Throttle Position Sensor (TPS) Test

Throttle Position Sensor Typical:

BACKGROUND AND APPLICATION INFORMATION:
The Throttle Position Sensor (TPS) is a variable resistor (potentiometer) that tells the Powertrain Control Module (PCM) the position of the throttle. This is an important sensor because its signal input to the PCM is used to calculate engine load, ignition timing, Exhaust Gas Recirculation (EGR) control (if equipped), idle control and other PCM controlled parameters such as transmission shift points. A bad TPS can cause hesitation and idle problems, among other driveability problems, and even I/M emission test failures. The TPS is a three wire sensor. One wire supplies the sensor with 5 volts via the PCM's VRef circuit, another wire provides the sensor ground, and the third wire is the sensor's signal output to the PCM.

Almost all TPS's from all vehicle manufacturers operate in the same way. Therefore the following DSO setup and test procedure will work for most makes and models with three wire Throttle Position Sensors. Generally, TPS's will produce just under one volt with the throttle closed and produce just under 5 volts with the throttle wide open.

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 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 position to wide open throttle and back to closed throttle 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.

Good Throttle Position Sensor Pattern (Typical):

EXPECT THIS WAVEFORM RESULT:
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 as it begins to rise (in the first 1/4 of throttle movement). This is the portion of the sensor's carbon track that is most often used while driving (during cruise, light throttle, etc.). The first 1/8 to 1/3 of the sensor's carbon track usually wears out first.

IMPORTANT NOTE: Beware of TPS's on 4.0L Jeep Cherokees. 4.0L Cherokees have two throttle position sensors: One is for the PCM and one is for the transmission controller. The transmission TPS operates opposite from the engine TPS. The transmission TPS produces just under 5 volts at idle and goes to just under one volt at wide open throttle. There are a few other instances where this might be encountered.

FOR MORE INFORMATION
Description of Automotive Signals
Diagnosis Using A Labscope
Labscope Quick Reference Appendix