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After Boil Coolant Pump Inoperative

After Boil Coolant Pump Inoperative

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.


Circuit/System Description

The auxiliary coolant pump and cooling fan system provides a measure of prevention against fuel vaporization due to excessive engine temperatures. The engine control module (ECM) commands the auxiliary or after boil coolant pump ON when the engine coolant temperature exceeds a calibrated value after engine shut down. The ECM applies ground to the coil side of the auxiliary pump circuit board (PCB) relay. This energizes the coil and applies voltage directly to the pump motor through the switch side of the relay.

Diagnostic Aids

The auxiliary coolant pump relay is a circuit board (PCB) relay. The relay is an integral component to the underhood bussed electrical center (UBEC). It cannot be removed and it is not visible.

Reference Information

Schematic Reference

Engine Cooling Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Cooling System Description and Operation

Electrical Information Reference


• Circuit Testing

• Connector Repairs

• Testing for Intermittent Conditions and Poor Connections

• Wiring Repairs


Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification


1. Ignition ON, observe the DTC information with a scan tool. Verify that DTC P2600, P2602, or P2603 is not set.

2. If any auxiliary coolant pump DTC is set, repair the DTC first. Refer to DTC P2600-P2603 .

3. Ignition ON, command the auxiliary coolant pump relay ON and OFF with a scan tool. Verify that the pump turns ON and OFF when changing between the commanded states.


Circuit/System Testing


1. Ignition OFF, disconnect the harness connector at the G8 auxiliary coolant pump.

2. Ignition ON, verify that a test lamp illuminates between the G8 pump ground circuit terminal 1 and B+.

3. If the test lamp does not illuminate, test the ground circuit for an open/high resistance.

4. Connect a test lamp between the G8 pump voltage supply circuit terminal 2 and ground. Command the G8 pump on with a scan tool. The test lamp should illuminate.

5. If the test lamp does not illuminate, test the voltage supply circuit for a short to ground or an open/high resistance. If the circuit tests normal and the AFTERBOIL fuse is OK, test or replace the X50A UBEC.

6. If the circuit tests normal and the AFTERBOIL fuse is open, test or replace the G8 auxiliary coolant pump.

7. If all of the circuits/connections test normal, replace the G8 auxiliary coolant pump.


Component Testing


1. Ignition OFF, disconnect and remove the X50A UBEC.

2. Remove the KR75 powertrain relay.

3. Test for 170-210 Ω between the powertrain relay switched output circuit terminal 87 and the auxiliary coolant pump relay control circuit at UBEC terminal X2-21.

4. If not within the specified range, replace the X50A UBEC.

5. Test for greater than 2 Ω between UBEC B+ supply cable and the AFTERBOIL PUMP Fuse 4.

6. If less than the specified range, replace the X50A UBEC.

7. Connect a 10 A fused jumper wire from the positive terminal of the battery to the powertrain relay switched output circuit terminal 87. Connect a jumper wire from the negative terminal of the battery to the auxiliary coolant pump relay control circuit at UBEC terminal X2-21. Test for less than 2 Ω between UBEC B+ supply cable and the AFTERBOIL PUMP Fuse 4.

8. If greater than the specified range, replace the X50A UBEC.


Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.


• Front Compartment Fuse Block Replacement

• Auxiliary Water Pump Replacement (LAU)