Chrysler RAM TRUCK 2500 1994 Service Manual page 284

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COOLING SYSTEM 7 - 29
pump pressure to force the valve to open. Coolant
leakage into the pellet container will cause the ther­
mostat to fail i n the open position. Thermostats very
rarely stick. Do not attempt to free a thermostat with
a prying device.
The same thermostat is used for winter and sum­
mer seasons. A n engine should not be operated with­
out a thermostat, except for servicing or testing.
Operating without a thermostat causes longer engine
warmup time, unreliable warmup performance, in­
creased exhaust emissions and crankcase condensa­
tion that can result i n sludge formation.
CAUTION: Do not operate an engine without a ther­
mostat, except for servicing or testing.
T H E R M O S T A T O P E R A T I O N — E X C E P T DIESEL
The thermostat controls the operating temperature
of the engine by controlling the amount of coolant
flow to the radiator. The thermostat is closed below
88°C (192°F). When the coolant reaches this temper­
ature, the thermostat begins to open, allowing cool­
ant flow to the radiator. This provides quick engine
warm-up and overall temperature control. The ther­
mostat is designed to provide a minimum engine op­
erating temperature of 88 to 93°C (192 to 199°F). I t
should be fully open for maximum coolant flow dur­
ing operation i n hot ambient temperatures of approx­
imately 104°C (220°F). Above 104°C (220°F), coolant
temperature is controlled by the radiator, fan and
ambient temperature.
CAUTION: Never operate the diesel engine without
a thermostat. Without a thermostat, coolant will not
flow to radiator causing engine overheating.
ON-BOARD DIAGNOSTICS
FOR COOLING SYSTEM COMPONENTS
The powertrain control module (PCM) (Fig. 41) con­
tains certain On-Board Diagnostics (OBD) for cooling
system components. OBD will be used to help detect
a thermostat open-too-soon failure condition.
The Malfunction Indicator Lamp (formerly called
the Check Engine Lamp) will not be l i t by an open-
too-soon condition. But, i f the thermostat has failed
open, a Diagnostic Trouble Code (DTC) number 17
will be set and stored i n the PCM. Do not change a
thermostat for lack of heat as indicated by the in­
strument panel gauge or heater performance unless a
DTC number 17 is present. Refer to the Diagnosis
section of this group for other probable causes.
The more common type of thermostat failure, usu­
ally found on high milage vehicles, is a thermostat
failed i n the shut position. The temperature gauge (if
equipped) will give an indication of this condition.
Depending upon length of time that vehicle is oper­
ated, pressure cap may vent. This will expel steam
and coolant to coolant reserve/overflow tank and to
surface below vehicle. Refer to the Diagnosis section
of this group. Also refer to the DRB scan tool and the
appropriate Powertrain Diagnostic Procedures man­
ual.
Vehicles equipped with a 5.9L diesel engine do not
have the DTC number 17 malfunction indicator lamp
(check engine lamp) feature. On-Board Diagnostics
for the 5.9L diesel engine can be obtained through
the DRB scan tool and appropriate Powertrain Diag­
nostic Procedures manual.
To access the cooling system DTC, refer to Access­
ing Diagnostic Trouble Codes i n the Diagnosis section
of this group. Also refer to On-Board Diagnostics
(OBD) i n Group 14, Fuel Systems for more informa­
tion on other Diagnostic Trouble Codes.
T H E R M O S T A T R E M O V A L — 3 . 9 L V - 6 O R
5.2/5.9L V-8 G A S POWERED ENGINES
WARNING: DO NOT LOOSEN THE RADIATOR
DRAINCOCK WITH THE SYSTEM HOT AND PRES­
SURIZED. SERIOUS BURNS FROM THE COOLANT
CAN OCCUR.
Do not waste reusable coolant. I f the solution is
clean, drain the coolant into a clean container for re­
use.
I f the thermostat is being replaced, be sure that
the replacement is the specified thermostat for the
vehicle model and engine type.
Factory installed thermostat housings on 3.9L V-6
or 5.2/5.9L V-8 engines are installed on a gasket with
an anti-stick coating. This will aid i n gasket removal
and clean-up.
(1) Disconnect negative battery cable at battery.