In order to release the parking brake, the pump motor must supply fluid pressure to the actuator. When this pressure overcomes the spring tension inside of the actuator, the actuator piston moves, releasing the parking brake. The park brake pressure sensor monitors the fluid pressure whenever the ignition is ON.
Thoroughly inspect connections or circuitry that may cause an intermittent malfunction. Refer to the following:
The numbers below refer to the step numbers on the diagnostic table.
Illumination of the LED on PBCM indicates the PBCM is attempting to energize the park brake pump motor relay.
The PBCM may be damaged if the park brake motor relay control circuit is shorted to ground.
Step | Action | Yes | No |
|---|---|---|---|
Schematic Reference: Park Brake System Schematics | |||
1 | Did you perform the Parking Brake Diagnostic System Check? | Go to Step 2 | |
2 |
Does the parking brake pump motor run? | Go to Diagnostic Aids | Go to Step 3 |
Observe the LED on the side of the PBCM. Does the LED illuminate? | Go to Step 4 | Go to Step 15 | |
4 |
Does the test lamp illuminate? | Go to Step 5 | Go to Step 6 |
5 | Test the ground circuit of the park brake pump motor for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 29 | Go to Step 13 |
6 |
Did you find and correct the condition? | Go to Step 29 | Go to Step 7 |
7 | Connect a test lamp between the battery positive voltage circuit at the park brake pump motor relay harness connector and a good ground. Does the test lamp illuminate? | Go to Step 8 | Go to Step 22 |
8 | Connect a test lamp between the park brake pump motor relay control circuit, at the park brake pump motor relay harness connector, and a good ground. Does the test lamp illuminate? | Go to Step 10 | Go to Step 9 |
Did you find and correct the condition? | Go to Step 29 | Go to Step 25 | |
10 |
Did you find and correct the condition? | Go to Step 29 | Go to Step 11 |
11 | Inspect for poor connections at the harness connector of the park brake pump motor relay. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 29 | Go to Step 12 |
12 | Replace the park brake pump motor relay. Refer to Park Brake Pump Relay Replacement . Did you complete the replacement? | Go to Step 29 | -- |
13 | Inspect for poor connections at the harness connector of the park brake pump motor. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 29 | Go to Step 14 |
14 | Replace the park brake pump motor. Refer to Park Brake Pump Replacement . Did you complete the replacement? | Go to Step 29 | -- |
15 |
Does the test lamp illuminate? | Go to Step 25 | Go to Step 16 |
16 | Test the park brake pressure valve solenoid supply voltage circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 29 | Go to Step 17 |
17 |
Important: For vehicles not equipped with a park brake relay, proceed to Step 19 . Connect a test lamp between the battery positive voltage circuit, at the park brake relay harness connector, and a good ground. Does the test lamp illuminate? | Go to Step 18 | Go to Step 22 |
18 | Connect a test lamp between the park brake relay control circuit, at the park brake relay harness connector, and a good ground. Does the test lamp illuminate? | Go to Step 21 | Go to Step 19 |
19 |
Does the test lamp illuminate? | Go to Step 20 | Go to Step 22 |
20 |
Important: For vehicles not equipped with a park brake relay, proceed to Step 27 . Test the park brake relay control circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 29 | Go to Step 27 |
21 | Test the ground circuit of the park brake relay for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 29 | Go to Step 23 |
22 | Repair the open in the battery positive voltage circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | Go to Step 29 | -- |
23 | Inspect for poor connections at the harness connector of the park brake relay. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 29 | Go to Step 24 |
24 | Replace the park brake relay. Refer to Park Brake Relay Replacement . Did you complete the replacement? | Go to Step 29 | -- |
25 | Inspect for poor connections at the harness connector of the PBCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 29 | Go to Step 26 |
26 | Replace the PBCM. Refer to Park Brake Control Module Replacement . Did you complete the replacement? | Go to Step 29 | -- |
27 | Inspect for poor connections at the harness connector of the park brake switch. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 29 | Go to Step 28 |
28 | Replace the park brake switch. Refer to Instrument Panel Park Brake Switch Replacement . Did you complete the replacement? | Go to Step 29 | -- |
29 |
Does the parking brake pump motor run? | System OK | Go to Step 3 |
In order to release the parking brake, the pump motor must supply fluid pressure to the actuator. When this pressure overcomes the spring tension inside of the actuator, the actuator piston moves, releasing the parking brake. The park brake pressure sensor monitors the fluid pressure whenever the ignition is ON.
Thoroughly inspect connections or circuitry that may cause an intermittent malfunction. Refer to the following:
The numbers below refer to the step numbers on the diagnostic table.
The gear selector must be placed in any gear other than park or neutral in order for the PBCM to command the parking brake to release. Place the gear selector in reverse or drive with the wheels blocked for the duration of this diagnostic procedure.
Illumination of the LED on PBCM indicates the PBCM is attempting to energize the park brake pump motor relay.
The PBCM may be damaged if the park brake pump motor relay control circuit is shorted to ground.
This step ensures the PBCM is receiving a voltage signal requesting the parking brake be released.
This step tests for a shorted PNP switch.
A shorted transmission shift select switch signal circuit sends a signal to the PBCM indicating the gear selector is always in park or neutral.
Step | Action | Yes | No |
|---|---|---|---|
Schematic Reference: Park Brake System Schematics | |||
1 | Did you perform the Parking Brake Diagnostic System Check? | Go to Step 2 | |
Does the parking brake pump motor run? | Go to Diagnostic Aids | Go to Step 3 | |
Observe the LED on the side of the PBCM. Does the LED illuminate? | Go to Step 4 | Go to Step 15 | |
4 |
Does the test lamp illuminate? | Go to Step 5 | Go to Step 6 |
5 | Test the ground circuit of the park brake pump motor for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 33 | Go to Step 13 |
6 |
Did you find and correct the condition? | Go to Step 33 | Go to Step 7 |
7 | Connect a test lamp between the battery positive voltage circuit (at the park brake pump motor relay harness connector) and a good ground. Does the test lamp illuminate? | Go to Step 8 | Go to Step 32 |
8 | Connect a test lamp between the park brake pump motor relay control circuit (at the park brake pump motor relay harness connector) and a good ground. Does the test lamp illuminate? | Go to Step 10 | Go to Step 9 |
Did you find and correct the condition? | Go to Step 33 | Go to Step 29 | |
10 |
Did you find and correct the condition? | Go to Step 33 | Go to Step 11 |
11 | Inspect for poor connections at the harness connector of the park brake pump motor relay. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 33 | Go to Step 12 |
12 | Replace the park brake pump motor relay. Refer to Park Brake Pump Relay Replacement . Did you complete the replacement? | Go to Step 33 | -- |
13 | Inspect for poor connections at the harness connector of the park brake pump motor. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 33 | Go to Step 14 |
14 | Replace the park brake pump motor. Refer to Park Brake Pump Replacement . Did you complete the replacement? | Go to Step 33 | -- |
Does the test lamp illuminate? | Go to Step 29 | Go to Step 16 | |
16 |
Does the test lamp illuminate? | Go to Step 17 | Go to Step 18 |
17 | Test the park brake pressure valve solenoid supply voltage circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 33 | Go to Step 28 |
Does the pump motor run? | Go to Step 20 | Go to Step 19 | |
Test the transmission shift select switch signal circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 33 | Go to Step 21 | |
20 | Adjust or replace the PNP switch. Refer to Park/Neutral Backup Switch Adjustment or Park/Neutral Backup Switch Replacement in Automatic Transmission - Allison. Did you complete the repair? | Go to Step 33 | -- |
21 |
Does the test lamp illuminate? | Go to Step 23 | Go to Step 22 |
22 | Test the column shift relay control circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 33 | Go to Step 29 |
23 | Use a test lamp connected to ground in order to verify power at the ignition 1 circuit and at the battery positive voltage circuit of the column shift relay. Does the test lamp illuminate when connected to each circuit, one at a time? | Go to Step 25 | Go to Step 24 |
24 | Repair the open in the circuit which did not illuminate the test lamp. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | Go to Step 33 | -- |
25 | Test the park brake switch supply voltage circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 33 | Go to Step 26 |
26 | Inspect for poor connections at the connector of the column shift relay. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 33 | Go to Step 27 |
27 | Replace the column shift relay. Did you complete the replacement? | Go to Step 33 | -- |
28 | Inspect for poor connections at the harness connector of the park brake switch. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 33 | Go to Step 31 |
29 | Inspect for poor connections at the harness connector of the PBCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 33 | Go to Step 30 |
30 | Replace the PBCM. Refer to Park Brake Control Module Replacement . Did you complete the replacement? | Go to Step 33 | -- |
31 | Replace the park brake switch. Refer to Instrument Panel Park Brake Switch Replacement . Did you complete the replacement? | Go to Step 33 | -- |
32 | Repair the open in the battery positive voltage circuit. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you complete the repair? | Go to Step 33 | -- |
33 |
Does the parking brake pump motor run? | System OK | Go to Step 3 |