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

The multifunction alarm module (MFAM) performs a 4 second bulb check on the AUX BRAKE indicator immediately after the engine is cranked or started. The MFAM illuminates the indicator by supplying voltage to the lamp. During the bulb check, the MFAM also performs a diagnostic which tests the power brake booster pump motor relay, the power brake booster fluid flow switch and the power brake booster pump circuitry for electrical faults. The AUX BRAKE indicator remains illuminated if the MFAM detects an electrical fault in the system. The MFAM also sounds an audible warning when the AUX BRAKE indicator illuminates.

### Test Description

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

1. This step tests if the multifunction alarm module is supplying voltage to the brake warning indicator supply voltage 2 circuit.

2. This step tests for a 12 volt signal from the MFAM.

3. This step tests if the MFAM is capable of supplying ground to the brake booster fluid flow switch signal circuit.

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 | Crank or start the engine while observing the AUX BRAKE indicator. Does the AUX BRAKE indicator illuminate for 4 seconds and then turn OFF and remain OFF? | Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems | Go to Step 3
3 |

1. Turn OFF the ignition.
2. Disconnect the MFAM harness connector.
3. Turn ON the ignition.

Does the AUX indicator illuminate?

Go to Step 14

Go to Step 4

4 |

1. Turn OFF the ignition.
2. Reconnect the MFAM harness connector.
3. Apply the brake.

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

Go to Step 5

Go to Brake Booster Pump Inoperative

5 |

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. Crank or start the engine.

Does the test lamp illuminate for 4 seconds and then turn OFF?

Go to Step 7

Go to Step 6

6 |

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

Did you find and correct the condition?

Go to Step 18

Go to Step 16

7 |

1. Turn OFF the ignition.
2. Disconnect the power brake booster fluid flow switch harness connector.
3. Connect a test lamp between the brake switch signal circuit and the brake booster fluid flow switch signal circuit at the power brake booster pump motor relay harness connector.
4. Crank or start the engine.

Does the test lamp illuminate for 4 seconds and then turn OFF?

Go to Step 9

Go to Step 8

8 | Test the brake booster fluid flow switch signal circuit for an open between the power brake booster pump motor relay and the MFAM. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 18 | Go to Step 16
9 |

1. Turn OFF the ignition.
2. Reconnect the power brake booster pump motor relay harness connector.
3. Reconnect the power brake booster fluid flow switch harness connector.
4. Remove the circuit breaker from the brake booster pump motor voltage supply circuit.
5. Connect a test lamp between the brake booster pump motor voltage supply circuit, in the circuit breaker cavity and a good ground.
6. Crank or start the engine.

Does the test lamp illuminate for 4 seconds and then turn OFF?

Go to Step 11

Go to Step 10

10 | Repair the open in the brake booster pump motor voltage supply circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | Go to Step 18 | --
11 | Use a DMM in order to test the circuit breaker, removed in step 8, for continuity. Refer to Circuit Testing in Wiring Systems. Did you find and correct the condition? | Go to Step 18 | Go to Step 12
12 |

1. Turn OFF the ignition.
2. Reinstall the circuit breaker for the brake booster pump motor voltage supply circuit.
3. Use a test lamp connected to ground in order to back-probe the brake booster pump motor voltage supply circuit at the MFAM harness connector. Refer to Probing Electrical Connectors in Wiring Systems.
4. Crank or start the engine.

Does the test lamp illuminate for 4 seconds and then turn OFF?

Go to Step 16

Go to Step 13

13 | Repair the open or short to ground in the brake booster pump motor voltage supply circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you complete the repair? | Go to Step 18 | --
14 |

1. Turn OFF the ignition.
2. Disconnect from the instrument panel cluster, the harness connector containing the brake warning indicator voltage supply 2 circuit. Refer to Instrument Cluster Replacement in Instrument Panel, Gages, and Console.
3. Turn ON the ignition.
4. Test the brake warning indicator voltage supply 2 circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 18

Go to Step 15

15 | Replace the instrument panel cluster. Refer to Instrument Cluster Replacement in Instrument Panel, Gages, and Console. Did you complete the replacement? | Go to Step 18 | --
16 | Inspect for poor connections at the MFAM harness connector. 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 18 | Go to Step 17
17 | Replace the MFAM. Refer to Multifunction Alarm Module Replacement in Instrument Panel, Gages, and Console. Did you complete the replacement? | Go to Step 18 | --
18 | Crank or start the engine while observing the AUX BRAKE indicator. Does the AUX BRAKE indicator illuminate for 4 seconds and then turn OFF and remain OFF? | System OK | Go to Step 2