GM Full-Size Trucks 1988-1998 Repair Information

Accelerator Pedal Position (APP) Sensor Module

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OPERATION



The APP sensor module contains three potentiometers (a device for measuring unknown voltage or potential difference by comparison to a standard voltage). Each of the APP sensors send a varying voltage to the ECM. By monitoring the output voltage from the Accelerator Pedal Position (APP) module, the ECM can determine fuel delivery based on the accelerator pedal position (driver demand).

TESTING



See Figure 1

  1. Backprobe with a high impedance voltmeter at APP sensor terminals G and A.
  2.  
  3. With the key ON and engine off, the voltmeter reading should be approximately 5.0 volts.
  4.  
  5. If the voltage is not as specified, either the wiring to the APP sensor or the ECM may be faulty. Correct any wiring or ECM faults before continuing test.
  6.  
  7. Backprobe with a high impedance voltmeter at terminals F and A.
  8.  
  9. With the key ON and engine idling, the APP sensor voltage should be approximately 0.5 volts.
  10.  
  11. Verify that the APP voltage increases or decreases smoothly as the throttle is opened or closed. Make sure to open and close the throttle very slowly in order to detect any abnormalities in the APP sensor voltage reading.
  12.  
  13. If the APP sensor voltage is not as specified, replace the APP sensor.
  14.  
  15. Backprobe with a high impedance voltmeter at APP sensor terminals D and B.
  16.  
  17. With the key ON and engine off, the voltmeter reading should be approximately 5.0 volts.
  18.  
  19. If the voltage is not as specified, either the wiring to the APP sensor or the ECM may be faulty. Correct any wiring or ECM faults before continuing test.
  20.  
  21. Backprobe with a high impedance voltmeter at terminals C and B.
  22.  
  23. With the key ON and engine idling, the APP sensor voltage should be approximately 4.5 volts.
  24.  
  25. Verify that the APP voltage decreases smoothly as the throttle is opened. Make sure to open and close the throttle very slowly in order to detect any abnormalities in the APP sensor voltage reading.
  26.  
  27. If the APP sensor voltage is not as specified, replace the APP sensor.
  28.  
  29. Backprobe with a high impedance voltmeter at APP sensor terminals E and J.
  30.  
  31. With the key ON and engine off, the voltmeter reading should be approximately 5.0 volts.
  32.  
  33. If the voltage is not as specified, either the wiring to the APP sensor or the ECM may be faulty. Correct any wiring or ECM faults before continuing test.
  34.  
  35. Backprobe with a high impedance voltmeter at terminals K and J.
  36.  
  37. With the key ON and engine idling, the APP sensor voltage should be approximately 4.0 volts.
  38.  
  39. Verify that the APP voltage decreases smoothly to about 2.0 volts as the throttle is opened. Make sure to open and close the throttle very slowly in order to detect any abnormalities in the APP sensor voltage reading.
  40.  
  41. If the APP sensor voltage is not as specified, replace the APP sensor module.
  42.  



Click image to see an enlarged view

Fig. Fig. 1: Accelerator Pedal Position (APP) sensor wiring diagram

REMOVAL & INSTALLATION



See Figure 2

  1. Disconnect the negative battery cable.
  2.  
  3. Unplug the APP module electrical connection.
  4.  
  5. Unfasten the module retaining bolts.
  6.  
  7. Remove the module.
  8.  

To install:
  1. Place the module in position and install the mounting bolts. Tighten the bolts to 10 inch lbs. (1 Nm).
  2.  
  3. Attach the module electrical connection.
  4.  
  5. Connect the negative battery cable.
  6.  



Click image to see an enlarged view

Fig. Fig. 2: The APP module (2) is retained to its mounting surface (1) by three bolts (3)-1996-98 diesel engines shown, others similar

 
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