Volvo 240/740/760/780/940/960 1990-1998

System Components

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Description

Wheel Speed Sensors

See Figure 1

The speed signal is sent to the control unit via a sensor and pulse wheel assembly. Each front wheel has its own assembly. Rear wheel speed is measured by one sensor and pulse wheel assembly on all vehicles except for the 850/C70/S70/V70 models which have one on each wheel.

As the teeth of the pulse generator pass the tip of the sensor, the changes from peak to valley to peak generate a small AC voltage in the sensor. The frequency of the voltage-which increases with wheel speed-is used by the CU to determine wheel speed. By comparing wheel speed during braking, the control unit determines impending wheel lock.



Click image to see an enlarged view

Fig. Fig. 1: Various wheel speed sensors

Electronic Control Unit (CU)

The CU is located below the left dashboard, above the left kick panel on the 240 and most of the 700 series. It is located on the right front wheel housing on 760 models with a B280 engine. On most other models, it is attached to the hydraulic modulator and is alongside the brake master cylinder. The unit is a microprocessor that receives and processes signals according to predetermined logic, and supplies a control signal to the solenoid valves located on the hydraulic modulator.

The control unit contains the safety or monitor circuitry which will disable the ABS system if any electrical fault is detected. The safety circuit also monitors battery voltage and will disable the system if voltage becomes too high or too low.

The electronic control unit cannot be repaired or serviced; if an internal fault is detected, the unit must be replaced.

Hydraulic Modulator

See Figure 2

Usually located under or near the brake master cylinder in the engine compartment, the hydraulic modulator contains the solenoid valves, the system recirculation pump, the solenoid control relay and the pump control relay. Certain engine and body combinations will locate the hydraulic modulator on the left wheel arch.

The 3 internal solenoids are electrically operated valves. When no current is applied, the valves are in the normal or open position, allowing brake fluid pressure to be controlled by the brake pedal. Engagement of ABS causes the solenoid to move through part of its travel, blocking the brake fluid passage. This puts the valve in the hold position, maintaining existing brake line pressure. If the CU still senses a wheel locking or about to lock, the solenoid is moved to the full extent of its travel, opening the pressure relief passage. Brake fluid in the circuit is allowed to escape, reducing line pressure and releasing the brake. The released fluid is momentarily held in a pressure accumulator which serves to reduce the pedal pounding sensation to the operator. Once released from the accumulator, the fluid is pumped back into the system for reuse.

With the exception of the two relays mounted on the unit, the hydraulic unit has no serviceable parts and cannot be repaired. If an internal fault is found, the unit must be replaced.



Click image to see an enlarged view

Fig. Fig. 2: Cutaway view of the hydraulic modulator assembly

Brake Lamp

The red dashboard BRAKE warning lamp functions in a manner identical to non-ABS vehicles. If the fluid level should drop below an acceptable level, the lamp will light as a warning to the operator. If the BRAKE lamp is lit, braking function on the vehicle may be impaired; do not operate the vehicle until the status and reliability of the braking system is determined.

Anti-Lock Lamp

See Figure 3

The Anti-Lock Braking System (ABS) warning lamp is coupled to the ABS CU. The lamp will light briefly during engine start-up as the CU performs an initial check of the system. If no faults are found, the CU will extinguish the lamp within a few seconds. After this initial test, the lamp should not come on during vehicle operation.

If the ABS lamp does come on during operation, the CU has detected a fault and disabled the system. If only the ABS warning lamp is lit, the vehicle retains normal braking characteristics and may be safely driven. If both warning lamps are lit, the braking capacity of the vehicle may be impaired.



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Fig. Fig. 3: Location of the ABS light on the instrument cluster

SYSTEM PRECAUTIONS





If the vehicle is equipped with an air bag system, always properly disable the system before commencing work on the ABS system. Refer to Chassis Electrical for the correct disabling procedure.
 
Certain components within the ABS system are not intended to be serviced or repaired individually. Only those components with removal and installation procedures should be serviced.
 
Do not use rubber hoses or other parts not specifically specified for the ABS system. When using repair kits, replace all parts included in the kit. Partial or incorrect repair may lead to functional problems and require the replacement of other components.
 
Lubricate rubber parts with clean, fresh brake fluid to ease assembly. Do not use lubricated shop air to clean parts; damage to rubber components may result.
 
Use only brake fluid from an unopened container. Use of suspect or contaminated brake fluid can reduce system performance and/or durability.
 
A clean repair area is essential. Perform repairs after components have been thoroughly cleaned. Do not allow ABS components to come into contact with any substance containing mineral oil; this includes used shop rags.
 
The control unit is a microprocessor similar to other computer units in the vehicle. Insure that the ignition switch is OFF before removing or installing controller harnesses. Avoid static electricity discharge at or near the controller.
 
Never disconnect any electrical connection with the ignition switch ON unless instructed to do so in a test.
 
Avoid touching connector pins with fingers.
 
Leave new components and modules in the shipping package until ready to install them.
 
To avoid static discharge, always touch a vehicle ground after sliding across a vehicle seat or walking across carpeted or vinyl floors.
 
If any arc welding is to be done on the vehicle, the ABS control unit should be disconnected before welding operations begin.
 
Never allow welding cables to lie on, near or across any vehicle electrical wiring.
 
If the vehicle is to be baked after paint repairs, disconnect and remove the control unit from the vehicle.
 

REMOVAL & INSTALLATION



Hydraulic Modulator
  1. Disconnect the negative battery cable.
  2.  
  3. Remove the cover (if equipped) from the hydraulic modulator.
  4.  
  5. Remove both relays (if equipped) from the top of the unit; disconnect the wiring connector at the unit.
  6.  
  7. Remove the air inlet hose.
  8.  
  9. Unplug any connectors from the unit.
  10.  
  11. Place rags or towels around the unit to absorb brake fluid which will be spilled.
  12.  
  13. Clean the line connections thoroughly.
  14.  
  15. Label each line for installation.
  16.  
  17. Remove the brake lines from the modulator.
  18.  
  19. Remove the bolts from the modulator support and push the support to the right.
  20.  
  21. Remove the hydraulic modulator.
  22.  

To install:
  1. If a new modulator is being installed, remove the hexagonal plugs from the old unit and install on the new unit. Check that the rubber pads are not damaged; install the rubber pads onto the hexagonal plugs.
  2.  
  3. Install the modulator and tighten the support. If installing a new unit, remove the plugs from the brake line ports.
  4.  
  5. Reconnect the brake lines according to the labels made at removal. The lines must be in their exact original positions.
  6.  
  7. Remove the rags from the work area and dispose of them properly.
  8.  
  9. Plug in any removed connectors.
  10.  
  11. Install the air inlet hose.
  12.  
  13. Install the relays (if equipped) on the hydraulic modulator.
  14.  
  15. Install the cover on the unit.
  16.  
  17. Bleed the brake system. Vehicles with hydraulic clutches may require bleeding of the clutch system as well.
  18.  
  19. When bleeding is complete, test the brake system by having an assistant press hard on the brake pedal; keep it depressed for 30 seconds. During the 30 second period, check that no leakage occurs at the brake line connections on the hydraulic modulator.
  20.  
  21. Connect the negative battery cable.
  22.  
  23. Test drive the vehicle, confirming system function.
  24.  

Control Unit
1990-93 Models

See Figure 4

The control unit is located under the steering wheel, behind the knee bolster panel.

  1. Disconnect the negative battery cable.
  2.  
  3. Remove the knee bolster panel under the steering wheel..
  4.  
  5. Loosen or remove the clips and retainers holding the control unit.
  6.  
  7. Lift out the unit.
  8.  
  9. Remove the electrical harness from the unit.
  10.  

To install:
  1. Attach the harness to the control module.
  2.  
  3. Place the control module into place and install the clips and retainers.
  4.  
  5. Install the knee bolster panel.
  6.  
  7. Connect the negative battery cable.
  8.  



Click image to see an enlarged view

Fig. Fig. 4: Control unit location on 1990-93 vehicles

1994-98 Models

See Figure 5

The control unit is located in the engine compartment, near the master cylinder/brake booster.

  1. Disconnect the negative battery cable.
  2.  
  3. Clean the connector of any dirt, grime and/or oil.
  4.  
  5. Remove the connector from the control unit.
  6.  
  7. Remove the control unit retaining bolts.
  8.  
  9. Lift the control unit out of the engine compartment.
  10.  

To install:
  1. Lower the control unit into place aligning the bolts holes.
  2.  
  3. Tighten the retaining bolts.
  4.  
  5. Install the connector on the control unit.
  6.  
  7. Connect the negative battery cable.
  8.  



Click image to see an enlarged view

Fig. Fig. 5: The control unit and hydraulic modulator assembly-850 series model shown

Wheel Speed Sensors
FRONT

See Figures 6, 7, 8 and 9

  1. Raise and safely support the front of the vehicle.
  2.  
  3. Remove the tire and wheel.
  4.  
  5. With the ignition switch OFF , disconnect the wheel speed sensor lead from the ABS harness. Remove any retaining bolts or clips holding the harness in place.
  6.  

Clips and retainers must be reinstalled in their exact original location. Take careful note of the position of each retainer and of the correct harness routing during removal.

  1. Remove the single bolt holding the speed sensor.
  2.  
  3. Carefully remove the sensor straight out of its mount. Do not subject the sensor to shock or vibration; protect the tip of the sensor at all times.
  4.  

To install:
  1. Fit the sensor into position. Make certain the sensor sits flush against the mounting surface; it must not be crooked.
  2.  
  3. Install the retaining bolt.
  4.  
  5. Route the sensor cable correctly and install the harness clips and retainers. The cable must be in its original position and completely clear of moving components.
  6.  
  7. Connect the sensor cable to the ABS harness.
  8.  
  9. Install the wheel.
  10.  
  11. Lower the vehicle to the ground.
  12.  



Click image to see an enlarged view

Fig. Fig. 6: Unplug the sensor connector



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Fig. Fig. 7: Remove the sensor retaining bolt



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Fig. Fig. 8: Carefully lift the sensor straight up and remove it



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Fig. Fig. 9: If the sensor is going to be reused, clean the tip off before installing

REAR WITHOUT MULTI-LINK SUSPENSION-EXCEPT 850

See Figure 10

  1. Raise and safely support the vehicle.
  2.  
  3. Disconnect the sensor connector from the harness.
  4.  
  5. Remove the clips and retainers holding the sensor wire to the axle. Take note of the routing of the sensor wire; exact reinstallation is required.
  6.  
  7. Remove the retaining bolt holding the sensor to the differential housing.
  8.  
  9. Remove the sensor straight from its housing; protect the tip from impact.
  10.  

To install:
  1. Install the sensor into the housing.
  2.  
  3. Tighten the retaining bolt.
  4.  
  5. Route the wire harness and attach the connectors.
  6.  
  7. Install the clips and retainers to the harness.
  8.  
  9. Lower the vehicle.
  10.  



Click image to see an enlarged view

Fig. Fig. 10: Sensor connector location

REAR WITH MULTI-LINK SUSPENSION-EXCEPT 850
  1. Remove the spare tire and fold back the trunk carpet to expose the fuel filler pipe.
  2.  
  3. Remove the cover(s) from the filler pipe.
  4.  
  5. Break the seal on the speed sensor harness connector and disconnect the sensor from the ABS harness.
  6.  
  7. Press the rubber grommet free of the bodywork and feed the sensor harness to the outside of the vehicle.
  8.  
  9. Raise and safely support the vehicle.
  10.  
  11. Install a jack with support fixture 5972 or its equivalent under the rear axle.
  12.  
  13. Remove the 2 bolts on each side of the rear axle assembly which hold the member to the body.
  14.  
  15. Lower the rear axle slightly, but do not allow the drive shaft to press against the fuel tank.
  16.  
  17. Disconnect the right brake wire from its attachment.
  18.  
  19. Remove the sensor cable from the retaining clips and clamps. Take note of the routing of the cable; it must be reinstalled in its exact original position.
  20.  
  21. Clean the sensor area; remove the retaining bolts and remove the sensor. Protect the tip from damage or impact.
  22.  

To install:
  1. Apply a light coat of oil to the O-ring on the new sensor.
  2.  
  3. Fit the sensor into place without damaging the tip.
  4.  
  5. Tighten the retaining bolts to 90 inch lbs. (10 Nm).
  6.  
  7. Install the sensor harness into the cable retainers, making certain it is routed correctly and out of the way of all moving parts.
  8.  
  9. Feed the cable through the body and secure the grommet.
  10.  
  11. Connect the right brake wire.
  12.  
  13. Raise the rear axle assembly and install the 4 bolts. Tighten each bolt to 52 ft. lbs. (70 Nm), then angle tighten each an additional 60 degrees.
  14.  
  15. Lower the vehicle.
  16.  
  17. Connect the sensor wiring harness to the ABS harness in the trunk and reseal the connector.
  18.  
  19. Clamp the cable to the filler pipe.
  20.  
  21. Install the filler covers, reposition the carpet and install the spare tire.
  22.  
  23. Test drive the vehicle, confirming correct function of the ABS system and the dashboard warning lamp.
  24.  

850/C70/S70/V70 rear sensors

See Figure 11

  1. Disconnect the negative battery cable.
  2.  
  3. On sedans and the C70:
    1. Fold the passenger side rear seat forward.
    2.  
    3. Remove the luggage compartment mat.
    4.  
    5. Remove the fuel pump and level sensor covers.
    6.  

  4.  
  5. On wagons:
    1. Fold up the floor panel, and bend mat back to access the fuel pump and level sensor covers.
    2.  

  6.  
  7. Remove the wheel sensor connector.
  8.  
  9. Raise and safely support the vehicle.
  10.  
  11. Note the direction and installation of the sensor harness for reinstallation. Pull the wheel sensor harness through from underneath the vehicle.
  12.  
  13. Remove the wheel(s).
  14.  
  15. Remove the sensor attaching bolt, and remove the sensor.
  16.  

To install:
  1. Install the new sensor into place, and tighten the retaining bolt.
  2.  
  3. Route the harness through the suspension and place into opening in floor.
  4.  
  5. Install the wheel(s).
  6.  
  7. Lower the vehicle.
  8.  
  9. Attach the sensor connector.
  10.  
  11. Install the removed covers and trim.
  12.  
  13. Connect the negative battery cable.
  14.  



Click image to see an enlarged view

Fig. Fig. 11: The rear wheel speed sensor as viewed with the brake disc removed

Pulse Wheel
integral with the rotor
  1. Raise and safely support the vehicle.
  2.  
  3. Remove the wheel(s) of the pulse wheel being replaced.
  4.  
  5. Remove the brake rotor.
  6.  
  7. Remove the grease seal and the wheel bearings from the rotor.
  8.  
  9. Place the rotor in a shop vise or other suitable holding fixture.
  10.  
  11. Using a suitable puller, remove the pulse wheel from the rotor.
  12.  

To install:
  1. Install the new pulse wheel onto the rotor, and using handle 1801 and driver 5276 or equivalent bearing/race/seal driver, press the pulse wheel onto the rotor.
  2.  
  3. Repack wheel bearings and install them into the rotor, and install a new grease seal.
  4.  
  5. Install the brake rotor onto the vehicle.
  6.  
  7. Install the wheel(s).
  8.  
  9. Lower the vehicle.
  10.  

integral with the hub
  1. Raise and safely support the vehicle.
  2.  
  3. Remove the wheel(s) of the pulse wheel being replaced.
  4.  
  5. Remove the brake rotor.
  6.  
  7. Remove the hub from the vehicle.
  8.  
  9. Place the hub in a vise or other suitable holding fixture.
  10.  
  11. Using a suitable puller, remove the pulse wheel from the hub.
  12.  

To install:
  1. Place the hub into a shop press, and using the suitable adapter press the new wheel onto the hub.
  2.  
  3. Install the hub onto the vehicle.
  4.  
  5. Install the brake rotor.
  6.  
  7. Install the wheel(s).
  8.  
  9. Lower the vehicle.
  10.  

Integral with the axle shaft
  1. Raise and safely support the vehicle.
  2.  
  3. Remove the wheel(s) of the pulse wheel being replaced.
  4.  
  5. Remove the brake rotor.
  6.  
  7. Remove the axle shaft from the vehicle.
  8.  
  9. Install the axle shaft in a vise or suitable fixture, ensuring not to damage the axle, or rip the CV-joint boot.
  10.  
  11. Using a suitable puller, remove the pulse wheel from the outer CV-joint.
  12.  

To install:
  1. Place the axle shaft into a shop press and, using the suitable adapter, press the new wheel onto the outer joint.
  2.  
  3. Install the axle shaft into the vehicle.
  4.  
  5. Install the brake rotor.
  6.  
  7. Install the wheel(s).
  8.  
  9. Lower the vehicle.
  10.  

on the differential

The pulse wheel is located on the ring gear inside the differential. Teardown of the differential is necessary to replace the pulse wheel. This job requires many special tools and is not recommended for the do-it-yourselfer.

 
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