Pinpoint Test M: Poor Vehicle Tracking During Anti-lock Function
Anti-Lock Control
Pinpoint Test M: 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, Restraints Control Module (RCM), etc.) 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 (Hydraulic Control Unit) or an issue with the base brake system (caliper, tubes, etc.) 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 (Battery Junction Box) 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:
- Base brake system
- HCU (Hydraulic Control Unit)
PINPOINT TEST M : POOR VEHICLE TRACKING DURING ANTI-LOCK FUNCTION
M1 BLEED THE BRAKE SYSTEM
- Bleed the HCU (Hydraulic Control Unit). Refer to Brake System Bleeding in Brakes and Traction Control.
- 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 M2.
M2 VERIFY VEHICLE TRACKING DURING NON-ABS BRAKING
- Test drive the vehicle and verify vehicle tracking during normal braking.
Does the vehicle track correctly during normal braking?
Yes
GO to M3.
No
The condition could be caused by a base brake component or vehicle alignment. REFER to Brakes and Traction Control to verify base brake components and REFER to Suspension &/or Alignment to verify suspension components.
M3 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 transaxle must be in NEUTRAL).
- Apply moderate brake pedal effort.
- Enter the following diagnostic mode on the scan tool: DataLogger- ABS.
- 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 with the brake pedal still applied.
Does the LF wheel rotate?
Yes
RELEASE the brake pedal. TOGGLE the LF_INLET active command OFF. GO to M4.
No
RELEASE the brake pedal. 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.
M4 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
RELEASE the brake pedal. INSTALL a new HCU (Hydraulic Control Unit). REFER to Hydraulic Control Unit (HCU) . TEST the system for normal operation.
No
RELEASE the brake pedal. GO to M5.
M5 CHECK THE ABS OUTLET VALVE (OPEN POSITION)
NOTICE: Do not allow the pump motor to run for more than 30 seconds or damage to the motor may occur.
- Apply moderate brake pedal effort.
- Toggle the following active commands ON:
- Left Front Inlet Valve (LF_INLET)
- Left Front Outlet Valve (LF_OUTLET)
- ABS Pump Motor (PMP_MTR)
- At the same time, have an assistant attempt to rotate the LF wheel with the brake pedal still applied.
Does the LF wheel rotate?
Yes
RELEASE the brake pedal. 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
RELEASE the brake pedal. TOGGLE all active commands OFF. GO to M6.
M6 CHECK THE ABS 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 (beginning with Step M3) for the RF, LR and RR wheels using the appropriate active commands. If no failure occurs, the system is operating normally.