4WABS Valves
The 4-wheel anti-lock brake system (ABS) prevents wheel lockup by automatically modulating the brake pressure during an emergency stop. By not locking the wheels, the driver can improve steering control during hard braking and stop the vehicle in the shortest possible distance under most conditions.
The ABS controls each front and both rear brakes separately (3 channels). The brake pedal force required to engage the ABS function may vary with the road surface conditions. A dry surface requires greater force, while a slippery surface requires much less force.
During the ABS operation, the driver will sense a pulsation in the brake pedal, accompanied by a slight up and down movement in the pedal height. In addition, a mechanical noise from the engine compartment may be heard. The pedal effort and pedal feel during normal braking are similar to that of a conventional power brake system.
The ABS consists of the following major components:
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> Anti-Lock Hydraulic Control Unit (2C215) located in the front of the engine compartment on the left frame rail.
> Anti-lock brake electronic control module located in the engine compartment behind the driver's side headlamp on a bracket mounted on the surface of the plastic fender apron.
> Two front brake anti-lock sensors (2C204) are attached to the suspension knuckles. The front brake anti-lock sensor indicators (2C182) are pressed onto the backside of the rotors.
> Rear axle speed sensor integrated into the rear axle housing.
> Acceleration sensor attached to the left frame rail under the brake booster.
> Stoplight switch (13480) located at the brake pedal (2455).
> Pump motor relay (14N089) located in the power distribution box.
> Main relay (14N089) located in the trailer relay box.
> System diode located in the trailer relay box.
> Fuses for system power and the pump motor, both located in the power distribution box.
The anti-lock hydraulic control unit (2C215) is located in the front of the engine compartment on the left frame rail. It consists of a valve body assembly, a pump and a motor assembly.
When the brakes are applied, fluid is forced from the master cylinder outlet ports to the anti-lock hydraulic control unit (2C215) inlet ports. This pressure is transmitted through three normally open solenoid valves contained inside the anti-lock hydraulic control unit, then through the outlet ports of the anti-lock hydraulic control unit to the wheels. The primary (rear) circuit of the master cylinder feeds the rear brakes. The secondary (front) circuit of the brake master cylinder (2140) feeds the front brakes. If the anti-lock brake electronic control module senses that a wheel is about to lock, based on wheel speed sensor data, it pulses the normally open solenoid valve closed, for that circuit. This prevents any more fluid from entering that circuit. The anti-lock brake electronic control module then looks at the sensor signal from the affected wheel again. If that wheel is still decelerating, it opens the normally closed solenoid valve for that circuit. This dumps any pressure that is trapped between the normally open valve and the brake back to the reservoir. Once the affected wheel comes back up to speed, the anti-lock brake electronic control module returns the valves to their normal condition allowing fluid flow to the affected brake.
The anti-lock brake electronic control module monitors the electromechanical components of the system. Malfunction of the Anti-Lock Brake System will cause the Anti-lock brake electronic control module to shut off or inhibit the system. However, normal power assisted braking remains. Malfunctions are indicated by the amber ABS warning light inside the vehicle.
The 4-wheel anti-lock brake system is self monitoring. When the ignition switch is placed in the RUN position, the anti-lock brake electronic control module will perform a preliminary self check on the anti-lock electrical system indicated by a momentary illumination of the amber ABS warning light in the instrument cluster. During vehicle operation, including normal and anti-lock braking, the anti-lock brake electronic control module monitors all electrical anti-lock functions and some hydraulic operations.
In most malfunctions of the anti-lock brake system, the amber ABS warning light will be illuminated. However, most malfunctions are recorded as a coded number in the anti-lock brake electronic control module memory and assist in pinpointing the component needing service. If system is OK, Code 16 will be present.
The anti-lock brake electronic control module is located in the engine compartment behind the driver's side head lamp on a bracket mounted on the surface of the plastic fender apron.
It is an on-board diagnostic, non-repairable unit consisting of two microprocessors and the necessary circuitry for their operation. These microprocessors are programmed identically. The anti-lock brake electronic control module monitors system operation during normal driving as well as during anti-lock braking.
In addition, the anti-lock brake electronic control module provides a speed signal to the programmable speedometer/odometer module (PSOM).
Under normal driving conditions, the microprocessors produce short test pulses to check the electrical system without any mechanical reaction. The anti-lock brake electronic control module continuously monitors the speed of each wheel to determine if any wheel is beginning to lock. When a wheel locking tendency is detected, the anti-lock brake electronic control module signals the appropriate solenoid valve in the anti-lock hydraulic control unit (2C215) to open or close as well as the pump motor to turn on and recycle brake fluid back to the brake master cylinder (2140). This results in moderate pulsations of the brake pedal (2455) and pump noise which may be heard in the passenger compartment. During normal braking, the brake pedal feel will be identical to a standard brake system.
Most concerns which occur to the anti-lock brake system will be stored as a coded number in the Keep-Alive Memory (KAM) of the anti-lock brake electronic control module. This means that once a code is stored, it will be retained by the anti-lock brake electronic control module even with the key in the OFF position. The codes can be retrieved by following the on-board diagnostic procedures.
A new light truck 4-Wheel ABS was introduced as standard equipment on the 1993 Explorer and Bronco, and it was revised in 1994 on Broncos, requiring a different control module revision for '94-96 Broncos, compared to the original '93 Bronco module.
Although this system shares some of the features of passenger car 4-Wheel ABS and some of the features of light truck Rear ABS (RABS), it has its own unique features, especially when it comes to diagnosing concerns.
Here are some general features of the light truck 4-Wheel ABS, some of which are common to the other systems as well:
> System prove-out at vehicle start-up of the light truck 4-Wheel ABS is the same as for passenger car and 4-Wheel light truck ABS.
> The amber anti-lock warning light will come on as soon as the vehicle is turned on or started to verify bulb operation, and will go out after 2-3 seconds, provided everything is normal.
> The 4-Wheel ABS operates in 4 x 4 as well as 4 x 2.
> In ABS mode, the brake pedal will pulsate and the pump motor noise may be heard from the engine compartment.
> When ABS codes are read out, code 16 will be present if the system is OK. This code is similar to the EEC's code 111. It basically means that the 4-Wheel ABS Electronic Control Unit (ECU) is indicating that everything is functioning properly electrically. In other words, the components, such as the pump motor and the valves, will activate when needed.
> Conventional bleed is all that is required for service on the new 4WABS including unhooking brake lines from the hydraulic controller (HCU). However, if the HCU is replaced, a special "bleed box" and electrical jumper will be required to bleed the replacement HCU. This procedure is not the same as for the passenger car systems.
There are differences between the Rear ABS and the light truck 4-Wheel ABS.
Front Wheel Control
In the 4WABS system, the front wheels, are now also under ABS control. This is accomplished by adding sensors at each front wheel while retaining the rear axle sensor that has been used since 1987 for light truck Rear ABS.
Open vs Closed Systems
There is an important difference in how the 4-Wheel ABS handles a low fluid condition and reacts to the red brake warning light.
RABS Open System
The Rear ABS is what is called an "open" hydraulic system. When an ABS stop is initiated and wheel lock is detected, the isolation or inlet valve closes, preventing any more fluid from going to the rear wheels.
Then, if necessary to prevent lockup, an outlet or dump valve opens to bleed fluid out of the rear brake lines. That fluid goes to an accumulator and is held there for the remainder of the stop.
In RABS "open" system, if there is a low fluid condition and the red warning light comes on, the ABS electronic control unit will detect this condition, turn the amber ABS warning light on and disable the system. The vehicle operator will still have basic brake system performance, but the benefit of RABS is lost.
The reason for disabling the RABS is that the low fluid condition could be caused by a leak in the brake system, decreasing its performance.
4WABS Closed System
In the 4-Wheel ABS the red brake light is not monitored. This is because the system is a "closed" hydraulic system.
If you are in a 4WABS stop and a wheel goes to lock, the isolation valve will activate, just as with the RABS. No more fluid will be directed to the wheel that may lock up.
If it still appears that a wheel is going to lock, the dump valve will open, allowing the fluid to bleed to an accumulator similar to the one in the RABS.
However, in this "closed" 4-Wheel ABS there is a pump that will move fluid from the accumulator, back to the line between the master cylinder and the hydraulic control unit, where it may be recycled again through the isolation and dump valves.
Since the fluid in this closed system is still available to both front and rear circuits, the number of cycles is unlimited. The closed 4WABS isn't shut down when the low fluid light comes on, as was the case with the open system.
Stoplamp Switch Signal
The new 4WABS uses the signal from the stoplamp switch (located on the brake pedal) differently than RABS. RABS is turned on when it receives a signal from the stoplamp switch indicating that the brakes are applied. 4WABS is already turned on and will activate when impending wheel lockup is detected.
4WABS first looks at the wheel-speed sensors. If a wheel appears to be going to lock, the system will activate the ABS with or without the stoplamp switch signal.
![4wabs-valves.jpg 4WABS Valves
The 4-wheel anti-lock brake system (ABS) prevents wheel lockup by automatically modulating the brake pressure during an emergency stop. By not locking the wheels, the driver can improve steering control during hard braking and stop the vehicle in the shortest possible distance under most conditions.
The ABS controls each front and both rear brakes separately (3 channels). The brake pedal force required to engage the ABS function may vary with the road surface conditions. A dry surface requires greater force, while a slippery surface requires much less force.
During the ABS operation, the driver will sense a pulsation in the brake pedal, accompanied by a slight up and down movement in the pedal height. In addition, a mechanical noise from the engine compartment may be heard. The pedal effort and pedal feel during normal braking are similar to that of a conventional power brake system.
The ABS consists of the following major components:
[url=https://www.supermotors.net/registry/media/1164812][img]https://www.supermotors.net/getfile/1164812/thumbnail/4wabscomponents.jpg[/img][/url] . [url=http://www.supermotors.net/registry/media/903627][img]http://www.supermotors.net/getfile/903627/thumbnail/fuses93gas.jpg[/img][/url]
> Anti-Lock Hydraulic Control Unit (2C215) located in the front of the engine compartment on the left frame rail.
> Anti-lock brake electronic control module located in the engine compartment behind the driver's side headlamp on a bracket mounted on the surface of the plastic fender apron.
> Two front brake anti-lock sensors (2C204) are attached to the suspension knuckles. The front brake anti-lock sensor indicators (2C182) are pressed onto the backside of the rotors.
> Rear axle speed sensor integrated into the rear axle housing.
> Acceleration sensor attached to the left frame rail under the brake booster.
> Stoplight switch (13480) located at the brake pedal (2455).
> Pump motor relay (14N089) located in the power distribution box.
> Main relay (14N089) located in the trailer relay box.
> System diode located in the trailer relay box.
> Fuses for system power and the pump motor, both located in the power distribution box.
The anti-lock hydraulic control unit (2C215) is located in the front of the engine compartment on the left frame rail. It consists of a valve body assembly, a pump and a motor assembly.
When the brakes are applied, fluid is forced from the master cylinder outlet ports to the anti-lock hydraulic control unit (2C215) inlet ports. This pressure is transmitted through three normally open solenoid valves contained inside the anti-lock hydraulic control unit, then through the outlet ports of the anti-lock hydraulic control unit to the wheels. The primary (rear) circuit of the master cylinder feeds the rear brakes. The secondary (front) circuit of the brake master cylinder (2140) feeds the front brakes. If the anti-lock brake electronic control module senses that a wheel is about to lock, based on wheel speed sensor data, it pulses the normally open solenoid valve closed, for that circuit. This prevents any more fluid from entering that circuit. The anti-lock brake electronic control module then looks at the sensor signal from the affected wheel again. If that wheel is still decelerating, it opens the normally closed solenoid valve for that circuit. This dumps any pressure that is trapped between the normally open valve and the brake back to the reservoir. Once the affected wheel comes back up to speed, the anti-lock brake electronic control module returns the valves to their normal condition allowing fluid flow to the affected brake.
The anti-lock brake electronic control module monitors the electromechanical components of the system. Malfunction of the Anti-Lock Brake System will cause the Anti-lock brake electronic control module to shut off or inhibit the system. However, normal power assisted braking remains. Malfunctions are indicated by the amber ABS warning light inside the vehicle.
The 4-wheel anti-lock brake system is self monitoring. When the ignition switch is placed in the RUN position, the anti-lock brake electronic control module will perform a preliminary self check on the anti-lock electrical system indicated by a momentary illumination of the amber ABS warning light in the instrument cluster. During vehicle operation, including normal and anti-lock braking, the anti-lock brake electronic control module monitors all electrical anti-lock functions and some hydraulic operations.
In most malfunctions of the anti-lock brake system, the amber ABS warning light will be illuminated. However, most malfunctions are recorded as a coded number in the anti-lock brake electronic control module memory and assist in pinpointing the component needing service. If system is OK, Code 16 will be present.
The anti-lock brake electronic control module is located in the engine compartment behind the driver's side head lamp on a bracket mounted on the surface of the plastic fender apron.
It is an on-board diagnostic, non-repairable unit consisting of two microprocessors and the necessary circuitry for their operation. These microprocessors are programmed identically. The anti-lock brake electronic control module monitors system operation during normal driving as well as during anti-lock braking.
In addition, the anti-lock brake electronic control module provides a speed signal to the programmable speedometer/odometer module (PSOM).
Under normal driving conditions, the microprocessors produce short test pulses to check the electrical system without any mechanical reaction. The anti-lock brake electronic control module continuously monitors the speed of each wheel to determine if any wheel is beginning to lock. When a wheel locking tendency is detected, the anti-lock brake electronic control module signals the appropriate solenoid valve in the anti-lock hydraulic control unit (2C215) to open or close as well as the pump motor to turn on and recycle brake fluid back to the brake master cylinder (2140). This results in moderate pulsations of the brake pedal (2455) and pump noise which may be heard in the passenger compartment. During normal braking, the brake pedal feel will be identical to a standard brake system.
Most concerns which occur to the anti-lock brake system will be stored as a coded number in the Keep-Alive Memory (KAM) of the anti-lock brake electronic control module. This means that once a code is stored, it will be retained by the anti-lock brake electronic control module even with the key in the OFF position. The codes can be retrieved by following the on-board diagnostic procedures.
A new light truck 4-Wheel ABS was introduced as standard equipment on the 1993 Explorer and Bronco, and it was revised in 1994 on Broncos, requiring a different control module revision for '94-96 Broncos, compared to the original '93 Bronco module.
Although this system shares some of the features of passenger car 4-Wheel ABS and some of the features of light truck Rear ABS (RABS), it has its own unique features, especially when it comes to diagnosing concerns.
Here are some general features of the light truck 4-Wheel ABS, some of which are common to the other systems as well:
> System prove-out at vehicle start-up of the light truck 4-Wheel ABS is the same as for passenger car and 4-Wheel light truck ABS.
> The amber anti-lock warning light will come on as soon as the vehicle is turned on or started to verify bulb operation, and will go out after 2-3 seconds, provided everything is normal.
> The 4-Wheel ABS operates in 4 x 4 as well as 4 x 2.
> In ABS mode, the brake pedal will pulsate and the pump motor noise may be heard from the engine compartment.
> When ABS codes are read out, code 16 will be present if the system is OK. This code is similar to the EEC's code 111. It basically means that the 4-Wheel ABS Electronic Control Unit (ECU) is indicating that everything is functioning properly electrically. In other words, the components, such as the pump motor and the valves, will activate when needed.
> Conventional bleed is all that is required for service on the new 4WABS including unhooking brake lines from the hydraulic controller (HCU). However, if the HCU is replaced, a special "bleed box" and electrical jumper will be required to bleed the replacement HCU. This procedure is not the same as for the passenger car systems.
There are differences between the Rear ABS and the light truck 4-Wheel ABS.
Front Wheel Control
In the 4WABS system, the front wheels, are now also under ABS control. This is accomplished by adding sensors at each front wheel while retaining the rear axle sensor that has been used since 1987 for light truck Rear ABS.
Open vs Closed Systems
There is an important difference in how the 4-Wheel ABS handles a low fluid condition and reacts to the red brake warning light.
RABS Open System
The Rear ABS is what is called an "open" hydraulic system. When an ABS stop is initiated and wheel lock is detected, the isolation or inlet valve closes, preventing any more fluid from going to the rear wheels.
Then, if necessary to prevent lockup, an outlet or dump valve opens to bleed fluid out of the rear brake lines. That fluid goes to an accumulator and is held there for the remainder of the stop.
In RABS "open" system, if there is a low fluid condition and the red warning light comes on, the ABS electronic control unit will detect this condition, turn the amber ABS warning light on and disable the system. The vehicle operator will still have basic brake system performance, but the benefit of RABS is lost.
The reason for disabling the RABS is that the low fluid condition could be caused by a leak in the brake system, decreasing its performance.
4WABS Closed System
In the 4-Wheel ABS the red brake light is not monitored. This is because the system is a "closed" hydraulic system.
If you are in a 4WABS stop and a wheel goes to lock, the isolation valve will activate, just as with the RABS. No more fluid will be directed to the wheel that may lock up.
If it still appears that a wheel is going to lock, the dump valve will open, allowing the fluid to bleed to an accumulator similar to the one in the RABS.
However, in this "closed" 4-Wheel ABS there is a pump that will move fluid from the accumulator, back to the line between the master cylinder and the hydraulic control unit, where it may be recycled again through the isolation and dump valves.
Since the fluid in this closed system is still available to both front and rear circuits, the number of cycles is unlimited. The closed 4WABS isn't shut down when the low fluid light comes on, as was the case with the open system.
Stoplamp Switch Signal
The new 4WABS uses the signal from the stoplamp switch (located on the brake pedal) differently than RABS. RABS is turned on when it receives a signal from the stoplamp switch indicating that the brakes are applied. 4WABS is already turned on and will activate when impending wheel lockup is detected.
4WABS first looks at the wheel-speed sensors. If a wheel appears to be going to lock, the system will activate the ABS with or without the stoplamp switch signal.](https://www.supermotors.net/getfile/280776/fullsize/4wabs-valves.jpg)