Antilock Brake System Description
DESCRIPTION - ANTILOCK BRAKE SYSTEMThis covers the physical and operational descriptions, and the on-car service procedures for the Mark 20e Antilock Brake System (ABS) with traction control. It is the only antilock brake system available on this vehicle.
All vehicles equipped with ABS use Electronic Variable Brake Proportioning (EVBP) to balance front-to rear braking when the brakes are applied in the partial braking range.
The traction control system reduces wheel slip and maintains traction at the driving speeds below 56 km/h (35 mph) when road conditions call for traction assistance.
This system shares most base brake hardware used on vehicles without ABS. A vehicle equipped with ABS, however, uses a different master cylinder and brake tubes. Also included in the ABS system is an Integrated Control Unit (ICU) and four wheel speed sensors. All vehicles with ABS come standard with four-wheel-disc brakes and traction control. The antilock brake system prevents wheel lockup under braking conditions on virtually any type of road surface. Antilock braking is desirable because a vehicle that is stopped without locking the wheels retains directional stability and some steering capability. This allows the driver to retain greater control of the vehicle during braking.
DESCRIPTION - ELECTRONIC VARIABLE BRAKE PROPORTIONING
Vehicles equipped with ABS use Electronic Variable Brake Proportioning (EVBP) to balance front-to-rear braking. The EVBP is used in place of a rear proportioning valve. The EVBP system uses the ABS system to control the slip of the rear wheels in partial braking range. The braking force of the rear wheels is controlled electronically by using the inlet and outlet valves located in the Integrated Control Unit (ICU).
DESCRIPTION - TRACTION CONTROL SYSTEM
Traction control reduces wheel slip and maintains traction at the driving wheels at speeds below 56 km/h (35 mph) when road surfaces are slippery. The traction control system reduces wheel slip by braking the wheel that is losing traction.
OPERATION - ANTILOCK BRAKE SYSTEM
There are a few performance characteristics of the Mark 20e Antilock Brake System that may at first seem abnormal, but in fact are normal. These characteristics are described below.
NORMAL BRAKING
Under normal braking conditions, the ABS functions the same as a standard base brake system with a diagonally split master cylinder and conventional vacuum assist.
ABS BRAKING
ABS operation is available at all vehicle speeds above 5 - 8 km/h (3 - 5 mph). If a wheel locking tendency is detected during a brake application, the brake system enters the ABS mode. During ABS braking, hydraulic pressure in the four wheel circuits is modulated to prevent any wheel from locking. Each wheel circuit is designed with a set of electric solenoids to allow modulation, although for vehicle stability, both rear wheel solenoids receive the same electrical signal. Wheel lockup may be perceived at the very end of an ABS stop and is considered normal.
During an ABS stop, the brakes hydraulic system is still diagonally split. However, the brake system pressure is further split into four control channels. During antilock operation of the vehicle's brake system, the wheels are controlled independently and are on separate control channels.
The system can build, hold and release pressure at each wheel, depending on signals generated by the Wheel Speed Sensors (WSS) at each wheel and received at the Controller Antilock Brake (CAB).
NOISE AND BRAKE PEDAL FEEL
During ABS braking, some brake pedal movement may be felt. In addition, ABS braking will create ticking, popping, or groaning noises heard by the driver. This is normal and is due to pressurized fluid being transferred between the master cylinder and the brakes. If ABS operation occurs during hard braking, some pulsation may be felt in the vehicle body due to fore-and-aft movement of the suspension as brake pressures are modulated.
At the end of an ABS stop, ABS is turned off when the vehicle is slowed to a speed of 5 - 6 km/h (3 - 4 mph). There may be a slight brake pedal drop anytime that the ABS is deactivated, such as at the end of the stop when the vehicle speed is less than 5 km/h (3 mph) or during an ABS stop where ABS is no longer required. These conditions exist when a vehicle is being stopped on a road surface with patches of ice, loose gravel, or sand on it. Also, stopping a vehicle on a bumpy road surface activates ABS because of the wheel hop caused by the bumps.
TIRE NOISE AND MARKS
Although the ABS system prevents complete wheel lockup, some wheel slip is desired in order to achieve optimum braking performance. Wheel slip is defined as follows: O percent slip means the wheel is rolling freely and 100 percent slip means the wheel is fully locked. During brake pressure modulation, wheel slip is allowed to reach up to 25-30 percent. This means that the wheel rolling velocity is 25-30 percent less than that of a free rolling wheel at a given vehicle speed. This slip may result in some tire chirping, depending on the road surface. This sound should not be interpreted as total wheel lockup.
Complete wheel lockup normally leaves black tire marks on dry pavement. The ABS will not leave dark black tire marks since the wheel never reaches a fully locked condition. However, tire marks may be noticeable as light patched marks.
START-UP CYCLE
When the ignition is turned on, a popping sound and a slight brake pedal movement may be noticed. The ABS warning indicator lamp will also be on for up to 5 seconds after the ignition is turned on. These conditions occur as part of an ABS self-diagnosis test performed by the antilock brake system. The popping noise is the result of brief activation of the solenoids inside the Integrated Control Unit (ICU).
DRIVE-OFF CYCLE
When the vehicle is first driven off, a humming may be heard or felt by the driver at approximately 25 - 40 km/h (15 - 25 mph). This is caused by brief activation of the ABS pump motor on the ICU and is a normal function of ABS as the system is performing a diagnosis check.
PREMATURE ABS CYCLING
Symptoms of premature ABS cycling include: clicking sounds from the solenoid valves; pump/motor running; and pulsations in the brake pedal. Premature ABS cycling can occur at any braking rate of the vehicle and on any type of road surface. Neither the red BRAKE warning indicator lamp, nor the amber ABS warning indicator lamp, illuminate and no fault codes are stored in the Controller Antilock Brake (CAB). Premature ABS cycling is a condition that needs to be correctly assessed when diagnosing problems with the antilock brake system. It may be necessary to use a DRBIII scan tool to detect and verify premature ABS cycling.
Check the following common causes when diagnosing premature ABS cycling: damaged tone wheels; incorrect tone wheels; damaged steering knuckle wheel speed sensor mounting bosses; loose wheel speed sensor mounting bolts; excessive tone wheel runout; excessively large tone wheel-to-wheel speed sensor air gap, or a damaged speed sensor head face. Give special attention to these components when diagnosing a vehicle exhibiting premature ABS cycling. After diagnosing the defective component, repair or replace it as required. When the component repair or replacement is completed, test drive the vehicle to verify that premature ABS cycling has been corrected.
OPERATION - ELECTRONIC VARIABLE BRAKE PROPORTIONING
Upon entry into EVBP the inlet valve for the rear brake circuit is switched on so that the fluid supply from the master cylinder is shut off. In order to decrease the rear brake pressure, the outlet valve for the rear brake circuit is pulsed. This allows fluid to enter the Low Pressure Accumulator (LPA) in the Hydraulic Control Unit (HCU) resulting in a drop in fluid pressure to the rear brakes. In order to increase the rear brake pressure, the outlet valve is switched off and the inlet valve is pulsed. This increases the pressure to the rear brakes. This back-and-forth process will continue until the required slip difference is obtained. At the end of EVBP braking (brakes released) the fluid in the LPA drains back to the master cylinder by switching on the outlet valve and draining through the inlet valve check valve. At the same time the inlet valve is switched on in case of another brake application.
The EVBP will remain functional during many ABS fault modes. If both the red BRAKE and amber ABS warning indicators are illuminated, the EVBP may not be functioning.
OPERATION - TRACTION CONTROL SYSTEM
The CAB monitors wheel speed. During acceleration, if the CAB detects front (drive) wheel slip and the brakes are not applied, the CAB enters traction control mode. Traction control operation proceeds in the following order:
1. Close the normally open Traction Control (TC) valves.
2. Start the pump/motor and supply volume and pressure to the front (drive) hydraulic circuit. (The pump/motor runs continuously during traction control operation.)
3. Open and close the build and decay solenoid valves to maintain minimum wheel slip and maximum traction.
The cycling of the build and decay valves during traction control is similar to that during antilock braking, except the valves work to control wheel spin by applying the brakes, whereas the ABS function is to control wheel skid by releasing the brakes.
Two pressure relief shuttle valves allow pressure and volume to return to the master cylinder reservoir when not consumed by the build and decay valves. These valves are necessary because the pump/motor supplies more volume than the system requires.
If the brakes are applied at anytime during a traction control cycle, the brake lamp switch triggers the control module to switch off traction control.
The traction control function indicator illuminates during a traction control cycle, displaying TRAC on the instrument panel. The traction control system is enabled at each ignition cycle. It may be turned off by depressing the traction control switch button. The traction control function indicator (TRAC OFF) illuminates immediately upon depressing the button. Pressing this button again or turning off and restarting the vehicle will enable the traction control system.
If the CAB calculates that the brake temperatures are high, the traction control system becomes inoperative until a time-out period has elapsed. During this "thermo-protection mode," the traction control function indicator illuminates TRAC OFF; note that no trouble code is registered.