GM Full-Size Trucks 1988-1998 Repair Information

Component Replacement

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REMOVAL & INSTALLATION



RWAL Electronic Control Unit (ECU)

See Figure 1

The RWAL ECU is a non-serviceable unit. It must be replaced when diagnosis indicates a malfunction.

  1. Turn the ignition switch OFF .
  2.  



Click image to see an enlarged view

Fig. Fig. 1: Control module-rear wheel anti-lock brake system

  1. Disconnect the wiring harness to the RWAL ECU.
  2.  
  3. Gently pry the tab at the rear of the ECU; remove the control unit toward the front of the vehicle.
  4.  

Do not touch the electrical connectors or pins; do not allow them to contact brake fluid. If contaminated with brake fluid, clean them with water followed by isopropyl alcohol.

  1. Install the RWAL ECU by sliding it into the bracket until the tab locks into the hole.
  2.  
  3. Connect the wiring harness to the ECU.
  4.  

Isolation/Dump Valve (Control Valve)

See Figure 2



Click image to see an enlarged view

Fig. Fig. 2: Anti-lock pressure valve and module assembly-rear wheel anti-lock brake system

  1. Disconnect the brake line fittings at the valve. Protect surrounding paintwork from spillage.
  2.  
  3. Disconnect the electrical connectors from both the ECU and the isolation valve.
  4.  
  5. Remove the bolts holding the valve to the bracket.
  6.  
  7. Carefully remove the isolation/dump valve from the vehicle.
  8.  

Do not touch the electrical connectors or pins; do not allow them to contact brake fluid. If contaminated with brake fluid, clean them with water followed by isopropyl alcohol.

To install:
  1. Place the valve in position and install the retaining bolts. Tighten the bolts to 21 ft. lbs. (29 Nm).
  2.  
  3. Connect the electrical connector to the RWAL ECU.
  4.  
  5. Install the brake lines; tighten the fittings to 20 ft. lbs. (27 Nm)
  6.  
  7. Bleed the brake system at all 4 wheels.
  8.  

Speed Sensor

The speed sensor is not serviceable and must replaced if malfunctioning. The sensor is located in the left rear of the transmission case on 2WD vehicles and on the transfer case of 4WD vehicles.

The speed sensor may be tested with an ohmmeter; the correct resistance is 900-2000 ohms. To remove the speed sensor:

  1. Disconnect the electrical connector from the speed sensor.
  2.  
  3. Remove the sensor retaining bolt if one is used.
  4.  
  5. Remove the speed sensor; have a container handy to catch transmission fluid when the sensor is removed.
  6.  
  7. Recover the O-ring used to seal the sensor; inspect it for damage or deterioration.
  8.  

To install:
  1. When installing, coat the new O-ring with a thin film of transmission fluid.
  2.  
  3. Install the O-ring and speed sensor.
  4.  
  5. If a retaining bolt is used, tighten the bolt to 8 ft. lbs. (11 Nm) in automatic transmissions or 9 ft. lbs. (12 Nm) for all other transmissions.
  6.  
  7. If the sensor is a screw-in unit, tighten it to 32 ft. lbs. (43 Nm).
  8.  
  9. Connect the wire harness to the sensor.
  10.  

 
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