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Cooling Fan Always On (LCD, LBX or LBS)

Cooling Fan Always On (LCD, LBX or LBS)

Diagnostic Instructions


• Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• Diagnostic Procedure Instructions provides an overview of each diagnostic category.


Diagnostic Fault Information

Cooling Fan Left High Speed Relay

Circuit Short to Ground High resistance Open Short to Voltage Signal Performance
Relay Coil Ignition

P0691 00

P0691 00

P0480 00

-

-

Relay Switch B+

1

1

1

-

-

Relay Coil Control

P0691 00

P0691 00

P0480 00

P0692 00

-

Relay Output Control

1

1

1

2

-

1. Low and medium speed fan inoperative without a DTC.

2. Medium speed fan always ON without a DTC.



Cooling Fan Left Low Speed Relay, Cooling Fan Speed Control Relay and Cooling Fan Right Low Speed Relay

Circuit Short to Ground High resistance Open Short to Voltage Signal Performance
Relay Coil Ignition

P0693 00

P0693 00

P0481 00

-

-

Relay Switch B+

1

1

1

-

-

Relay Coil Control

P0693 00

P0693 00

P0481 00

P0694 00

-

Relay Output Control

1

1

1

2

-

1. Fans inoperative without a DTC.

2. Medium speed fan always ON without a DTC.



Cooling Fan Medium Speed Relay

Circuit Short to Ground High resistance Open Short to Voltage Signal Performance
Relay Coil Ignition Circuit

P0695 00

P0695 00

P0482 00

-

-

Relay Switch B+ Circuit

1

1

1

-

-

Relay Coil Control

P0695 00

P0695 00

P0482 00

P0696 00

-

Relay Output Control

1

1

1

2

-

1. High speed fan inoperative without a DTC.

2. High speed fan always ON without a DTC.



Circuit/System Description

The engine control module (ECM) commands the fans ON in either high speed, medium speed, or low speed, depending on cooling requirements.

Low Speed

In low speed operation, the ECM applies ground to the coil side of the following relays with the cooling fan relay 1 control circuit:


• Cooling Fan Left Low Speed Relay

• Cooling Fan Right Low Speed Relay

• Cooling Fan Speed Control Relay


This energizes the relay coils. The low speed cooling fan relays supply power to the appropriate cooling fan motor through an internal fan resistor. The fan speed control relay contacts close and supply a direct ground path for the left cooling fan motor. The right cooling fan motor is connected to a dedicated ground. This results in both fans operating at low speed.

Medium Speed

In medium speed operation, the ECM commands the low speed fan relays, and the speed control relay off. The ECM simultaneously commands both of the cooling fan high speed relays on with the cooling fan relay 2 control circuit. The cooling fan motors are supplied power directly from the cooling fan high speed relays. The right cooling fan is connected to a dedicated ground, and operates at high speed. The left cooling fan is supplied a ground through the right cooling fan resistor and fan motor, which is connected to the de-energized cooling fan speed control relay. This results in the right cooling fan operating at a reduced speed.

High Speed

In high speed operation, the ECM applies a ground to the coil side of all five cooling fan relays. When energized, the high speed relays apply voltage directly to the cooling fans through the switch side of the relays. The energized cooling fan speed control relay provides a direct ground circuit for the left cooling fan. This arrangement allows each fan to receive full voltage and operate at high speed.

Reference Information

Schematic Reference

Engine Cooling Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Cooling Fan Description and Operation (LHU/A20NFT, LAU/A28NER)

Electrical Information Reference


• Circuit Testing

• Connector Repairs

• Testing for Intermittent Conditions and Poor Connections

• Wiring Repairs


Circuit/System Verification


1. Ignition OFF, disconnect the appropriate cooling fan relay.

2. If any of the above DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle .

3. Ignition ON, command the appropriate fan relay ON and OFF with a scan tool. Verify that the fans turn ON and OFF when changing between the commanded states.


Circuit/System Testing


1. Ignition OFF, disconnect the appropriate cooling fan relay.

2. Ignition ON, verify that the cooling fan is not ON.

3. If the fan is ON, test for a short to voltage in the relay controlled output circuit terminal 8.

4. If all circuits test normal, test or replace the cooling fan relay.


Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.


• Relay Replacement (Attached to Wire Harness)

• Engine Coolant Fan Motor Replacement (Diesel) Engine Coolant Fan Motor Replacement (LAU/A28NER)