For 1990-2009 cars

### Circuit Description

The Auxiliary Brake System provides hydraulic fluid flow within the brake booster if the engine is OFF and the brake is applied. The system consists of the following 3 basic components:

• | The Power Brake Booster Pump Motor Relay

• | The Power Brake Booster Fluid Flow Switch

• | The Power Brake Booster Pump Motor

### Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. This step tests for proper function of the stop lamps relay.

2. This step tests the power brake booster fluid flow switch and the switch circuitry.

3. This step tests for proper function of the power brake booster pump motor relay.

Step | Action | Yes | No
--- | --- | --- | ---
Schematic Reference: Brake Warning System Schematics
1 | Did you review the Hydraulic Brake System Description and Operation and perform the necessary inspections? | Go to Step 2 | Go to Symptoms - Hydraulic Brakes
2 |

1. Turn OFF the ignition.
2. Apply the brake.

Does the power brake booster pump motor run when the brake is applied?

Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems

Go to Step 3

3 |

1. Disconnect the harness connector of the power brake booster pump motor relay, which is located in the Maxi-Fuse center.
2. Connect a test lamp between the brake switch signal circuit and a good ground.
3. Apply the brake.

Does the test lamp illuminate?

Go to Step 4

Go to Step 13

4 | Connect a test lamp between the battery positive voltage circuit of the power brake booster pump motor relay and a good ground. Does the test lamp illuminate? | Go to Step 5 | Go to Step 15
5 |

1. Disconnect the multifunction alarm module (MFAM) harness connector.
2. Connect a test lamp between the battery positive voltage circuit of the power brake booster pump motor relay and brake booster fluid flow switch signal circuit.

Does the test lamp illuminate?

Go to Step 6

Go to Step 7

6 |

1. Reconnect the MFAM and hydraulic brake relay harness connectors.
2. Back-probe the brake booster pump motor supply voltage circuit at the power brake booster pump motor relay harness connector with a test lamp connected to ground. Refer to Probing Electrical Connectors in Wiring Systems.
3. Apply the brake.

Does the test lamp illuminate?

Go to Step 10

Go to Step 22

7 | Test the brake booster fluid flow switch signal circuit and the ground circuit of the power brake booster fluid flow switch for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 24 | Go to Step 8
8 | Inspect for poor connections at the harness connector of the power brake booster fluid flow 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 24 | Go to Step 9
9 | Replace the power brake booster fluid flow switch. Refer to Fluid Flow Switch Replacement . Did you complete the replacement? | Go to Step 24 | --
10 | Test the brake booster pump motor 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 24 | Go to Step 11
11 | Test the ground circuit of the brake booster 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 24 | Go to Step 12
12 | Replace the brake booster pump motor. Refer to Hydraulic Brake Booster Pump Assembly Replacement . Did you complete the replacement? | Go to Step 24 | --
13 |

1. Disconnect the stoplamps relay harness connector.
2. Test the brake switch signal circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 24

Go to Step 14

14 | Connect a test lamp between the battery positive voltage circuit of the stop lamps relay and a good ground. Does the test lamp illuminate? | Go to Step 16 | Go to Step 15
15 | Repair the open or high resistance in the battery positive voltage circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | Go to Step 24 | --
16 | Connect a test lamp between the battery positive voltage circuit and the ground circuit of the stop lamps relay. Does the test lamp illuminate? | Go to Step 18 | Go to Step 17
17 | Repair the open in the ground circuit of the stop lamps relay. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | Go to Step 24 | --
18 |

1. Connect a test lamp between the stop lamp supply voltage circuit and a good ground.
2. Apply the brake.

Does the test lamp illuminate?

Go to Step 20

Go to Step 19

19 | Repair the open in the stop lamp supply voltage circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | Go to Step 24 | --
20 | Inspect for poor connections at the harness connector of the stop lamps 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 24 | Go to Step 21
21 | Replace the stop lamps relay. Did you complete the replacement? | Go to Step 24 | --
22 | Inspect for poor connections at the harness connector of the power brake booster 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 24 | Go to Step 23
23 | Replace the power brake booster pump motor relay. Did you complete the replacement? | Go to Step 24 | --
24 | Apply the brake. Does the power brake booster pump motor run when the brake is applied? | System OK | Go to Step 3