For 1990-2009 cars

### Auxiliary Brake Indicator Lamp

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 multi-function 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.

The most likely causes of AUX BRAKE indicator illumination are as follows:

• | Fluid flow is detected in the hydro-boost system within 4 seconds of engine start. For example, the power brake booster fluid flow switch is stuck open.

• | The MFAM does not detect any voltage on the brake booster pump motor supply voltage circuit when the power brake booster pump motor relay is supposed to be energized. For example, the power brake booster pump motor relay contacts are corroded or stuck open.

### Primary Brake Indicator Lamp

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 multifunction 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.

The most likely causes of PRIMARY BRAKE indicator illumination are as follows:

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

• | The power brake fluid flow switch remains closed after 4 seconds of engine running due to the fluid switch being stuck closed or a low fluid flow condition existing in the power steering/brake assist system.