# Rear Wheel Anti-lock Brake System (RABS)

## General Information

The Rear Wheel Anti-lock Brake System (RABS) is an electronically operated, hydraulically controlled rear wheel brake control system. Major components include the vacuum power brake booster, master cylinder, the speed sensor, and the control valve or Hydraulic Control Unit (HCU) and the control module.

The system is designed to retard rear wheel lockup during periods of high wheel slip when braking. Retarding the rear wheel lockup is accomplished by modulating fluid pressure to the rear wheel brake circuit. When the control module detects a variation in voltage across the speed sensor, the RABS is activated. The control module opens and closes various valves located inside the HCU/control unit. These valves, called dump and isolation valves, modulate the hydraulic pressure to the rear wheels by applying and venting the pressure to the brake fluid circuits.

## Diagnosis and Testing

### DIAGNOSTIC TROUBLE CODE RETRIEVAL

#### Except 1999-00 Super Duty Models

RABS diagnostic trouble codes—Except 1999-00 Super Duty models

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WARNING
Care must be taken to connect only the black/orange stripe wire to ground. Attaching the mating connector wire to ground will result in a blown fuse.

Verify the ignition switch is in the RUN position (engine does not need to be running). Next, locate the black RABS II diagnostic connector. The diagnostic connector has two mating halves, one of which has a black/orange wire connected to it. Detach the two connector halves. Attach one end of a jumper wire to the black/orange wire side of the diagnostic connector. Ground the opposite end of the jumper wire by connecting it to a ground until the ABS light begins to flash. Grounding this wire should start the yellow ABS warning indicator flashing.

The code consists of a number of short flashes and ends with a long flash. Count the short flashes and include the following long flash in the count to obtain the code number. For example, three short flashes followed by one long flash indicates diagnostic trouble code 4. The code will continue to repeat itself until the key is turned off. It is recommended that the code be verified by reading it several times. This code will be used later for system repair instructions. It should be written down for future use. A diagnostic trouble code 16 is obtained when the anti-lock brake control module detects normal system operation.

#### 1999-00 Super Duty Models

The Diagnostic Trouble Codes (DTC) are retrieved using a scan tool, such as Rotunda NGS Tester 007-00500 or its equivalent. Refer to the manufacturer's instructions for operating the tool and retrieving the codes.

The Data Link Connector (DLC) for the ABS is the OBDII connector located under the driver's side of the instrument panel, underneath the steering column.

Click to enlarge

### ERASING DIAGNOSTIC TROUBLE CODES

#### Except 1999-00 Super Duty Models

The last step of the system pre-check always includes clearing the keep alive memory (KAM). Turn off the ignition while the diagnostic connector halves are separated as described in retrieving diagnostic trouble codes in this section. The diagnostic connector should be re-assembled to provide KAM power to the anti-lock brake control module.

#### 1999-00 Super Duty Models

The Diagnostic Trouble Codes (DTCs) are cleared using a scan tool, such as Rotunda NGS Tester 007-00500 or its equivalent. Refer to the manufacturer's instructions for operating the tool and clearing the codes.

## Speed Sensor

### REMOVAL & INSTALLATION

Detach the connector on the speed sensor

Remove the sensor hold-down bolt and ...

Remove the sensor

Click to enlarge

The sensor is located on the rear axle housing.

1. Raise and safely support the vehicle securely on jackstands.
2. Carefully clean the axle surface to keep dirt from entering the housing.
3. Detach the connector on the speed sensor.
4. Remove the sensor hold-down bolt.
5. Remove the sensor.
6. If a new sensor is being installed, lubricate the O-ring with clean engine oil. Carefully push the sensor into the housing aligning the mounting flange hole with the threaded hole in the housing. Tighten the hold-down bolt to 30 ft. lbs. (41 Nm)—If the old sensor is being installed, clean it thoroughly and install a new O-ring coated with clean engine oil.
7. Attach the connector on the speed sensor.
8. Lower the vehicle.

## Tone (Exciter) Ring

### REMOVAL & INSTALLATION

The exciter ring is located in the differential case inside the axle housing on the rear gear. The rear differential must be disassembled to replace the ring. Refer to Section 7.

## Control Valve

### REMOVAL & INSTALLATION

#### 1997-98 F-250HD, F-350 and F-Super Duty; F-150 and F-250

Rear anti-lock braking system valve—1997-98 F-250HD, F-350 and F-Super Duty

Click to enlarge

CAUTION
Brake fluid contains polyglycol ethers and polyglycols. Avoid contact with the eyes and wash your hands thoroughly after handling brake fluid. If you do get brake fluid in your eyes, flush your eyes with clean, running water for 15 minutes. If eye irritation persists, or if you have taken brake fluid internally, IMMEDIATELY seek medical assistance.

The valve is located on the frame rail, under the vehicle on the driver's side on the 1997-98 F-250HD, F-350 and F-Super Duty. The valve is located in the brake lines, below the master cylinder.

1. Disconnect the brake lines from the valve and plug the lines.
2. Disconnect the wiring harness at the valve.
3. Remove the 3 nuts retaining the valve to the frame rail and lift out the valve.
4. Installation is the reverse of removal. Don't overtighten the brake lines. Bleed the brakes.

#### 1999-00 F-250SD, F-350 and F-Super Duty

Remove rubber dust cover

Detach the anti-lock brake control module electrical connectors

Disconnect the hydraulic brake lines

Remove the HCU bracket and remove the HCU

WARNING
Electronic modules are sensitive to static electrical charges. If exposed to these charges, damage may result.

1. On diesel engine equipped vehicles, remove the battery tray.
2. On gasoline engine equipped vehicles, disconnect the battery tray ground cable.
3. On gasoline engine equipped vehicles, remove the engine air cleaner assembly.
4. On gasoline engine equipped vehicles, remove the EVAP canister.
5. Remove rubber dust cover.
6. Remove pushpin.
7. Detach the anti-lock brake control module electrical connectors.

CAUTION
Brake fluid contains polyglycol ethers and polyglycols. Avoid contact with the eyes and wash your hands thoroughly after handling brake fluid. If you do get brake fluid in your eyes, flush your eyes with clean, running water for 15 minutes. If eye irritation persists, or if you have taken brake fluid internally, IMMEDIATELY seek medical assistance.

WARNING
Plug each port to prevent brake fluid from spilling.

8. Disconnect the hydraulic brake lines.
9. Remove the HCU bracket
10. Remove the HCU.
11. The installation is the reverse of the removal.
12. Bleed the RABS system.

## Control Module

### REMOVAL & INSTALLATION

#### 1997-98 F-250HD, F-350 and F-Super Duty

Rear anti-lock braking system module

Click to enlarge

The module is located on the firewall just inboard of the master cylinder.

1. Disconnect the wiring harness.
2. Remove the 2 attaching screws and lift out the module.
3. Installation is the reverse of removal.

### F-150 and F-250

WARNING
Electronic modules are sensitive to static electrical charges. If exposed to these charges, damage may result.

1. Disconnect the negative battery cable.
2. Remove the instrument panel cluster panel. Refer to Section 6.
3. Detach the anti-lock brake control module electrical connector.
4. Remove the module retaining bolts.
5. Remove the anti-lock brake control module.
6. The installation is the reverse of the removal.

#### 1999-00 F-250SD, F-350 and F-Super Duty

Control module mounting—1999-00 F-250SD, F-350 and F-Super Duty

WARNING
Electronic modules are sensitive to static electrical charges. If exposed to these charges, damage may result.

1. On diesel engine equipped vehicles, remove the battery tray.
2. On gasoline engine equipped vehicles, disconnect the battery ground cable.
3. On gasoline engine equipped vehicles, remove the engine air cleaner.
4. On gasoline engine equipped vehicles, remove the EVAP canister.
5. Remove rubber dust cover.
6. Remove pushpin.
7. Detach the three anti-lock brake control module electrical connectors.
8. Remove the screw.
9. Lift and remove the anti-lock brake control module.
10. The installation is the reverse of the removal.

## Bleeding the ABS System

The RABS system is bleed in the same manner as a conventional brake system. Refer to brake bleeding in this section.