GM Astro/Safari 1985-1996 Repair Guide

Throttle Position Sensor (TPS)

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OPERATION



See Figure 1



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Fig. Fig. 1: Internal workings of a TPS

The Throttle Position Sensor (TPS) is connected to the throttle shaft on the throttle body. It is a potentiometer with one end connected to 5 volts from the ECM and the other to ground.

A third wire is connected to the ECM to measure the voltage from the TPS. As the throttle valve angle is changed (accelerator pedal moved), the output of the TPS also changes. At a closed throttle position, the output of the TPS is low (approximately .5 volts). As the throttle valve opens, the output increases so that, at wide-open throttle, the output voltage should be approximately 4.5 volts.

By monitoring the output voltage from the TPS, the ECM can determine fuel delivery based on throttle valve angle (driver demand).

TESTING



See Figure 2



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Fig. Fig. 2: Most testing of the TPS can be done while it is still mounted on the throttle body

  1. Backprobe with a high impedance voltmeter at TPS terminals A and B.
  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 TPS 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 C and B.
  8.  
  9. With the key ON and engine off and the throttle closed, the TPS voltage should be approximately 0.5-1.2 volts.
  10.  
  11. Verify that the TPS 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 TPS voltage reading.
  12.  
  13. If the sensor voltage is not as specified, replace the sensor.
  14.  

REMOVAL & INSTALLATION



See Figures 3, 4, 5, 6 and 7



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Fig. Fig. 3: Throttle position sensor-TBI 700 and CMFI



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Fig. Fig. 4: Throttle position sensor-TBI 220



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Fig. Fig. 5: Throttle position sensor-CSFI



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Fig. Fig. 6: Even though the TPS is not adjustable, marking its location before removal is a good idea



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Fig. Fig. 7: Use the correct size bit in the mounting screws. This one just might strip out the screw

  1. Disconnect the negative battery cable and remove the air cleaner and gasket.
  2.  
  3. Disengage the electrical connector.
  4.  
  5. Unfasten the one or two TPS attaching screw assemblies.
  6.  
  7. Remove the TPS from the throttle body assembly.
  8.  
  9. Remove the TPS seal.
  10.  

To install:
  1. Install the TPS seal over the throttle shaft.
  2.  
  3. With the throttle valve closed, install the TPS on the throttle shaft. Rotate it counterclockwise, to align the mounting holes.
  4.  
  5. Install the 2 TPS attaching screw assemblies.
  6.  
  7. Engage the electrical connector.
  8.  
  9. Install the air cleaner and gasket.
  10.  
  11. Connect the negative battery cable.
  12.  

 
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