ABS System Self-Diagnostics

Fig. 33 - Power Up Circuitry.:

DURING POWER UP
The anti-lock system is in stand-by mode with the ignition switch in "Off" or "Accessory" position. When the switch is moved to the "Start" position, voltage through the switch activates the indicator lights. The Electronic Control Unit (ECU) and other system components are still in the stand-by mode. The indicator lights illuminate as part of the self test feature and remain on until the ignition switch is moved to the "run/on" position (fig. 33).

While in the "run/on" position, battery voltage is supplied through the ignition switch to power up the system ECU. While the vehicle is motionless (no wheel speed inputs), parking brake released and immediately after the switch is moved to the run/on" position, the pressure modulator solenoid valves (isolation, decay and build) are activated and briefly exercised by the ECU. This serves two purposes:

^ It checks solenoid valve function as part of the self diagnostic feature.

^ It exercises the solenoid valves. This is important because other than during system power up mode the solenoid valves only get energized by the ECU during anti-lock braking mode of operation.

It should be noted that if the brake pedal is depressed during the power up sequence the brake pedal may exhibit a slight pulsation. Also three clicks may be heard as the solenoids are activated. This is considered normal and does not require service.