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Cooling Fan Description and Operation (Diesel)

Cooling Fan Description and Operation (Diesel)



The engine cooling fan system consists of 2 electrical cooling fans and 5 fan relays, 2 in UEC and 3 in the relay box, providing 4 different speed levels. The relays are arranged in a series parallel (S/P) configuration that allows the engine control module (ECM) to operate both fans together at low or high speeds, depending on cooling requirements. The cooling fans and fan relays receive battery positive voltage from the underhood fuse block. The ground path is provided at G104.

The engagement of the different steps is depending either on coolant temperature or A/C pressure. If the coolant temperature is too high at ignition off there will be an afterrun for maximum 2 minutes.

At step 1 (lowest speed) ECM activates relay K9 in UEC and the fan motors are in serial circuit. One motor is without pre-resistor, the other one with pre-resistor.

At step 2 ECM activates the relays in the relay box and both fan motors are connected in parallel. Both motors have pre-resistors.

At step 3 ECM activates relay K9 in UEC and the relays in the relay box and the motors are in serial circuit, one with and one without pre-resistor.

At step 4 ECM activates K9 and K13 in UEC as well as the relays in the relay box and the motors will be in parallel with no pre-resistor.

Refer to Engine Cooling Schematics .