Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairMedium
A technician is performing a fluid flush on an ABS-equipped vehicle. After connecting a pressure bleeder, they are advised to cycle the ABS solenoids and pump motor using a scan tool. Why is this step necessary for a complete fluid flush in some ABS systems?
- ATo purge old fluid from the ABS hydraulic control unit's internal passages and accumulators.
- BTo reset the brake fluid level sensor in the reservoir.
- CTo calibrate the wheel speed sensors after the fluid change.
- DTo prevent air from entering the master cylinder during flushing.
Show answer & explanationAnswer & explanation
Correct answer: A. To purge old fluid from the ABS hydraulic control unit's internal passages and accumulators.
The ABS HCU contains internal passages, valves, and accumulators that hold old fluid. Simply bleeding the calipers will not remove this fluid. Cycling the ABS components with a scan tool forces the old fluid out of these internal areas, ensuring a complete flush.
Why the other options are wrong
- B. The brake fluid level sensor is a passive component and does not require reset during a fluid flush.
- C. Wheel speed sensor calibration is not directly related to a fluid flush procedure.
- D. A pressure bleeder helps prevent air entry, but cycling ABS components is for fluid removal from the HCU itself.
ABS HCU Fluid Flush
A procedure requiring a scan tool to activate the ABS pump motor and solenoid valves during a brake fluid flush, ensuring all old fluid is removed from the Hydraulic Control Unit's internal passages.
- ABS HCU traps old fluid
- Scan tool activation is required
- Ensures complete fluid replacement
Memory trick: ABS needs cycles, scan tool's the key, to flush out old fluid from the HCU!