Anti-Lock Brake System Ford Bronco II I

DESCRIPTION & OPERATION

The Rear Anti-Lock Brake System (RABS) is designed to prevent vehicle from skidding due to rear brake lock up. This is done by controlling the amount of hydraulic fluid pressure to the rear wheel cylinders. The system consists of a warning light, computer module, an electro-hydraulic valve, speed sensor and a excitor ring. (Scheme 1)

As vehicle is moving, speed sensor mounted on rear axle signals the computer module. This is done by the speed sensor generating an AC frequency signal as the teeth on exciter ring, located on differential case, passes by the speed sensor probe. When the brake pedal is depressed, control module senses drop in vehicle speed.

If rate of deceleration indicates lock-up is occurring, the control module activates the electro-hydraulic valve which closes the internal isolation valve. If speed sensor signal still indicates lock-up occurrence, the control module activates the dump solenoid which bleeds off brake fluid pressure. When the control module determines that lock-up condition no longer exists, all valves and solenoids open allow brake system to return to normal operation.

Scheme 1

SAFETY PRECAUTIONS

1. NEVER open a bleeder valve or loosen a hydraulic line while ABS is pressurized.
2. NEVER disconnect or reconnect any electrical connectors while ignition is on. Damage to ABS control unit may result.
3. DO NOT attempt to bleed hydraulic system without first referring to the appropriate article.
4. Only use specially designed brake hoses/lines on ABS-equipped vehicles.
5. DO NOT tap on speed sensor components (sensor, sensor rings). Speed rings must be pressed, NOT hammered into hubs. Striking these components can cause demagnetization or a loss of polarization, affecting the accuracy of the speed signal returning to the ABS control unit.
6. DO NOT mix tire sizes. Increasing the width, as long as tires remain close to the original diameter, is acceptable. Rolling diameter must be identical for all 4 tires. Some manufacturers recommend tires of the same brand, style and type. Failure to follow this precaution may cause inaccurate wheel speed readings.
7. DO NOT contaminate speed sensor components with grease. Only use recommended anti-corrosion coating.
8. When speed sensor components have been removed, ALWAYS check sensor-to-ring air gaps when applicable. These specifications can be found in each appropriate article.
9. ONLY use recommended brake fluids. DO NOT use silicone brake fluids in an ABS-equipped vehicle.
10. When installing transmitting devices (CB's, telephones, etc.) on ABS-equipped vehicles, DO NOT locate the antenna near the ABS control unit (or any control unit).
11. Disconnect all on-board computers, when using electric welding equipment.
12. DO NOT expose the ABS control unit to prolonged periods of high heat (185°F/85°C for 2 hours is generally considered a maximum limit).

DIAGNOSTIC SYSTEM

Rear Anti-Lock Brake System has a self test capability. There are 2 warning lights, located on instrument panel, to inform driver of malfunction. Red warning light indicates low fluid level, parking brake on or low vacuum (diesel models). A Yellow light indicates a malfunction in the system. This light should come on for approximately 2 seconds when ignition is turned on or engine is cranking. When control module detects a malfunction, the anti-lock brake system becomes inoperative and the Yellow warning light will come on.

RETRIEVING CODES

To obtain stored trouble code, momentarily ground diagnostic connector and record number of times light flashes. (Scheme 2) DO NOT turn ignition off before grounding connector or code will be lost. Only the first code stored will appear until that problem is corrected.

Scheme 2

Scheme 3

Scheme 4

Note. The test connector is usually located behind left door pillar panel.

INOPERATIVE YELLOW LIGHT/IGNITION ON & RED LIGHT OFF

1. Check computer module connector to ensure it is plugged in. Check 20-amp fuse. Unplug computer module connector. Check resistance between terminal No. 4 and ground. (Scheme 5) If resistance is one ohm or more, check terminals. If terminals are okay, repair open module ground wire. If resistance is less than one ohm, check voltage between terminal No. 7 and ground, with ignition on.
2. If there is no voltage, check 15-amp anti-lock fuse. If fuse is good or voltage is between 11-13 volts, check bulb. Replace bulb if defective. If bulb is good, check voltage at bulb socket. If there is voltage, replace bulb. If there is no voltage, check for bad bulb socket, bad connections or defective wiring.

Scheme 5

YELLOW LIGHT OKAY & RED LIGHT ON

1. Ensure parking brake is not set. If light is still on, unplug parking brake switch connector. If light goes off, replace parking brake switch.
2. If light is still on, go to step 3) on gasoline engine models. On models equipped with diesel engine, operate vehicle until proper vacuum is built up. If light is still on, unplug vacuum warning switch. If light goes out, replace vacuum switch. If light stays on, go to step 3).
3. On all models, unplug computer module connector. If light goes out, replace computer module. If light stays on, check wiring for shorts or ground.

YELLOW & RED LIGHTS ON STEADY OR INTERMITTENT

1. Check brake fluid level. Fill if necessary. If brake fluid is okay and lights are still on, ensure parking brake is not on. On models equipped with diesel engine, build vacuum up. If both lights goes off, replace isolator diode in wiring harness. Isolator diode is located in engine compartment by the left fender apron in the fluid level indicator wiring on master cylinder.
2. Voltage at fluid level sensor should be 11-13 volts. If both lights are still on, temporarily ground diagnostic lead. If code 4 flashes, go to «CODE 4»(/ford/bronco-ii/i-1984-1990/remont/anti-locktraction-control/#anti-lock-brake-system__code-4-electro-hydraulic-valve-switch) in this article. If any other code was flashed, replace computer module. If both lights stayed on, unplug connector from master cylinder switch. Connect a jumper wire between 2 Purple and White wires on connector.
3. Turn ignition on. If both lights go off, check master cylinder switch connector. If okay, replace switch. If both lights stayed on, unplug computer module connector. Turn ignition on. If both lights go out, replace computer module. If Yellow light goes off and Red light stays on, check for short in brake warning light circuit. If okay, check ignition switch.

YELLOW LIGHT STAYS ON & RED LIGHT OFF

1. Apply parking brake to ensure Red light is operative. Temporarily ground diagnostic lead. If trouble code appears, go to appropriate code. If no trouble code appears, disconnect computer module. Check for voltage between terminal No. 1 on connector and ground.
2. If there is no voltage, check 20-amp fuse. If fuse is okay, check power lead from fuse block to computer module. If there is 11-13 volts, check master cylinder switch. Ensure connector is plugged in. If light is still on, unplug connector. If light is still on with connector unplugged, check resistance between terminal No. 12 and ground on connector.
3. If resistance is less than 100,000 ohms, repair ground wire to computer module. Ensure there is no loose or dirty terminals. If resistance is 100,000 ohms or more, plug in computer module connector. Turn ignition switch to "RUN" position. Check voltage at Purple and White wires at master cylinder switch with connector plugged in.
4. If voltage is 8-13 volts, unplug computer module connector. Check for voltage between terminal No. 2 and ground. Turn ignition on. If voltage is below 8 volts or above 13 volts, check for open wire between computer module and master cylinder switch. If voltage is 8-13 volts, replace computer module.
5. If in step 4), voltage was 7 or less volts, connect a test light to positive battery post and to Purple and White wire which had low voltage reading at master cylinder connector. Turn ignition on. If Yellow light goes off and test lights comes on, check for contaminated master cylinder switch terminals or defective switch. If Yellow light and test light does not come on, replace isolator diode.

FLASHING YELLOW LIGHT & RED LIGHT OFF

Check all connections for looseness or contamination. If connections are okay, replace computer module.

LIGHTS OKAY BUT REAR WHEELS LOCK-UP

Check vehicle stoplight operation. If stoplights do not operate, repair as necessary. If stoplights operate, check rear brakes. Repair or replace brake if necessary. If brakes are okay, replace computer module and retest system. If system is still not correct, check speed sensor. See CODE 6 under DIAGNOSTIC CODES in this article.

CODE 1 - YELLOW LIGHT FLASHES CONT. & EVENLY (RED LIGHT OFF)

If trouble code is 16 or higher, replace computer module.

CODE 2 - OPEN ISOLATE CIRCUIT

1. Unplug computer module connector. Check resistance between terminal No. 13 and ground. If resistance is 3-6 ohms, replace computer module. If resistance is greater than 6 ohms, unplug electro-hydraulic valve connector. Check resistance between terminal No. 3 and ground.
2. If resistance is one ohm or more, repair open circuit. If resistance is less than one ohm, check resistance between terminals No. 2 and 3. If resistance is 3-6 ohms, repair open in wire between terminal No. 2 and computer module. If resistance is over 6 ohms, replace electro-hydraulic valve.

CODE 3 - OPEN DUMP VALVE CIRCUIT

1. Unplug computer module connector. Check resistance between ground and terminals No. 8 and 14. Terminals No. 8 and 14 are connected together. If resistance is 1-3 ohms, replace computer module. If resistance is over 3 ohms, unplug electro-hydraulic valve connector.
2. Check resistance between terminals No. 1 and 3. If resistance is 1-3 ohms, repair open dump solenoid wire at terminal No. 1. If resistance is over 3 ohms, replace electro-hydraulic valve.

CODE 4 - ELECTRO-HYDRAULIC VALVE SWITCH CLOSED (1987)

1. Unplug electro-hydraulic connector. Check resistance between Electro-Hydraulic Valve Connector (terminal No. 4) and ground. (Scheme 5) Place ohmmeter on 20,000 ohms scale. If resistance is less than 10,000 ohms, replace electro-hydraulic valve. If resistance is 10,000 ohms or more, unplug computer module connector.
2. Check resistance between Computer Module Connector (terminal No. 6) and ground. (Scheme 5) Place ohmmeter on 20,000 ohms scale. If resistance is 10,000 ohms or more, replace computer module. If resistance is less than 10,000 ohms, repair shorted wire from electro-hydraulic valve to computer module.

CODE 4 - ELECTRO-HYDRAULIC VALVE SWITCH CLOSED (1988)

1. Unplug electro-hydraulic connector. Check resistance between Electro-Hydraulic Valve Connector (terminal No. 4) and ground. (Scheme 5) Place ohmmeter on 20,000 ohms scale. If resistance is less than 10,000 ohms, replace electro-hydraulic valve. If resistance is 10,000 ohms or more, go to step 2.
2. Place ohmmeter on 20,000 ohms scale. Check resistance between Electro-Hydraulic Valve Connector (terminal No. 4) and valve ground pin (terminal 3). (Scheme 5) If resistance is less than 10,000 ohms, replace electro-hydraulic valve. If resistance is 10,000 ohms or more, go to step 3.
3. Disconnect battery. Place ohmmeter on 200,000 ohms scale. Unplug electro-hydraulic connector. Check resistance between Computer Module Connector (terminal No. 6) and ground. (Scheme 5) If resistance is 100,000 ohms or more, replace computer module. If resistance is less than 100,000 ohms, repair shorted wire from electro-hydraulic valve to computer module.

CODE 5 - BRAKES LOCKING

1. If rear brakes are grabbing, unplug computer module and road test vehicle. If rear brakes are still grabbing, repair rear brakes. If rear brakes operate okay, go to step 2) on 4WD models. On 2WD models, replace electro-hydraulic valve.
2. Ensure transfer case system is operating properly. Unplug computer module. Check voltage between terminal No. 5 and ground. Voltage should be 11-13 volts with vehicle in 2WD. When shifted to 4WD, voltage should be less than one volt. If voltage is above one volt when shifted to 4WD, repair 4WD switch or wiring. If voltage was less than one volt, ensure transfer case is shifting properly. If not, repair as necessary. If transfer case is shifting properly, replace electro-hydraulic valve.

CODE 6 - ERRATIC SENSOR SIGNAL

1. Aggressively move sensor wire from differential connector. Check for loose connections. If all connections appear tight, unplug computer module connector. Check resistance between terminals No. 3 and 10. Move sensor wire while check resistance. If resistance reading is erratic, repair loose connection.
2. If resistance is 1000-2000 ohms, remove sensor from differential. Check sensor for metal chips. If there are metal chips, drain and clean differential. Ensure exciter ring does not have any broken or chipped teeth. If there is no metal chips, check AC output voltage at sensor diagnostic sensor connector. Connector is located on left inside fender panel in engine compartment.
3. Raise vehicle until rear wheels clear the floor. Start engine. Operate vehicle so wheels rotate at 3 MPH. Place voltmeter on "2000 mV" AC scale. If reading is 200 mV or more and does not vary, replace computer module. If reading is less and 200 mV or varies, replace sensor and recheck AC output.

CODE 7 - NO VALVE ISOLATION

Unplug computer module connector. Check resistance between terminal No. 13 and ground. If resistance is 3-6 ohms, replace computer module. If resistance is less than 3 ohms, unplug electro-hydraulic connector. Check resistance between terminals No. 2 and 3. If resistance is less than 3 ohms, replace electro-hydraulic valve. If resistance is 3-6 ohms, repair shorted wire between terminal No. 2 on solenoid connector and computer module.

CODE 8 - NO DUMP VALVE

1. Unplug computer module connector. Check resistance between terminals No. 14 and 8, and ground. If resistance is 1-3 ohms, replace computer module. If resistance is less than one ohm, unplug electro-hydraulic valve connector.
2. Check resistance between terminals No. 1 and 3. If resistance is less than one ohm, replace electro-hydraulic valve. If resistance is 1-3 ohms, repair shorted wire between terminal No. 1 on electro-hydraulic connector and computer module.

CODE 9 - HIGH SENSOR RESISTANCE

1. Unplug sensor connector at differential. Check resistance between sensor terminals. If resistance is over 2500 ohms, replace sensor. If resistance is 1000-2500 ohms, reconnect sensor connector. Unplug computer module connector.
2. Check resistance between terminals No. 3 and 10. If resistance is 1000-2500 ohms, replace computer module. If resistance is over 2500 ohms, repair wire between sensor and computer module. ALWAYS use high flex wire from manufacturer.

CODE 10 - LOW SENSOR RESISTANCE

1. Unplug computer module connector. Check resistance between terminals No. 3 and 10. If resistance is 1000-2500 ohms, replace control module. If resistance is less than 1000 ohms, unplug sensor connector at differential. Check resistance between sensor terminals.
2. If resistance is less than 1000 ohms, replace sensor. If resistance is 1000-2500 ohms, check resistance between terminal No. 10 and ground. If resistance is 100,000 ohms or less, repair wire between sensor and computer module. ALWAYS use high flex wire from manufacturer.

CODE 11 - STOPLIGHT OPERATION

Check stoplight fuse, stoplight switch adjustment and wiring. If system is okay, unplug computer module connector. Check voltage between terminal No. 11 and ground with brake pedal is depressed. If there is no voltage, repair open wire. If voltage is 8-13 volts, check stoplight and turn signal wiring. Repair as necessary.

CODES 12-16 - COMPUTER MODULE MALFUNCTIONS

If code 12-16 appears, replace computer module.

TORQUE SPECIFICATIONS

TORQUE SPECIFICATIONS

Bronco II RABS Wiring Diagram. Scheme 6

Bronco & F Series RABS Wiring Diagram. Scheme 7

See also:
• CODE 4
• CODE 6