Automobile & Light Truck Certification Test A5 BrakesAnti-lock Brake System (ABS) Diagnosis and RepairMedium
During an ABS system bleeding procedure, a technician is using a scan tool to actuate the ABS pump motor and solenoid valves. Why is this step often necessary after replacing a component like the ABS hydraulic control unit?
- ATo reset the ABS control module's memory of past fault codes.
- BTo purge air trapped within the ABS hydraulic control unit's internal passages.
- CTo calibrate the wheel speed sensors to the new unit.
- DTo ensure proper brake pedal feel and travel.
Show answer & explanationAnswer & explanation
Correct answer: B. To purge air trapped within the ABS hydraulic control unit's internal passages.
Replacing an ABS hydraulic control unit introduces air into its internal passages, including the pump and valve circuits. Traditional bleeding methods cannot remove this trapped air. Actuating the pump and valves with a scan tool cycles fluid through these passages, allowing the air to be purged from the system.
Why the other options are wrong
- A. Resetting fault codes is a diagnostic step, not part of the hydraulic bleeding process, although it may be done afterward.
- C. Wheel speed sensors are passive or active components and do not require calibration to the ABS unit during bleeding.
- D. While proper bleeding contributes to pedal feel, the primary reason for scan tool actuation is specifically to remove air from the ABS HCU, which is inaccessible otherwise.
ABS Scan Tool Bleeding
A procedure using a diagnostic scan tool to actuate the ABS pump and solenoid valves, forcing air out of the hydraulic control unit.
- Necessary after replacing ABS HCU, master cylinder, or extended system opens.
- Traditional bleeding methods cannot remove air from internal ABS components.
- Ensures proper brake pressure modulation and prevents a spongy pedal.
Memory trick: After a module swap, bleed the air with the scan tool's clap!