Technician A correctly states that scanning for codes is crucial when diagnosing an ABS system. Technician B is also correct that some wheel speed sensors produce an AC voltage. Both statements are accurate.
ABS (anti-lock braking systems), first developed in 1947, is a critical safety feature designed to provide controlled stops, rather than just shorter distances. The system relies on an ABS control module, motor relay, and wheel speed sensors, but requires functional “base brakes” to operate.
ABS works by monitoring wheel speed sensors. If a wheel stops spinning during braking, the electronic brake control module takes control, using pressure hold, increase, or decrease functions to allow the wheel to spin again. This process happens fifteen times per second, effectively pulsing the brakes faster than a human driver.
One type of wheel speed sensor generates an AC signal as a toothed ring spins, creating a high and low signal. This can be tested with a digital volt ohmmeter (DVOM) on the AC scale, measuring approximately 250 millivolts when the wheel is spun. Ohms of resistance can also be measured.
Diagnosing ABS problems often requires a scan tool to read chassis (C) codes and access data like wheel speeds and pressures. Bleeding ABS systems can be more complex, sometimes requiring a scan tool. Understanding these basics is important for passing the A-5 ASE brake exam.
ABS with John Gardner
