Pinpoint Test S: Poor Vehicle Tracking During Anti-Lock Function

Anti-Lock Control

Pinpoint Test S: Poor Vehicle Tracking During Anti-Lock Function


Normal Operation

The ABS module uses input from several different sensors (wheel speed sensors, Brake Pedal Position (BPP) switch, sensor cluster, etc.) to determine if an ABS event is necessary. If an ABS event is necessary, the ABS module commands the hydraulic pump motor on and the Hydraulic Control Unit (HCU) to actuate the appropriate solenoids. During an ABS event, 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. Other items that could cause uneven braking during an ABS event are; an internal issue with the HCU (Hydraulic Control Unit), an issue with the base brake system (caliper, brake pads, etc.) or worn out suspension components.

The operating voltage required for ABS module, hydraulic pump and isolation valve operation is between 10 and 17 volts.

This pinpoint test is intended to diagnose the following:

- Air in hydraulic brake system

- Low tire pressure

- Base brake system

- HCU (Hydraulic Control Unit)

PINPOINT TEST S : POOR VEHICLE TRACKING DURING ANTI-LOCK FUNCTION

S1 VERIFY VEHICLE TRACKING DURING NON-ABS BRAKING

- NOTE: Before road testing the vehicle, check the tire pressure and confirm tire pressures are set to the pressure indicated on the VC (Vehicle Certification) label.

Test drive the vehicle and verify vehicle tracking during normal braking.


Does the vehicle track correctly during normal braking?

Yes
GO to S2.
No
The condition can be caused by a base brake concern or vehicle alignment. REFER to Brakes and Traction Control to verify the base brake system and REFER to Suspension &/or Alignment to verify suspension components.

S2 BLEED THE BRAKE SYSTEM

- Bleed the HCU (Hydraulic Control Unit).

- Test drive the vehicle and verify vehicle tracking during an ABS stop.

Does the vehicle track correctly during an ABS stop?

Yes
The brake system is operating correctly. The concern may have been caused by air in the hydraulic system or a sticky valve.
No
GO to S3.

S3 CHECK THE ABS INLET VALVE (CLOSED POSITION)

- Connect the scan tool.

- Ignition ON.

- With the vehicle in NEUTRAL, position it on a hoist.

- Rotate all the wheels to make sure they rotate freely (the transmission must be in NEUTRAL).

- Apply moderate brake pedal effort.

- Enter the following diagnostic mode on the scan tool: DataLogger- ABS Module.

- Toggle the left front inlet valve (LF_INLET) active command ON and at the same time, have an assistant attempt to rotate the LF wheel.

Does the LF wheel rotate?

Yes
TOGGLE the LF_INLET active command OFF. INSTALL a new HCU (Hydraulic Control Unit). REFER to Hydraulic Control Unit (HCU) . TEST the system for normal operation.
No
TOGGLE the LF_INLET active command OFF. GO to S4.

S4 CHECK THE ABS INLET VALVE (OPEN POSITION)

- Apply moderate brake pedal effort.

- Have an assistant attempt to rotate the LF wheel.

Does the LF wheel rotate?

Yes
INSTALL a new HCU (Hydraulic Control Unit). REFER to Hydraulic Control Unit (HCU) . TEST the system for normal operation.
No
GO to S5.

S5 CHECK THE ABS OUTPUT VALVE (OPEN POSITION)

- Apply moderate brake pedal effort.

- Toggle the left front outlet valve (LF_OUTLET) and the LF_INLET active commands ON and, at the same time, have an assistant attempt to rotate the LF wheel.

Does the LF wheel rotate?

Yes
TOGGLE all active commands OFF. INSTALL a new HCU (Hydraulic Control Unit). REFER to Hydraulic Control Unit (HCU) . TEST the system for normal operation.
No
TOGGLE all active commands OFF. GO to S6.

S6 CHECK THE ABS INLET AND OUTLET VALVE (CLOSED POSITION)

- Apply moderate brake pedal effort.

- Have an assistant attempt to rotate the LF wheel.

Does the LF wheel rotate?

Yes
INSTALL a new HCU (Hydraulic Control Unit). REFER to Hydraulic Control Unit (HCU) . TEST the system for normal operation.
No
REPEAT this procedure (GO to S3) for the RF, LR and RR wheels using the appropriate active commands. If no failure occurs, the system is operating normally.