Power-up Self-Test

The EBCM is able to detect many malfunctions whenever the ignition is ON. However, certain failures cannot be detected unless active diagnostic tests are performed on the components. Shorted solenoid coil or motor windings, for example, cannot be detected until the components are commanded ON by the EBCM. Therefore, a power-up self-test is required at the beginning of each ignition cycle to verify correct operation of components before the various control systems can be enabled. The EBCM performs the first phase of the power-up self-test when the ignition is first turned ON. The system relay, solenoids and the ABS pump motor are commanded ON and OFF to verify proper operation and the EBCM verifies the ability to return the system to base braking in the event of a failure. The master cylinder pressure sensor performs a self-test by sending a series of specific voltage signals to the EBCM, each for a predetermined amount of time. This phase of the power-up self-test may be heard by the driver, depending on how soon the engine is cranked and started after turning ON the ignition. The second phase of the power-up self-test begins when the vehicle is driven at a speed greater than 16 km/h (10 mph) and the EBCM has not detected any TCS/VSES related malfunctions thus far. During this phase, the precharge pump is tested to verify the ability to build adequate pressure to perform brake pressure application during certain TCS and VSES events. When the brake switch indicates that the brake is not applied and the master cylinder pressure is detected as being low, the EBCM proceeds with the test. The EBCM isolates all of the wheels by closing the four isolation valves. The precharge pump is then commanded ON while the EBCM monitors the master cylinder pressure sensor input. The precharge pump must build approximately 248 kPa (36 psi) of hydraulic pressure within 1 second or the test is failed. If the EBCM uses brake pressure application to perform TCS or VSES prior to the second phase of the power-up self-test, the precharge pump is tested at this time and the second phase of the test is not required. Due to the fact that all of the wheels are isolated during the second phase of the test, the test must be aborted if the brake is applied while the test is being performed. Occasionally, the driver may detect this by experiencing a momentary hard pedal.