Dynasty L4-153 2.5L SOHC VIN K TBI (1992)
Sensor Block: Description and Operation
Booster Piston Travel Switch Operation
Two switches, designated S2a and S2b, are located in the hydraulic assembly reed block for the purpose of signaling booster piston travel to the
Anti-lock Brake Control Module (ABCM). These two switches are used by the ABCM along with the control pressure switch S1 and the brake light
switch to determine if a proper brake application has occurred. If the switches do not indicate a proper brake application, the ABCM records a fault
code, and turns on the BRAKE warning lamp.
A two pin connector from the sensor block to the reed block is located behind the sensor block cover and is orange in color. The reed block is an integral
part of the booster/master cylinder and is not serviceable.
Normal switch sequence is as follows:
1. The brake light switch is activated upon application of the brake.
2. As the master cylinder pistons begin to move, it causes switch S2a to open, indicating initial piston travel.
3. As brake pedal force is increased, the master cylinder pistons continue to travel and pressure builds in the booster servo circuit. When booster
servo pressure reaches approximately 400 psi, the control pressure switch Sl opens.
4. As the pistons approach the bottom of their available travel, switch S2b opens.
By the time the S2b switch opens, often the wheels have reached a locked position and the system has entered anti-lock mode. However, if the vehicle
has not entered anti-lock mode, opening of switch S2b will cause the replenishing valve to be activated by the ABCM. This will cause fluid to be
redirected to the master cylinder circuits from the booster servo circuit in order to provide additional brake fluid displacement capability. There may be a
noticeable "clunk" in the brake pedal due to slight pressure differences between the master cylinder circuit and the booster servo circuit as the
replenishing valve is activated.
The settings of the switches have been carefully designed and optimized to the specific characteristics of the vehicle. Since the sequence is sensitive to
piston travel (fluid displacement), anything that affects fluid displacement may trigger erroneous fault detection. This usually causes switch S2b to open
(deep piston travel) prior to switch S1 (normal boost pressure). These conditions include:
1. Soft brake linings (replace pads with high quality or OEM parts)
2. Air in the wheel brake circuits (bleed brakes to correct problem)
3. Setting brake pads by pumping the brake pedal with the ignition on after a brake lining change (leave key off when pumping pedal to set caliper
pistions)
