Pinpoint Test F: Poor Vehicle Tracking During Anti-Lock Function
Refer to appropriate SYSTEM WIRING DIAGRAMS article, Vehicle Dynamic Systems for schematic and connector information.
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 13 (10A) along circuit 1003 (GY/YE). 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 F: POOR VEHICLE TRACKING DURING ANTI-LOCK FUNCTION
- F1 BLEED THE BRAKE SYSTEM
- Bleed the brake system. Refer to BRAKE SYSTEM - GENERAL INFORMATION article.
- Place the air suspension switch in the ON position and test drive the vehicle.
- Does the vehicle track poorly?
WARNING: Before servicing a vehicle equipped with a fire suppression system, depower the system by following the procedure in FIRE SUPPRESSION SYSTEM article. Failure to follow the instructions may result in serious personal injury. - F2 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 article. Failure to follow the instructions may result in serious personal injury.
- With the vehicle in NEUTRAL, position it on a hoist. Refer to JACKING & LIFTING article.
- Connect the diagnostic tool.
- Key in ON position.
- 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 diagnostic tool: ABS DataLogger: Pump Motor Output Command (PMP_MOTOR)
- Toggle the PMP_MOTOR output 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) . TEST the system for normal operation. If equipped with fire suppression system, Go to F11.
- NO : TOGGLE the PMP_MOTOR command OFF. Go to F3.
- F3 CHECK THE LF HCU VALVE RELEASE
- Enter the following diagnostic mode on the diagnostic tool: ABS DataLogger Left Front Inlet Valve and the Left Front Outlet Valve Output Command (LF_INLET and LF_OUTLET)
- Toggle the LF_INLET and LF_OUTLET output commands ON.
- Immediately after toggling the output command ON, have an assistant spin the LF wheel using a 2-foot lever, such as a breaker bar.
- Does the LF wheel spin?
- F4 CHECK THE RF HCU VALVE OPERATION
- Enter the following diagnostic mode on the diagnostic tool: ABS DataLogger Pump Motor Output Command (PMP_MOTOR)
- Toggle the PMP_MOTOR output 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) . TEST the system for normal operation. If equipped with fire suppression system, Go to F11.
- NO : TOGGLE the PMP_MOTOR command OFF. Go to F5.
- F5 CHECK THE RF HCU VALVE RELEASE
- Enter the following diagnostic mode on the diagnostic tool: ABS DataLogger Right Front Inlet Valve and the Right Front Outlet Valve Output Command (RF_INLET and RF_OUTLET)
- Toggle the RF_INLET and RF_OUTLET output commands ON.
- Immediately after toggling the output command ON, have an assistant spin the RF wheel using a 2-foot lever, such as a breaker bar.
- Does the RF wheel spin?
- F6 CHECK THE LR HCU VALVE OPERATION
- Enter the following diagnostic mode on the diagnostic tool: ABS DataLogger Pump Motor Output Command (PMP_MOTOR)
- Toggle the PMP_MOTOR output 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 command OFF. INSTALL a new HCU. REFER to Hydraulic Control Unit (HCU) . TEST the system for normal operation. If equipped with fire suppression system, Go to F11.
- NO : TOGGLE the PMP_MOTOR command OFF. Go to F7.
- F7 CHECK THE LR HCU VALVE RELEASE
- Enter the following diagnostic mode on the diagnostic tool: ABS DataLogger Right Front Inlet Valve and the Right Front Outlet Valve Output Command (LR_INLET and LR_OUTLET)
- Toggle the LR_INLET and LR_OUTLET output commands ON.
- Immediately after toggling the output command ON, have an assistant spin the LR wheel using a 2-foot lever, such as a breaker bar.
- Does the LR wheel spin?
- F8 CHECK THE RR HCU VALVE OPERATION
- Enter the following diagnostic mode on the diagnostic tool: ABS DataLogger Pump Motor Output Command (PMP_MOTOR)
- Toggle the ABS module PMP_MOTOR output 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) . TEST the system for normal operation. If equipped with fire suppression system, Go to F11.
- NO : TOGGLE the PMP_MOTOR command OFF. Go to F9.
- F9 CHECK THE RR HCU VALVE RELEASE
- Enter the following diagnostic mode on the diagnostic tool: ABS DataLogger Right Front Inlet Valve and the Right Front Outlet Valve Output Command (RR_INLET and RR_OUTLET)
- Toggle the ABS module RR_INLET and RR_OUTLET output commands ON.
- Immediately after toggling the output command ON, have an assistant spin the RR wheel using a 2-foot lever, such as a breaker bar.
- Does the RR wheel spin?
- F10 CHECK FOR DTCs
- Retrieve and record any ABS module DTCs.
- Are any DTCs present?
- YES : Go to ABS MODULE DIAGNOSTIC TROUBLE CODE INDEX .
- NO : INSTALL a new HCU. REFER to Hydraulic Control Unit (HCU) . TEST the system for normal operation. If equipped with fire suppression system, go to F11.
- F11 REPOWER THE FIRE SUPPRESSION SYSTEM
- With the vehicle in NEUTRAL, position it on a hoist. Refer to JACKING & LIFTING article.
- Verify that the fire suppression system is repowered. Refer to FIRE SUPPRESSION SYSTEM article.
- Is the fire suppression system repowered?
WARNING: Before servicing a vehicle equipped with a fire suppression system, depower the system by following the procedure in FIRE SUPPRESSION SYSTEM article. Failure to follow the instructions may result in serious personal injury.- YES : Fire suppression system repowering is complete.
- NO : REFER to FIRE SUPPRESSION SYSTEM article for fire suppression depowering and repowering procedure.