Roadmaster 1992

Air/Fuel Ratio Sensor

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Location





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Fig. Oxygen sensor location-1991-1993

Operation



This sensor(s) protrudes into the exhaust stream and monitors the oxygen content of the exhaust gases. The difference between the oxygen content of the exhaust gases and that of the outside air generates a voltage signal to the ECM. The ECM monitors this voltage and, depending upon the value of the signal received, issues a command to adjust for a rich or a lean condition.

Air-Fuel Ratio Sensor Heater

In order to achieve operating temperature and enter closed loop more quickly, modern oxygen sensors have built-in heating elements.

Component Description

The Air-Fuel Ratio (AFR) Sensor, also known as a Wideband Oxygen Sensor is a planar zirconium electronic device that measures the proportion of Hydrocarbons (HC) and oxygen (O2) content in the exhaust gas. The function of the AFR sensor is to constantly monitor the air-fuel ratio of the engine, determine if the mixture is rich or lean, and provide feedback (real-time information) to the Powertrain Control Module (PCM) so that the air-fuel ratio may be automatically adjusted.

Fuel Trim

Fuel trim is the PCM parameter that is used to adjust the air-fuel mixture. The ideal AFR Sensor feedback value is 2.5 volts DC, which is indicated when air and fuel are at the stoichiometric ratio of 14.7:1. This value is determined by AFR sensor feedback, and provides the final data for the PCM in determining the correct air-fuel mixture.

Removal & Installation



The sensor may be difficult to remove when the engine temperature is below 120° F (48 ° C). Excessive removal force may damage the threads in the exhaust manifold or pipe; follow the removal procedure carefully.

  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. Locate the oxygen sensor. It protrudes from the center of the exhaust manifold at the front of the engine compartment (it looks somewhat like a spark plug).
  6.  
  7. Disconnect the electrical connector from the oxygen sensor.
  8.  
  9. Spray a commercial solvent onto the sensor threads and allow it to soak in for at least five minutes.
  10.  
  11. Carefully unscrew and remove the sensor.

    Click image to see an enlarged view

    Fig. Oxygen sensor and related components-1991-1993

  12.  

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. Installation is the reverse of the removal procedure.
    NOTE
    Coat the sensor's threads with GM anti-seize compound No. 5613695 or the equivalent. Most new sensors come pre-coated and do not require additional coating. This is not a conventional anti-seize paste. The use of a regular compound may electrically insulate the sensor, rendering it inoperative. You must coat the threads with an electrically conductive anti-seize compound.

  2.  
  3. Installation torque is 30 ft. lbs. (42 Nm). Do not overtighten.
  4.  
  5. Reconnect the electrical connector. Be careful not to damage the electrical pigtail.
  6.  
  7. Check the sensor boot for proper fit and installation.
  8.  
  9. 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.
  10.  

Testing




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

Air/Fuel Ratio Sensor

A properly operating heated Air-Fuel Ratio (AFR) sensor should display a level feedback signal of approximately 2.5 volts DC. If the feedback signal is not within the specified range, additional diagnosis is required in order to determine the proper function of the AFR sensor, or if a fault exists in any related sub-systems.

Air/Fuel Ratio Sensor Heater

The AFR sensor heater may be tested by performing a resistance test; the sensor must be disconnected from the main engine wiring harness. If the circuit resistance is greater than 5.0 ohms, replace the HO2S. Refer to the Electrical Wiring Diagram for specific circuit information and connector views.

Click image to see an enlarged view

Fig. Normal AFR sensor reading.

Air/Fuel Ratio Sensor Strategy

The AFR sensor operates most effectively when an operating temperature of 1,472° F (800° C) or higher is reached and maintained. When the AFR sensor reaches the specified temperature and begins sending the feedback signal, the PCM uses the information to enter closed loop mode, which is defined as the PCM having full control of the air-fuel ratio. In order to achieve operating temperature and enter closed loop more quickly, modern AFR sensors have built-in heating elements.

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


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Fig. Related Diagnostic Trouble Codes

 
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