The throttle position sensor, or TPS, monitors the position of the throttle. The TPS is a potentiometer that sends an electrical signal to the powertrain control module. The PCM uses this information to adjust fuel/air mixtures. A faulty TPS will cause erratic idling since it sends the wrong information to the PCM.
The TPS is located on the end of the throttle shaft on the throttle body. To find the throttle body look to the left of the intake cover. The intake cover has the name of the engine on it, for instance it may read "3400 SFI" in large letters. The throttle body is just to the left of the cover and close to the firewall. The TPS is a small black electrical unit mounted right beside the air intake duct.
The TPS sends different voltages to the PCM depending on the angle of the throttle valve. If the TPS is broken or loose it may cause erratic idling since the PCM is receiving different voltages, as if the throttle were moving.
You can check the TPS by first checking that the electrical connections are tight and not corroded. Check the wiring harness and wires for damage or breaks. Use a voltmeter to test the current coming from the TPS. With the engine off the TPS should be sending 5.0 volts of current. Check the current at idle and increasing RPM's. At idle the voltage should read 0.5 volts and at high rpm it should read 4.5 volts.
