GM Astro/Safari 1985-1996 Repair Guide

Throttle Position Sensor (TPS)



The Throttle Position Sensor (TPS) is located inside the carburetor. It is a potentiometer with one wire 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 accelerator pedal is moved, the output of the TPS also changes. At a closed throttle position, the output of the TPS is low (approximately 0.5 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).


  1. Backprobe with a high impedance voltmeter at TPS terminals A and B.
  3. With the key ON and engine off, the voltmeter reading should be approximately 5.0 volts.
  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.
  7. Backprobe with a high impedance voltmeter at terminals C and B.
  9. With the key ON and engine off and the throttle closed, the TPS voltage should be approximately 0.5-1.2 volts.
  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.
  13. If the sensor voltage is not as specified, replace the sensor.


The throttle position sensor is located in the carburetor. Please refer to Fuel System for the procedures to remove the TPS.