Brakes (Diagnostics): Description and operation

Diagnostics

While the ignition is on, the diagnostics function of the ABS ECU monitors the system for faults. In addition, the return pump is tested by pulsing it briefly immediately after the engine starts provided vehicle speed exceeded 4.4 mph (7 kph) during the previous ignition cycle. If a fault is detected at any time, the ABS ECU stores a related fault code in memory and illuminates the appropriate warning lamps in the instrument pack. If a fault exists in a warning lamp circuit, the lamp will not illuminate during the lamp check at ignition on, but, provided there are no other faults, the related function will otherwise be fully operational.

Checks performed by diagnostics

After detecting a fault, the ABS ECU selects an appropriate default strategy which, where possible, retains some operational capability. A shuttle valve switch fault, sticking throttle and implausible gear input are classified as permanent faults. If a permanent fault is detected, the related warning lamp illumination and default strategies are automatically employed in subsequent ignition cycles, even if the fault is intermittent, until the fault has been rectified and cleared from memory. If a non permanent fault is detected, the related warning lamp illumination and default strategies will only be employed in subsequent ignition cycles if the fault is still present.

After rectification of an ABS sensor fault, the ABS and ETC functions are disabled, and their warning lamps remain illuminated, until vehicle speed exceeds 9.4 mph (15 kph) (to allow additional checks to be performed).

Fault retrieval and diagnosis of the ABS ECU are done using Testbook.