DTC 14

The EBCM monitors every brake apply for a proper sequence of events in the hydraulic unit. The following switches are used to verify each apply.

BRAKE LIGHT SWITCH - BLS

The brake light switch, located on the brake pedal bracket, is normally open and is provided power by the Control Pressure Switch (CPS) fuse (fuse #8). When the brake pedal is applied, the switch closes and provides a 12 volt signal to EBCM pin 25 and Body Computer Module (BCM) pin 1D2.

PISTON TRAVEL SWITCHES - S2A AND S2B

Two piston travel switches are located in the booster reed block assembly. They are monitored on circuit 886 (EBCM pin 31), which is provided 5 volts by the EBCM. When the brakes are not applied, this line is held to ground by the piston travel switches as shown in the above schematic.

When the brakes are partially applied, switch S2A opens. Due to internal resistance in the reed block, EBCM pin 31 is then held at 2.5 volts (S2A open and S2B closed).

When the brake pedal approaches full stroke, switch S2B opens. With both S2A and S2B open, a path to ground no longer exists for circuit 886 and EBCM pin 31 is forced to 5 volts. The EBCM monitors voltage on pin 31 to determine status of the piston travel switches.

CONTROL PRESSURE SWITCH - S1

The EBCM provides the control pressure switch circuit (circuit 848, EBCM pin 16) 5 volts at all times. When the brakes are not applied, S1 is closed, completing a path to ground for circuit 848 and holding EBCM pin 16 at 0 volts ground.

When the brakes are applied enough to generate approximately 400 psi in the booster circuit, S 1 opens, removing the path to ground and forcing circuit 848 to 5 volts. The EBCM monitors voltage on pin 16 to determine the position of the control pressure switch.

SWITCH SEQUENCE

When a normal brake apply is made, the following sequence of events should take place:

1. As soon as the pedal is depressed, the brake light switch should close, providing a 12 volt signal to EBCM pin 25.

2. As the booster piston continues to move, switch S2A opens, forcing EBCM pin 31 to 2.5 volts.

3. As the brake is applied further, the control pressure switch (S1) opens, holding EBCM pin 16 at 5 volts.

4. As full pedal travel is reached, switch S2B opens forcing EBCM pin 31 to 5 volts.

FAILURE CONDITIONS

EBCM Codes 13, 14 and 15 will set if an improper switch sequence is detected. The specific conditions required to set each code are as follows:

Code 13 - S2B opens before S1. Code 14 - S1 opens before S2A. Code 15 - S1 opens before Brake Light Switch (BLS) closes or S2B opens before BLS closes.

Air in the brake system will sometimes cause ABS codes 13, 14 or 15 to set due to additional booster piston travel requirements.