Hummer H1 Service Manual page 185

4-2 Cooling System
TRANSFER CASE
OIL COOLER
TRANSMISSION OIL
COOLER BYPASS
VALVE
When the thermostat detects the coolant is hot, the thermostat
opens allowing coolant to be pumped to the radiator where it is
routed through a series of fins and tubes. A fan draws in out-
side air to help dissipate the heat before the coolant is recircu-
lated back through the engine (Figure 4-3).
AIR CONDITIONING
CONDENSER
(IF EQUIPPED)
POWER
FAN
STEERING
RADIATOR
OIL
SHROUD
COOLER
Figure 4-3: Cooling System
The heater provides heat to warm the interior of the vehicle for
passenger comfort. Hot coolant leaves the engine and passes
through a heater coil. A fan draws air through the heated coil
which heats the air. The air is then channeled through a plenum
chamber that directs the flow of the heated air into the passen-
ger compartment.
If the engine is generating more heat than the cooling system
can effectively dissipate, there may be a coolant or cooling sys-
tem problem. The engine temperature sending unit transmits a
signal indicating the coolant temperature to the temperature
____________________________________________________________
ENGINE TEMPERATURE
SENDING UNIT
Figure 4-2: Cooling System Routing
ENGINE OIL/
TRANSMISSION
OIL COOLER
WATER
PUMP
DRAINCOCK
THERMOSTAT
SURGE TANK
HEATER
gauge on the instrument panel to let the driver know that the
system should be serviced as soon as possible. Engine coolant
is added to the system through the filler cap in the surge tank.
After adding or refilling the diesel engine cooling system, open
the valve on the water crossover and bleed the air from the sys-
tem.
Like the radiator, the oil cooler also uses a series of fins and
tubes to dissipate heat from fluid. The lower half of the oil
cooler dissipates heat from the engine oil while the upper half
cools transmission fluid.
Engine oil flows from the left rear of the engine block to the
bottom half of the oil cooler, then returns to the engine block.
NOTE: To ensure an accurate engine oil level measurement
check the engine oil within 1 to 2 minutes after shut down.
The transmission fluid is cooled by the top half of the split oil
cooler. Transmission fluid flows from the transmission oil out-
let to the top half of the oil cooler, then to the inside of the
transfer case, where it returns to the transmission (Figures 4-2
and 4-3). The transmission fluid that passes through the trans-
fer case does not mix with the transfer case fluid, but flows
through a separate oil (secondary) cooler located within the
transfer case.
A transmission cooler bypass valve is positioned between the
transmission and transfer case cooler in the fluid line. The unit
is rigidly mounted to the frame rail and is connected in parallel
by hoses to the transmission cooler. The valve consists of a
housing with inlet and outlet fittings, and an internal flow con-
trol valve, spring loaded in the closed position. The valve is de-
signed to begin opening at approximately 20 psi and be fully
open by 30 psi allowing fluid to bypass the transmission cooler
and regulate fluid pressure in the cooler.
At low temperatures, fluid viscosity and pressure are typically
high. By allowing fluid to bypass the cooler at low tempera-
tures the transmission warms to normal operating temperature
WATER
CROSSOVER
OIL COOLER
RADIATOR
®