Pinpoint Test H: Poor Vehicle Tracking During Anti-Lock Function
Anti-Lock Control
Pinpoint Tests
Pinpoint Test H: Poor Vehicle Tracking During Anti-Lock Function
Normal Operation
Fused battery voltage is provided to the ABS module from Battery Junction Box (BJB) fuses 109 (20A) and 106 (40A) along circuits 483 (RD) and 601 (LB/PK) respectively. Fused ignition voltage is provided from Central Junction Box (CJB) fuse 20 (10A) along circuit 1789 (VT/WH). When necessary, the ABS module supplies voltage to the Hydraulic Control Unit (HCU) assembly and the pump motor is activated. The ABS module is provided a ground along circuit 57 (BK) and the hydraulic pump motor is provided a ground internally through the HCU.
This pinpoint test is intended to diagnose the following:
- Base brake system
- HCU assembly
- Circuit shorted
PINPOINT TEST H: POOR VEHICLE TRACKING DURING ANTI-LOCK FUNCTION
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H1 BLEED THE BRAKE SYSTEM
- WARNING: Before servicing a vehicle equipped with a fire suppression system, depower the system by following the procedure in Fire Suppression System. Failure to follow the instructions may result in serious personal injury Procedures.
- Bleed the brake system. Refer to Brake System - General Information Brake System Bleeding.
- Place the air suspension switch in the ON position and test drive the vehicle.
- Does the vehicle track poorly?
Yes
GO to H2.
No
The brake system is operating correctly at this time. TEST the system for normal operation. If equipped with a fire suppression system, GO to H11.
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H2 CHECK THE LF HCU VALVE OPERATION
WARNING: Before servicing a vehicle equipped with a fire suppression system, depower the system by following the procedure in Fire Suppression System. Failure to follow the instructions may result in serious personal injury Procedures.
- With the vehicle in NEUTRAL, position it on a hoist. Refer to Jacking and Lifting Service and Repair.
- Connect the scan tool.
- Ignition ON.
- With the wheels off the ground, spin each wheel by hand to make sure that they spin freely (the transmission must be in the NEUTRAL position).
- Enter the following diagnostic mode on the scan tool: DataLogger - ABS Module.
- Toggle the ABS pump motor (PMP_MOTOR) active command ON for 4 seconds.
- Apply moderate brake pedal effort.
- Have an assistant attempt to rotate the LF wheel.
- Does the LF wheel rotate?
Yes
TOGGLE the PMP_MOTOR command OFF. INSTALL a new HCU. REFER to Hydraulic Control Unit (HCU) Service and Repair. TEST the system for normal operation. If equipped with a fire suppression system, GO to H11.
No
TOGGLE the PMP_MOTOR command OFF. GO to H3.
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H3 CHECK THE LF HCU VALVE RELEASE
- Toggle the following active commands ON:
- LF inlet valve (LF_INLET)
- LF outlet valve (LF_OUTLET)
- Immediately after toggling the active command ON, have an assistant spin the LF wheel using a 2-foot lever, such as a breaker bar.
- Does the LF wheel spin?
Yes
TOGGLE both commands OFF. GO to H4.
No
TOGGLE both commands OFF. GO to H10.
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H4 CHECK THE RF HCU VALVE OPERATION
- Toggle the PMP_MOTOR active command ON for 4 seconds.
- Apply moderate brake pedal effort.
- Have an assistant attempt to rotate the RF wheel.
- Does the RF wheel rotate?
Yes
TOGGLE the PMP_MOTOR command OFF. INSTALL a new HCU. REFER to Hydraulic Control Unit (HCU) Service and Repair. TEST the system for normal operation. If equipped with a fire suppression system, GO to H11.
No
TOGGLE the PMP_MOTOR active command OFF. GO to H5.
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H5 CHECK THE RF HCU VALVE RELEASE
- Toggle the following active commands ON:
- RF inlet valve (RF_INLET)
- RF outlet valve (RF_OUTLET)
- Immediately after toggling the active command ON, have an assistant spin the RF wheel using a 2-foot lever, such as a breaker bar.
- Does the RF wheel spin?
Yes
TOGGLE both active commands OFF. GO to H6.
No
TOGGLE both active commands OFF. GO to H10.
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H6 CHECK THE LR HCU VALVE OPERATION
- Toggle the PMP_MOTOR active command ON for 4 seconds.
- Apply moderate brake pedal effort.
- Have an assistant attempt to rotate the LR wheel.
- Does the LR wheel rotate?
Yes
TOGGLE the PMP_MOTOR active command OFF. INSTALL a new HCU. REFER to Hydraulic Control Unit (HCU) Service and Repair. TEST the system for normal operation. If equipped with a fire suppression system, GO to H11.
No
TOGGLE the PMP_MOTOR active command OFF. GO to H7.
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H7 CHECK THE LR HCU VALVE RELEASE
- Toggle the following active commands ON:
- LR inlet valve (LR_INLET)
- LR outlet valve (LR_OUTLET)
- Immediately after toggling the active command ON, have an assistant spin the LR wheel using a 2-foot lever, such as a breaker bar.
- Does the LR wheel spin?
Yes
TOGGLE both commands OFF. GO to H8.
No
TOGGLE both commands OFF. GO to H10.
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H8 CHECK THE RR HCU VALVE OPERATION
- Toggle the ABS module PMP_MOTOR active command ON for 4 seconds.
- Apply moderate brake pedal effort.
- Have an assistant attempt to rotate the RR wheel.
- Does the RR wheel rotate?
Yes
TOGGLE the PMP_MOTOR command OFF. INSTALL a new HCU. REFER to Hydraulic Control Unit (HCU) Service and Repair. TEST the system for normal operation. If equipped with a fire suppression system, GO to H11.
No
TOGGLE the PMP_MOTOR command OFF. GO to H9.
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H9 CHECK THE RR HCU VALVE RELEASE
- Toggle the following active commands ON:
- RR inlet valve (RR_INLET)
- RR outlet valve (RR_OUTLET)
- Immediately after toggling the active command ON, have an assistant spin the RR wheel using a 2-foot lever, such as a breaker bar.
- Does the RR wheel spin?
Yes
TOGGLE both commands OFF. The anti-lock brake system is operating correctly at this time. TEST the system for normal operation. If equipped with a fire suppression system, GO to H11.
No
TOGGLE both commands OFF. GO to H10.
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H10 CHECK FOR DTCs
- Enter the following diagnostic mode on the scan tool: Self Test - ABS Module.
- Retrieve and record any ABS module DTCs.
- Are any DTCs present?
Yes
GO to the ABS Module DTC Chart. Diagnostic Trouble Code Tests and Associated Procedures
No
INSTALL a new HCU. REFER to Hydraulic Control Unit (HCU) Service and Repair. TEST the system for normal operation. If equipped with a fire suppression system, GO to H11.
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H11 REPOWER THE FIRE SUPPRESSION SYSTEM
- WARNING: Before servicing a vehicle equipped with a fire suppression system, depower the system by following the procedure in Fire Suppression System. Failure to follow the instructions may result in serious personal injury Procedures.
- With the vehicle in NEUTRAL, position it on a hoist. Refer to Jacking and Lifting Service and Repair.
- Verify that the fire suppression system is repowered. Refer to Fire Suppression System Procedures.
- Is the fire suppression system repowered?
Yes
Fire suppression system repowering is complete.
No
REFER to Fire Suppression System Procedures for fire suppression depowering and repowering procedure.
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