Roadmaster 1992

Vehicle Speed Sensor

Print

Location





Click image to see an enlarged view

Fig. VSS location-1991-1993

Operation



The Vehicle Speed Sensor (VSS) is a permanent magnet output coil device that monitors the vehicle speed. A reluctor is attached to the transmission/transaxle final drive, and is used to generate a constant signal as it passes the VSS magnetic coil. The VSS Sensor utilizes system voltage (12 volts DC), or reference voltage (5 volts DC) to generate a digital output signal to the PCM that is based upon a 0.5 volt AC reference. The alternating magnetic field is used by the sensor output electronics to produce a generated wave form. The VSS waveform is used by the Powertrain Control Module (PCM) to calculate engine and vehicle load, ignition timing, fuel injector timing and pulse width, fuel cut, and the speedometer display.  

Removal & Installation



  1. Before servicing the vehicle, refer to the Precautions Section.
    NOTE
    Disconnecting the negative battery cable on some vehicles may interfere with the functions of the on-board computer systems and may require the computer to undergo a relearning process, once the negative battery cable is reconnected.


    NOTE
    If working near and/or around the SRS system and components, be sure to properly disable the SRS system. See disarming/arming the SRS system.

  2.  
  3. Disconnect the negative battery cable.
  4.  
  5. Raise and safely support the vehicle.
  6.  
  7. Disconnect the VSS electrical connector.
  8.  
  9. Remove the retaining bolt.
  10.  
  11. Have a clean container positioned under the component to catch the transmission fluid and remove the VSS.
  12.  
  13. Remove and discard the O-ring.

    Click image to see an enlarged view

    Fig. VSS and related components-1991-1993

  14.  

To install:


NOTE
Be sure to use new fasteners, as required.


NOTE
Use the correct fastener in the correct location. Replacement fasteners must be the correct part number for that application. Fasteners requiring replacement or fasteners requiring the use of thread sealer are identified. Do not use paints, lubricants or corrosion inhibitors on fasteners or fastener joint surfaces unless specified. These coatings affect fastener torque and joint clamping force and may damage the fastener. Use the correct tightening sequence and specifications when installing fasteners.

  1. Lubricate a new O-ring with a thin film of transmission fluid. Install the O-ring and VSS.
  2.  
  3. Install the retaining bolt.
  4.  
  5. Connect the electrical connector.
  6.  
  7. Lower the vehicle.
  8.  
  9. Connect the negative battery cable.
  10.  
  11. Refill transmission to proper level.
  12.  
  13. Using the GM diagnostic scan tool or aftermarket equivalent reprogram the necessary systems and components. Be sure to follow the scan tool manufacturer-s directions.
  14.  

Testing




NOTE
Refer to the Electrical Wiring Diagram for component and connector locations, connector views, and circuit-specific information.

Connection & Wiring Diagnosis

Refer to the Electrical Wiring Diagram for component and connector locations, connector views, and circuit-specific information.  

Many intermittent open or short circuits may be caused by wiring harness and connector movement due to vibration, engine torque, bumps and rough pavement, etc.

  1. Test the wiring harness and connectors by performing the following tests:

    Move the related connectors and wiring while monitoring the appropriate scan tool data.
     
    Move the related connectors and wiring with the component commanded ON and OFF. Using a suitable the scan tool, observe the component operation.
     
    With the engine running, move the related connectors and wiring while monitoring component operation.
     
    If harness or connector movement affects the data displayed, the component and system operation, or the engine operation, inspect and repair the harness or connections as necessary.
     

  2.  
  3. Test the connector terminal pins and/or wiring by performing the following tests:

    Inspect for incorrect mating of the connector halves, or terminals not fully seated in the connector body.
     
    Inspect for improperly formed or damaged terminals and test for incorrect terminal tension.
     
    Inspect for poor terminal to wire connections including terminals crimped over insulation. This requires removing the terminal from the connector body.
     
    Inspect for corrosion or water intrusion. Pierced or damaged insulation can allow moisture to enter the wiring. The conductor can corrode inside the insulation with little visible evidence. Look for swollen and/or brittle sections of wire in the suspect circuits.
     
    Inspect for wires that are broken inside the insulation by gently pulling on suspect sections of wiring.
     

  4.  

Related Diagnostic Trouble Codes


Click image to see an enlarged view

Fig. Related Diagnostic Trouble Codes

VSS Circuit Testing

Use a suitable scan tool or a Graphing Multi-Meter (GMM in order to view the VSS data.  

VSS logic is based on alternating lines of magnetic flux which generate an analog signal to the PCM, and are based on the vehicle speed.

Verify that the engine that there is no foreign material obstructing the path between the reluctor and the VSS. If any engine mechanical faults are evident, locate and repair as required before continuing.  

A Digital Volt-Ohm Meter (DVOM) may be used to verify the condition of the wiring: additional information may be acquired by taking measurements at the sensor connector as well as the PCM connector. If significant resistance is measured (greater than 5 ohms), check the wiring harness and connections for corrosion, poor pin connections, or damaged wires.

If all engine wiring and pin connections are confirmed, disconnect the VSS to verify the signal with a GMM, and verify PCM communication before replacing the VSS Sensor.

Click image to see an enlarged view

Fig. VSS Range Chart

VSS Strategy

The VSS provides a magnetically generated signal waveform to the Powertrain Control Module (PCM) for a number of engine control, drive train, and information display system calculations. The VSS signal frequency varies according to vehicle speed, and is compared with the Crankshaft Position (CKP) Sensor and Camshaft Position (CMP) Sensor signals for measured operating conditions. The magnetic waveform pattern is used by the Powertrain Control Module (PCM) to calculate engine and vehicle load, ignition timing, fuel injector timing and pulse width, and the speedometer display.

 
label.common.footer.alt.autozoneLogo