ABS System Description
TEVES MARK IVG ABS SYSTEMAn electronic control module is used to monitor wheel speeds and to modulate (control) hydraulic pressure in each brake channel to prevent wheel lock-up during braking.
During a non-ABS stop,the system functions as a standard front/rear split configuration.The primary supplies brake fluid pressure to the front brakes, and the secondary supplies the rear brakes. A conventional combination/proportioning valve is used.
During an ABS stop, the system still uses the front/rear hydraulic split; however, the brake system pressure is further split into three control channels.
During ABS operation, the front wheels are controlled independently and are on two separate control channels. The rear wheels are controlled together through one control channel. By using separate control channels for the front wheels, more steering control is maintained during maximum braking.
During an antilock stop,"wheel lock-up" does not necessarily mean that the wheel has locked, it means only that the wheel is turning slower than the vehicle speed. This is called "wheel slip" and is indicated as a percentage.0% slip means that the wheel is rolling free and 100% slip means that the wheel is locked. The antilock system maintains an average of approximately 20% wheel slip.
It is important to remember that the antilock brake system does not shorten the vehicle stopping distance under all driving conditions, but provides improved control of the vehicle while stopping.Vehicle stopping distance is still dependent on vehicle speed, weight, tires, road surface, and other factors.
System Components Antilock Brake System
controller - antilock brake (CAB)
vacuum booster
master cylinder (w/center valves)
hydraulic control unit (HCU)
valve block assembly: 6 valve solenoids (3 inlet valves, 3 outlet valves), 4 accumulators
pump/motor assembly:
1 motor
2 pumps
G (acceleration) switch
1 proportioning valve
4 wheel speed sensor/tone wheel assemblies
2 relays (main and pump/motor)
ABS warning lamp
fuses and wiring harness
fluid reservoir (integral part of master cylinder assembly)
The Teves Mark IVG Antilock Brake System (ABS) is an electronically operated, all wheel brake control system.
The system is designed to prevent wheel lockup and maintain steering control during periods of high wheel slip when braking. Preventing lockup is accomplished by modulating fluid pressure to the wheel brake units.
Antilock Brake System:
The hydraulic system is a three channel design. The front wheel brakes are controlled individually and the rear wheel brakes in tandem. The ABS electrical system is separate from other electrical circuits in the vehicle. A specially programmed controller antilock brake unit operates the system components.
SYSTEM COMPONENTS
ABS system components include:
- Controller Antilock Brake (CAB)
- Vacuum Booster
- Master Cylinder (w/center valves)
- Hydraulic Control Unit (HCU)
- Valve Block Assembly: 6 valve solenoids (3 inlet valves, 3 outlet valves), 4 accumulators
- Pump/Motor Assembly:
1 motor
2 pumps
- G (Acceleration) Switch
- 1 Proportioning Valve
- 4 Wheel Speed Sensor/Tone Wheel Assemblies
- ABS Warning Lamp
- Fuses and Wiring Harness
- Fluid Reservoir (integral part of master cylinder assembly)
PEDAL FEEL/VEHICLE CHARACTERISTICS
There are several pedal feel/vehicle characteristics that are considered normal for antilock braking that may require further explanation.
When stopping conditions activate the antilock brakes, the driver may feel some vibrations/pulsations in the brake pedal and may hear the electric solenoid valves clicking and the pump motor running. The vibrations/pulsations are caused by the isolating, building and decaying of brake fluid pressure within the brake lines. The ABS system prevents complete wheel lock-up, but some wheel slip is required for the best braking performance. This slip may result in some tire chirping, depending on the road surface. The chirping should not be interpreted as total wheel lock-up. While total wheel lock-up leaves black tire marks on dry pavement, antilock braking may leave some light marks.
At the end of an ABS stop, the ABS system may not function below 5 Km/h (3 mph). There may be a slight brake pedal drop anytime the ABS is deactivated.
In case of braking on a bumpy surface, the ABS system may detect wheel locking tendencies due to wheel hop and cycle ABS. In the scenario the brake pedal may pulsate with a perceived loss of declaration. ABS braking may also be activated at times while on dry pavement with sand, gravel, or other loose debris on the road.
It should be noted that the pulsating pedal feel characteristic will not illuminate the brake warning lamps or set a trouble code that is stored in the Controller Antilock Brake (CAB). When investigating a hard pedal feel, inspect the sensor and tone wheel teeth for chips/broken teeth, damaged sensor pole tips, excessive runout of the tone wheel, or excessive gap.
Hydraulic Control Unit
Mark IVG and Mark 20 Systems: The Hydraulic Control Unit (HCU) contains the valve block assembly, four accumulators, and the pump/motor assembly.
Valve Block Assembly: The valve block assembly contains 6 valves with three inlet valves and three outlet valves. The inlet valves are spring-loaded in the open position and the outlet valves are spring loaded in he closed position. During an anti lock stop, these valves are cycled to maintain the proper slip ration for each channel. If a wheel locks, the inlet valve is closed to prevent any further pressure increase. Then the outlet valve is opened to release the pressure to the accumulators until the wheel is no longer slipping. Once the wheel is no longer slipping, the outlet valve is closed and the inlet valve is opened to reapply pressure. If the wheel is decelerating within its predetermined limits (proper slip ratio), both valves will close to hold the pressure constant.
Pump/Motor Assembly: The pump/motor assembly provides the extra amount of fluid needed during antilock braking. The pump is supplied fluid that is released to the accumulators when the outlet valve is opened during an antilock stop. The pump is also used to drain the accumulator circuits after the anti lock stop is complete. The pump is operated by an integral electric motor. This DC-type motor is controlled by the CAB. The CAB turns on the motor when an antilock stop is detected. The pump continues to run during the antilock stop and is turned oft approximately 3-5 seconds after the stop is complete. The pump mechanism consists of two opposing pistons operated by an eccentric cam. One piston supplies the primary hydraulic circuit. The opposing piston supplies the secondary hydraulic circuit. In operation, one piston draws fluid from the accumulators. The opposing piston pumps fluid to the valve body solenoids. The CAB monitors the pump/motor operation internally.
Accumulators: The accumulators provide temporary fluid storage during an anti lock stop and are drained by the pump/motor. The 6mm (0.24") accumulator is used for the front brakes, the 3mm (0.12") accumulator is used for the rear brakes, the 1 mm (0.04") accumulator is used for the lip seal saver, and the 0.5mm (0.02") accumulator is used for noise reduction.
Relays/Switches (Mark IVG only)
The ABS main relay is located in the Power Distribution Center (PDC). The main relay has two functions. First, when the relay is energized, it supplies power to the valve/solenoids in the valve block assembly and the pump/motor relay coil. Second, when the relay is de-energized, it supplies a ground to turn on the ABS warning lamp. The main relay is controlled by the CAB. The CAB energizes the main relay when it receives an ignition input signal.
The ABS pump/motor relay supplies power to the pump/motor. It is also located in the power distribution center (PDC). The relay coil is powered by the main relay and the coil ground is controlled by the CAB. The pump/motor relay is a special five-terminal relay and should not be replaced with a standard relay.
This section covers the Teves Mark IVG antilock braking system (ABS) found on the 1996 Jeep Grand Cherokee.