CDL Air Brakes TestAir Brake System PartsMedium
A driver is performing a pre-trip inspection. After turning on the engine, they notice the Anti-lock Braking System (ABS) indicator light illuminates briefly and then turns off. What does this behavior indicate?
- AThe ABS system is performing a normal self-test and is functioning correctly.
- BThere is a low air pressure condition in the ABS modulator.
- CThe ABS system is disconnected and not operational.
- DThe ABS system is faulty and requires immediate attention.
Show answer & explanationAnswer & explanation
Correct answer: A. The ABS system is performing a normal self-test and is functioning correctly.
It is normal for the ABS indicator light to illuminate briefly when the engine starts (or ignition is turned on) as the system performs a self-test. If the light then turns off, it indicates that the ABS system has successfully completed its self-test and is functioning correctly.
Why the other options are wrong
- B. Low air pressure conditions are typically indicated by separate low air warning lights/buzzers, not directly by the ABS light in this manner.
- C. If the ABS system were disconnected, the light would likely remain off or on constantly, depending on the fault detection logic, but not perform a brief self-test and turn off.
- D. If the ABS system were faulty, the light would typically remain illuminated, not turn off.
ABS Indicator Light Self-Test
The ABS indicator light normally illuminates briefly when the ignition is turned on or the engine starts, then extinguishes, signifying a successful system self-test and proper operation.
- Standard diagnostic procedure for ABS.
- Light remaining on indicates a fault.
- Light never coming on indicates a circuit or bulb fault.
Memory trick: Brief flash, then off, means ABS is ready to take off!