For 1990-2009 cars

### Circuit Description

The primary brake system consists of the following:

• | 4 wheel disc hydraulic brake system with 4 channel ABS.

• | Hydro-boost brake power assist system which utilizes hydraulic pressure form the power steering pump.

The primary brake indicator illuminates when ground is applied to the lamp. A brake fluid level switch, located in the master cylinder fluid reservoir, supplies ground to the primary brake indicator if the brake fluid drops below a minimum level. The multi-function alarm module performs a diagnostic after the engine has been running for 4 seconds in order to verify the power brake booster fluid flow switch has opened, indicating hydraulic fluid flow. On Allison automatic transmissions, the EBCM may also illuminate the PRIMARY BRAKE indicator if DTC C0287 is set.

### Diagnostic Aids

The most likely causes of primary brake indicator illumination are as follows:

• | The brake fluid level switch is closed, possible worn brake pads or external brake fluid leak.

• | The power brake booster fluid flow switch remains closed after 4 seconds of engine running, power brake booster fluid flow switch is stuck closed or a low fluid flow condition exists in the power steering/brake assist system.

Thoroughly inspect connections or circuitry that may cause an intermittent malfunction. Refer to Testing for Electrical Intermittents , Testing for Intermittent Conditions and Poor Connections , Wiring Repairs and Connector Repairs in Wiring Systems.

### Test Description

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

1. This step is only necessary if the vehicle is equipped with an Allison automatic transmission. For vehicles not so equipped, go to step 4.

2. The AUX BRAKE indicator may remain ON when the power brake booster fluid flow switch is disconnected. This is a normal condition.

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

1. Turn OFF the ignition for 5 seconds.
2. Start the engine while observing the PRIMARY BRAKE indicator.

Does the PRIMARY BRAKE indicator illuminate for approximately 4 seconds and then turn OFF?

Go to Diagnostic Aids

Go to Step 3

3 |

1. Install a scan tool.
2. Select the Hydraulic 4WAL 4 Sensor Display DTCs function on the scan tool.

Is DTC C0287 set in EBCM memory?

Go to DTC C0287 in Antilock Brake System

Go to Step 4

4 |

1. Turn OFF the ignition.
2. Disconnect the brake fluid level switch harness connector.
3. Start the engine.

Does the PRIMARY BRAKE indicator illuminate for approximately 4 seconds and then turn OFF?

Go to Step 10

Go to Step 5

5 |

1. Turn OFF the ignition.
2. Disconnect the power brake booster fluid flow switch harness connector.
3. Start the engine.

Does the PRIMARY BRAKE indicator illuminate for approximately 4 seconds and then turn OFF?

Go to Step 11

Go to Step 6

6 | Test the brake booster fluid flow 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 15 | Go to Step 7
7 |

1. Turn OFF the ignition.
2. Reconnect the power brake booster fluid flow switch harness connector.
3. Disconnect the 24-way EBCM harness connector.
4. Start the engine.

Does the PRIMARY BRAKE indicator illuminate for approximately 4 seconds and then turn OFF?

Go to Step 12

Go to Step 8

8 |

1. Turn OFF the ignition.
2. Disconnect the multifunction alarm module (MFAM) harness connector.
3. Start the engine.

Does the PRIMARY BRAKE indicator illuminate?

Go to Step 9

Go to Step 13

9 |

1. Turn OFF the ignition.
2. Disconnect from the instrument panel cluster, the harness connector containing the brake warning indicator control circuit. Refer to Instrument Cluster Replacement in Instrument Panel, Gages, and Console.
3. Test the brake warning indicator control 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 15

Go to Step 14

10 | Replace the brake fluid level switch. Refer to Brake Fluid Level Indicator Switch Replacement . Did you complete the replacement? | Go to Step 15 | --
11 | Replace the power brake booster fluid flow switch. Refer to Fluid Flow Switch Replacement . Did you complete the replacement? | Go to Step 15 | --
12 | Replace the EBCM. Refer to Electronic Brake Control Module Replacement in Antilock Brake System. Did you complete the replacement? | Go to Step 15 | --
13 | Replace the multifunction alarm module. Refer to Multifunction Alarm Module Replacement in Instrument Panel, Gages, and Console. Did you complete the replacement? | Go to Step 15 | --
14 | Replace the instrument panel cluster. Refer to Instrument Cluster Replacement in Instrument Panel, Gages, and Console. Did you complete the replacement? | Go to Step 15 | --
15 |

1. Turn OFF the ignition for 5 seconds.
2. Start the engine while observing the PRIMARY BRAKE indicator.

Does the PRIMARY BRAKE indicator illuminate for approximately 4 seconds and then turn OFF?

System OK

Go to Step 3