Example of Bad Throttle Position Sensor

Throttle Position Sensor, Typical:

BACKGROUND AND APPLICATION INFORMATION:
Symptom: The vehicle would buck and jerk at just less than half throttle and then run perfectly for a period of time. The Throttle Position Sensor (TPS) waveform above was captured from a sensor that would intermittently malfunction. The sensor did not exhibit the same magnitude of failure each time the throttle was opened and closed. Sometimes it would even get in a good mood and work perfect for half an hour.

CONNECT AND SETUP THE DIGITAL STORAGE OSCILLOSCOPE (DSO):
1. Connect the "COM" probe to the TPS ground circuit, engine block, or negative battery post.
2. Connect the "CH1" probe to the TPS signal wire to the Powertrain Control Module (PCM). Use a wiring diagram to obtain the PCM pin number or the color of the wire.
3. Set the voltage scaling to 1 V/div.
4. Set the time base to 500 ms/div (1/2 second per division).
5. Select "GND" coupling and position the trace on the second division line from the bottom of the DSO display.
6. After ground position is set, select "DC" input coupling.
7. Select "SPIKE DETECT" or "MIN/MAX" acquire mode.
8. "TRIGGER" mode is usually set automatically (Roll Mode).

EXERCISE THE SENSOR
With the key on, engine off, slowly open the throttle from closed to wide open throttle and back to closed again. Repeat this process several times. It is best to do this slowly so the waveform is spread out on the display like the example above.

Bad Throttle Position Sensor Pattern (Typical Failure):

EXPECT THIS WAVEFORM RESULT IF THE SENSOR IS BAD:
Check the manufacturer's specifications for exact voltage range specifications, but generally the sensor's voltage should range from just under one volt at idle to just under 5 volts at wide open throttle. There should be no breaks, spikes to ground or dropouts in the waveform. Pay particular attention to the waveform above as it begins to rise (as it reaches approximately. 2.8 volts). This is the portion of the sensor's carbon track that is usually damaged or broken. The worn out or broken carbon track in this sensor did not communicate with the PCM using the correct critical dimension for that throttle angle. Therefore, the PCM could not calculate a correct fuel mixture command for the given engine conditions, causing a driveability problem.

For More Information
Description of Automotive Signals
Diagnosis Using A Labscope
Labscope Quick Reference Appendix