine coolant is a solution made up of a 50-50 mixture of DEX-COOL and suitable
drinking water. The coolant solution carries excess heat away from the engine to the radiator,
where the heat is dissipated to the atmosphere.

Radiator

The radiator is a heat exchanger. It consists of a core and two tanks. The aluminum core is a tube
and fin crossflow design that extends from the inlet tank to the outlet tank. Fins are placed around
the outside of the tubes to improve heat transfer to the atmosphere.

The inlet and outlet tanks are a molded, high temperature, nylon reinforced plastic material. A
high temperature rubber gasket seals the tank flange edge to the aluminum core. The tanks are
clamped to the core with clinch tabs. The tabs are part of the aluminum header at each end of the
core.

The radiator also has a drain cock located in the bottom of the left hand tank. The drain cock unit
includes the drain cock and drain cock seal.

The radiator removes heat from the coolant passing through it. The fins on the core transfer heat
from the coolant passing through the tubes. As air passes between the fins, it absorbs heat and
cools the coolant.

Surge Tank

The surge tank is a plastic tank with a threaded pressure cap. The tank is mounted at a point
higher than all other coolant passages. The surge tank provides an air space in the cooling system
that allows the coolant to expand and contract. The surge tank provides a coolant fill point and a
central air bleed location.

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Cooling - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

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Chevrolet Silverado / GMC Sierra. Manual - part 1685