The throttle position sensor is one of many sensors that relay information to the computer. This information is necessary for proper engine management. The throttle position sensor's function is to relay to the computer the position of the throttle plates, from closed to wide-open throttle. This information is necessary for the engine to determine the driver's desired power and acceleration.
The throttle position sensor is always attached to the throttle plate support rod and on the opposite side of the throttle linkage. The throttle position sensor is a potentiometer, similar to a dimmer switch for overhead lights. At rest with the throttle closed, the voltage signal is .3 to .5 volts ideally, and increases as the throttle is opened to 4.5 to 5.0 volts.
Problems occur with the position sensor when a wear pattern occurs on the resistance surfaces. Most sensors will wear in the cruising area--the area of a certain speed range. When this happens, as soon as the position sensor passes over or lands on this worn section of resister, the signal becomes ambiguous. The reading will conflict with the signals from the mass air flow sensor and the crankshaft sensor. At this point, the computer is incapable of signaling the proper response by engine management. It must take a best guess on the information needed to be sent to the computer. It is possible for the throttle position sensor to fail completely, although not as common.
Some throttle position sensors are adjustable, to a small degree. If it is adjustable it will have elongated slots on the sides where it is secured to the throttle body. The adjustment is used to set the closed throttle voltage for a starting point. If a sensor is out of adjustment, the idle rpm will be out of specifications. The only other problem with the sensors is a bad connection or bent pins in the plug. These can be checked easily by looking into both ends for irregularities.
To check a suspect throttle position sensor, use a voltmeter. Connect the red lead to the signal wire and the black lead to a good ground. If it is unclear which is the signal wire, disconnect the plug at the sensor and turn the ignition switch to the on position with the engine off. There are three wires--a 5-volt terminal, a ground and a signal. Find the 5 volt input voltage on the harness side. The black wire will be the ground and the wire left is the signal. Plug the sensor in. With the key on and engine off, observe the voltage on the signal wire. It should be close to .5 volts--or under a volt in any case. If the voltage is not correct, check to see if the throttle position sensor is adjustable. If it is not adjustable, replace the sensor. If the voltage is okay, open the throttle slowly and watch the voltage climb. It should rise smoothly with no glitches or dropouts. Wide-open throttle should be a maximum of 5 volts. Replace the sensor if it fails any of these tests.