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Three-Wire Device (Potentiometer) Tests
A potentiometer is a variable resistor. The signal generated is used by the vehicle computer to determine the
position and direction of movement of a device within the component. Typical potentiometer applications are:
Throttle Position Sensor (TPS), Exhaust Gas Recirculation Valve Position Sensor (EGR), and Vane Air Flow
Meter (VAF), etc.
An analog Throttle Position Sensor (TPS) is found on many vehicles. The TPS informs the vehicle computer of
the following:
• Throttle opening
• Whether and how fast the throttle is opening
• Whether and how fast the throttle is closing
• When the throttle is wide open
• When the throttle is at idle
One of its most important functions is to tell the computer that the throttle is opening. It replaces the
accelerator pump found on carbureted engines, stopping the engine from stumbling when the throttle is opened
quickly. When that happens, manifold absolute pressure (MAP) quickly rises (vacuum drops), causing vaporized
gasoline to condense on the manifold walls. Since there is less fuel available to the cylinders, more fuel must
be added to the air stream.
Another important function is to tell the computer that the throttle is closing. To maintain acceptable
emissions, the computer must lean out the mixture when MAP drops (vacuum rises).
For best fuel economy, the computer completely shuts off fuel in some engines when vacuum is high and the
throttle is at idle. Therefore, the computer must know when the throttle is at idle.
Throttle position information is a variable resistance from a potentiometer attached to the throttle shaft. Wide-
open-throttle and throttle-closed signals come from switches attached to the TPS.
The TPS is really just a potentiometer or variable resistor. As you sweep the throttle, the resistance changes. As
its resistance changes, so does the voltage signal returning to the computer. The TPS can be tested either by
watching the voltage change or by watching the resistance change, using the bargraph on the meter.
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