The ECM commands the vacuum pump relay ON to activate the power brake booster vacuum pump to maintain the vacuum needed when the brakes are applied.

Ignition ON

The ECM detects a short to voltage, short to ground or an open/high resistance on the relay coil control circuit.

DTC P258C and P258D are Type B DTCs.

DTC P258C and P258D are Type B DTCs.

Schematic Reference

Hydraulic Brake Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Cooling Fan Description and Operation

Electrical Information Reference

Scan Tool Reference

Control Module References for scan tool information

1. Ignition OFF, disconnect the K8 vacuum pump relay.
2. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal 87 and ground.

⇒	If the test lamp illuminates, test the control circuit for a short to voltage.

3. Verify that a test lamp illuminates between the B+ circuit terminal 86 and ground.

⇒	If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the ignition circuit fuse is open, test or replace the relay.

4. Verify that a test lamp illuminates between the B+ circuit terminal 30 and ground.

⇒	If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.

5. Verify that a test lamp does not illuminate between the control circuit terminal 85 and ground.

⇒	If the test lamp illuminates, test the control circuit for a short to voltage.

6. Disconnect the harness connector at the vacuum pump.
7. Test for less than 2 ohms between the vacuum pump ground circuit terminal 2 and ground.

⇒	If greater than the specified range, test the ground circuit for an open/high resistance.

8. Connect the harness connector at the vacuum pump.
9. Connect a 20A fused jumper wire between the B+ circuit terminal 30 and the control circuit terminal 87. Verify the vacuum pump is activated.

⇒	If the vacuum pump does not activate, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, test or replace the power vacuum brake booster.

10. Ignition OFF, test for infinite resistance between the control circuit terminal 85 and ground.

⇒	If less than the specified value, test the control circuit for a short to ground. If the circuit tests normal, replace the ECM

11. Disconnect the X2 harness connector at the ECM.
12. Test for less than 2 ohms between the control circuit terminal 85 at the pump relay cavity and terminal 48 at the ECM harness connector.

⇒	If greater than the specified range, test the control circuit for an open/high resistance.

13. If all circuits test normal, test or replace the power vacuum brake booster.

1. Ignition OFF, disconnect the relay.
2. Test for 60-180 ohms between terminals 85 and 86.

⇒	If the resistance is not within the specified range, replace the relay.

3. Test for infinite resistance between the following terminals:

•

30 and 86

•

30 and 87

•

30 and 85

•

85 and 87

⇒	If not the specified value, replace the relay.

4. Install a 30A fused jumper wire between relay terminal 85 and B+. Install a jumper wire between relay terminal 86 and ground.
5. Test for less than 2 ohms between terminals 30 and 87.

⇒	If greater than the specified range, replace the relay.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.