Engineering Analysis: The insured’s vehicle was equipped with an anti-lock brake system. Like all such systems, the ABS senses wheel slip and “takes control” of vehicle braking when conditions warrant. Normally, the system applies and releases each individual brake mechanism many times each second until the slipping wheel or wheels regain traction. During this brief period, the system is essentially closed to additional brake pedal pressure, maintaining instead pedal pressure that was applied immediately before wheel slip. On the incident vehicle, however, a faulty wheel speed sensor reported wheel slippage when in fact, no such slippage was occurring. This caused the ABS to activate on each and every brake application. Moreover, because the system essentially “isolates” the operator from the brake system while the ABS is engaged, the operator could not increase braking effort beyond the initial application pressure. As a result, the operator could not respond to changing braking requirements without first releasing, then reapplying the brake pedal.
Result: Consistent with the operator’s statement, the investigation revealed a brake system malfunction that resulted in potentially increased stopping distance and reduced effectiveness.