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Home >> Lincoln >> 2012 >> MKT Base, AWD >> Repair and Diagnosis (Single Page) >> Brakes >> Traction Control >> Anti-Lock Brake System - Vehicle Dynamic Systems >> Diagnosis And Testing >> Anti-Lock Control >> Pinpoint Tests >> Pinpoint Test M: Poor Vehicle Tracking During Anti-lock Function >> Normal Operation

Normal Operation

The ABS module uses input from several different sensors (wheel speed sensors, Brake Pedal Position (BPP) switch, Restraints Control Module (RCM)) to determine if 1 or more wheels are about to lock up. If a wheel lockup event is about to occur, the ABS module will modulate fluid pressure to the affected brake caliper. The ABS module modulates fluid pressure by opening and closing 4 pairs of solenoid operated valves - a dump (or outlet) valve and an isolation (inlet) valve for each wheel. The dump valve is normally closed and the isolation valve is normally open. The ABS module will first close off pressure from the master cylinder to the affected wheel by closing the isolation valve. If this action does not bring the wheel back from the edge of lockup, the ABS module will open the dump valve, relieving all pressure to the affected wheel. The ABS module will then alternate between the isolation and dump valve to maintain a pressure below lockup. Depending on conditions, the ABS module may pulse the valves up to 18 times per second. Fluid released from the system by the dump valves is sent to the accumulators - 1 for the front chamber of the master cylinder and 1 for the rear chamber. This fluid is returned to the master cylinder by the pump in the Hydraulic Control Unit (HCU). The ABS module shuts the pump off when wheel speeds return to normal.

During an ABS event (and normal braking), the front and rear end of the vehicle should track true to the direction of travel along the vehicle centerline unless driver input dictates otherwise, there is an internal issue with the HCU or an issue with the base brake system (caliper, tubes) that allows one or more wheels to lock up.

The operating voltage required to supply the ABS module, hydraulic pump, isolation and dump valves is in a range between 10 and 16 volts. Fused battery voltage is supplied to the ABS module by Battery Junction Box (BJB) fuse 9 (40A) along circuit SBB09 (RD) and by BJB fuse 12 (20A) along circuit SBB12 (GN/RD). Fused ignition voltage is supplied to the ABS module by Smart Junction Box (SJB) fuse 34 (5A) along circuit CBP34 (VT/BN). Ground is provided along circuit GD120 (BK/GN).

This pinpoint test is intended to diagnose the following: 

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