10.24
ELECTRICAL
Fan Control Circuit Operation / Testing
The cooling fan is controlled through the Power Distribution
Module and thermistor. Perform the tests in the next two
sections if you are experiencing problems with the cooling fan
operation.
Fan Control Circuit Bypass Test
1. Disconnect harness from the coolant temperature sensor
(thermistor) on the engine. See image below for location
reference.
2. With the vehicle in neutral and the parking brake applied,
turn the ignition key on. The fan should start running.
3. If the fan does not run or runs slowly, check the fan motor
wiring, connections, ground path, 12V power input, and fan
motor condition. (Refer to “Fan Motor Current Draw” in
the next section of this chapter) Repair or replace as
necessary. If the fan runs with the sensor harness
disconnected, but will not turn on when the engine is hot,
verify the coolant level is correct and check the coolant
temperature sensor and connector terminals
Coolant Temperature Sensor Test
(Non-EFI Models)
If the thermistor circuit is open the engine hot light and fan will
both come on. With engine cold, disconnect lead and measure
resistance of sensor between the two Yellow/Black connector
terminals. There should be no continuity or very high resistance
(see chart below).
THERMISTOR READING DURING OPERATION
Situation
Ohms
Reading
Diagnosis / Action
Hot light ON
& Fan ON
Above 50k
Open Thermistor / Replace
Thermistor
Hot light ON Below 185
Engine Overheat or Bad
Thermistor
Thermistor Test
Condition
Approx. Reading
(Ohms)
Temperature
Hot Light On 185Ω ± 3% 215° F (102° C)
Fan On (Temp + 245Ω ± 3% 195° F (91° C)
Fan Off (Temp -) 307Ω ± 3% 180° F (82° C)
Room Temperature 2000Ω ± 3% 77° F (25° C)
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