See Figure 1
Found on 1991 Caravan/Voyager/Town and Country (AS body), the Bendix System 10 will prevent wheel locking under heavy braking. By preventing wheel lock-up, maximum braking effort is maintained while preventing loss of directional control. Additionally, some steering capability is maintained during the stop. The ABS system will operate regardless of road surface conditions.
There are conditions for which the ABS system provides no benefit. Hydroplaning is possible when the tires ride on a film of water, losing contact with the paved surface. This renders the vehicle totally uncontrollable until road contact is regained. Extreme steering maneuvers at high speed or cornering beyond the limits of tire adhesion can result in skidding which is independent of vehicle braking. For this reason, the system is named anti-lock rather than anti-skid.
Under normal braking conditions, the ABS system functions in the same manner as a standard brake system. The primary difference is that power assist is gained from hydraulic pressure rather than a conventional vacuum booster.
The system is a combination of electrical and hydraulic components, working together to control the flow of brake fluid to the wheels when necessary. The pump and motor assembly pressurizes brake fluid from the reservoir and stores it within an accumulator for use in both normal power-assisted and ABS braking. The Controller Anti-lock Brake (CAB) is the electronic brain of the system, receiving and interpreting speed signals from 4 sensors at the wheels. The CAB will enter anti-lock mode when it senses impending wheel lock at any wheel and immediately controls the line pressure(s) to the affected wheel(s). The hydraulic assembly serves as both an integral master cylinder and the hydraulic booster assembly for the brake system. It contains the wheel circuit valves used to control the brake fluid pressure to each wheel circuit.
During anti-lock braking, line pressures are controlled or modulated by the rapid cycling of electronic valves within the hydraulic assembly. These valves can allow pressures within the system to increase, remain constant or decrease depending on the needs of the moment as registered by the CAB. The front wheels are controlled individually while the rear wheels receive the same electrical signal, based on the wheel with the greatest locking tendency. Anti-lock function is available above 3-5 mph (4.5-8 km/h).
The operator may hear a popping or clicking sound as the pump and/or control valves cycle on and off during normal operation. The sounds are due to normal operation and are not indicative of a system problem; under most conditions, the sounds are only faintly audible. If ABS is engaged, the operator may notice some pulsation in the pedal. If additional force is applied to the pedal during an ABS-engaged stop, the operator will notice extremely hard pedal feel. This is due to isolation of the master cylinder during ABS operation. Some pulsing may also be felt in the body of the vehicle due to suspension movement as brake pressures apply and release at the individual wheels.
Although the ABS system prevents wheel lock-up under hard braking, as brake pressure increases, wheel slip is allowed to reach as high as 30%. This means that the rolling velocity of a given wheel is 30% less than that of a free-rolling wheel at a given speed. This slip will result in some tire chirp during ABS operation. The sound should not be interpreted as lock-up but rather as an indication of the system holding the wheel(s) just outside the point of lock-up. Additionally, since the ABS system turns off below 4 mph (7 km/h), the final few feet of an ABS-engaged stop may be completed with the wheels locked.
The Bendix system is equipped with built-in diagnostic capability. At every start-up, the CAB illuminates the dashboard warning lights and turns them off after checking the circuitry. When the vehicle reaches 3-5 mph (4.5-8 km/h), the CAB conducts a system check, briefly activating all the control valves to confirm their operation. This system check may be noticed by the operator as a series of rapid clicks during initial drive-off; the sound is normal and not indicative of a problem. Some fault conditions will cause the CAB to set and retain a trouble code which may be retrieved for diagnostic purposes. Stored fault codes will remain stored until cleared by the DRB II.
The CAB will illuminate the appropriate dashboard warning lamp according to the fault detected. It is possible to have a fault affecting only the ABS function; in this case, the ABS system will be disabled but the vehicle will retain normal braking capability.
Wheel Speed (WSS) Sensors
See Figure 2
The speed of each wheel is monitored by a wheel speed sensor (WSS). A toothed tone wheel rotates in front of the sensor, generating a small AC voltage which is transmitted to the CAB. Each speed sensor is individually removable; the tone wheels are permanently mounted to either the front outer constant velocity joint assemblies or the rear wheel hub assembly. The air gap between the sensor and the tone wheel is set by the correct installation of the sensor; the air gap is not adjustable.Controller Anti-Lock Brake (CAB) Module
The CAB is located in the engine compartment under the battery tray. This computer-operating separately from other on-board controllers-monitors wheel speed signals as well as several internal functions. The CAB controls the wheel circuit valves once a locking tendency is detected. This pressure modulation continues until the locking tendency is no longer detected.
The CAB receives inputs from the wheel speed sensors, the boost pressure transducer, the primary pressure transducer, the brake light switch, the brake fluid level sensor, the differential pressure switch, ignition switch, starter relay, system relay voltage and a ground signal. Outputs managed by the CAB include the 10 modulator valves (3 build valves, 3 decay valves and 4 isolation valves), both dashboard warning lamps, system relay actuation, low fluid/parking brake output, and diagnostic communication including transmitting fault codes.
The CAB found on 2WD Caravan, Voyager, and Town and Country vehicles is different from that on the AWD versions. The controllers must not be interchanged.Pump and Motor Assembly
See Figure 3
The integral motor and pump assembly is mounted on rubber isolators to a to a transaxle bracket below the hydraulic assembly. Fluid is taken from the master cylinder reservoir, pressurized and sent to storage in both the piston accumulator and the hydraulic bladder accumulator. The pump/motor assembly is serviceable only as a unit and should never be disassembled. Hoses running to and from the pump unit should never be repaired but replaced as an assembly.Hydraulic Assembly
See Figure 4
The integral hydraulic assembly is located on the firewall and provides the function of the power booster and master cylinder. Other components provide the brake pressure modulation and system monitoring required by the ABS system. The hydraulic assembly consists of several components with individual function as outlined in this section. Note that although the components and their function may be discussed and tested separately, most components are not serviced individually.BOOSTER/MASTER CYLINDER
The master cylinder uses a diagonally split configuration in normal braking. The 2 circuits are isolated so that a leak in one will not affect the other. During brake pedal application, the pushrod applies force to the boost control valve, allowing pressurized fluid from the accumulator to flow into the master cylinder chamber. This pressure within the booster servo applies pressure to the primary and secondary master cylinder pistons. The pressures generated by the primary and secondary pistons are used to apply the brakes during normal braking.
The pressure within the hydraulic booster is directly proportional to the pressure applied to the brake pedal. As with vacuum operated boosters, brake efficiency depends on road surface and the force applied to the pedal.HYDRAULIC ACCUMULATORS
See Figure 5
The external or bladder accumulator stores brake fluid from the pump under very high pressure; the pressurized fluid is available for hydraulic assist (boost) and/or ABS braking. The accumulator uses a diaphragm and nitrogen pre-charge of about 1000 psi (6895 kPa). Normally, the pump will charge the accumulator to a working pressure of 1600-2000 psi (11,030-13,790 kPa). For this reason, all safety precautions must be observed before working on the hydraulic system. The pressures within the accumulator are sufficient to cause serious personal injury or damage to the vehicle.
The piston accumulator is an integral part of the pump/motor assembly. It contains a pre-charge of approximately 460 psi (3171 kPa) nitrogen gas. This accumulator cannot be removed from the pump/motor assembly; component service is not possible.DUAL FUNCTION PRESSURE SWITCH
This switch is located on the bottom of the hydraulic assembly and monitors the pressure within the accumulator. When accumulator pressure falls below 1600 psi (11,030 kPa), the dual function pressure switch causes the pump/motor to energize. When accumulator pressure reaches 2000 psi (13,790 kPa), the pump/motor is shut off.
The second purpose of this switch is to provide a signal to the CAB when accumulator pressure falls below the 1000 psi (6895 kPa) minimum. An internal warning pressure switch is normally closed at working pressures, grounding pin 17 at the controller. Should the accumulator pressure drop below minimum, the switch opens, a voltage signal is detected at the CAB and is read as low pressure. At this warning pressure, the CAB disables the ABS function and illuminates the dash warning lamps. After 2 minutes of continuous low pressure, a low accumulator fault code is stored in memory.PRESSURE TRANSDUCERS
The boost pressure transducer is mounted on the bottom of the hydraulic assembly and monitors boost servo pressure. The primary pressure transducer is found on the left side of the hydraulic assembly and monitors the pressure within the primary master cylinder.
Both transducers generate a signal of 0.25-5.0 volts directly proportional to the fluid pressure. The CAB compares these signals and confirms proper operation. If either of the signals exceeds a pre-planned range, the CAB will disable the anti-lock system.DIFFERENTIAL PRESSURE SWITCH
The differential pressure switch is used to detect a pressure differential between the primary and secondary master cylinder hydraulic circuits. When the pressure difference is 300 psi (2068 kPa) or more, this switch grounds the output of the primary pressure transducer. The CAB receives 0.0 volts from the transducer and reacts by shutting down the ABS function and illuminating the dash warning lights.FLUID LEVEL SENSOR
A float and magnetic reed switch monitor the fluid level within the master cylinder hydraulic reservoir. Located in the reservoir cap, the sensor signal is used an input to the CAB. If low fluid level is detected, the BRAKE warning lamp on the dash is illuminated. If the vehicle is in motion above 3 mph (5 km/h), the ABS will be disabled and the anti-lock warning lamp will light. If the vehicle is not moving, or is below 3 mph (5 km/h) the anti-lock lamp will not come on.Dashboard Warning Lamps
BRAKE WARNING LAMP
The red BRAKE warning lamp will be illuminated to warn the operator of conditions which may result in reduced braking ability. These conditions include:
The lamp will also illuminate whenever the ignition switch is put in the START position or the ignition switch is turned to ON . Under these bulb test conditions, the lamp should stay illuminated for about 2 seconds.
ANTI-LOCK WARNING LAMP
See Figure 6
The amber ANTI-LOCK warning lamp is controlled by the CAB. If the controller detects a condition resulting in the shut-down of the ABS function, the ANTI-LOCK lamp will be lit. The ANTI-LOCK lamp is normally lit until the CAB completes its self-tests; if no faults are found, the lamp is turned off.
Display of the ANTI-LOCK warning lamp by itself indicates only that the ABS function has been disabled. Power-assisted normal braking is still available and the vehicle may be driven with reasonable care.
When starting the vehicle, the ANTI-LOCK lamp may stay on 1-to-30 seconds depending on the residual press the left front headlight (AS body) The relay controls the operation of the pump/motor assembly and is energized by a signal from the dual function pressure switch. The relay may be serviced individually.
The system relay controls the modulator valves and the anti-lock warning lamp relay. The system relay, near the pump/motor relay, controls power to the CAB after the start-up cycle.
The anti-lock warning lamp relay controls the dashboard warning lamp. When the relay is energized by the CAB, the dash lamp is held OFF . Thus, the lamp will light when the CAB fails, is disconnected or causes the ABS function to be discontinued. The CAB has the capability to turn the light on by itself by providing a separate ground.Proportioning Valves
In place of the usual differential pressure proportioning valve, this system incorporates individual valves in each rear brake line. Located in the brake outlet ports of the hydraulic assembly, these screw-in valves limit rear brake pressure after a certain pressure is reached. This improves front-to-rear brake balance during normal braking. Each proportioning valve may be serviced individually.Diagnostic Connector
The ABS diagnostic connector is located on the left side of the steering column under the dash. The blue 6-pin connector is used for connecting diagnostic tools such as the DRB II.