Initial Inspection and Diagnostic Overview

Pedal Feel/Vehicle Characteristics

There are several pedal feel/vehicle characteristics that are considered normal for antilock braking that may require further explanation.
When stopping conditions activate the antilock brakes, the driver may feel some vibrations/pulsations in the brake pedal and may hear the electric solenoid valves clicking and the pump motor running.
The vibrations/pulsations are caused by the isolating, building and decaying of brake fluid pressure within the brake lines. The ABS system prevents complete wheel lock-up, but some wheel slip is required for the best braking performance. This slip may result in some tire chirping, depending on the road surface. The chirping should not be interpreted as total wheel lock-up (although wheel lock-up may occur below 5 Km/h (3 mph). While complete wheel lock-up leaves black tire marks on dry pavement, antilock braking may leave some light marks.
At the end of an ABS stop, the ABS system may not function below 5 Km/h (3 mph). There may be a slight brake pedal drop anytime the ABS is deactivated.
In case of braking on a bumpy surface, the ABS system may detect wheel locking tendencies due to wheel hop and cycle ABS. In this scenario the brake pedal may pulsate with a perceived loss of deceleration. ABS braking may also be activated at times while on dry pavement with sand, gravel, or other loose debris on the road.
It should be noted that the pulsating pedal feel characteristic will not illuminate the brake warning lamps or set a trouble code that is stored in the Controller-Antilock Brake (CAB). When investigating a hard pedal feel, inspect the sensor and tone wheel teeth for chips/broken teeth, damaged sensor pole tips, excessive runout of the tone wheel, or excessive gap.