The TPS (Throttle Position Sensor) in your 1990 Ford F-150 is a type of variable resistor called a potentiometer, used by the on-board computer to determine the amount the throttle is open. A 5-volt reference signal goes to the sensor from the computer. As the throttle is opened, the sensor lowers its resistance and passes more of the reference voltage back to the computer through the signal-return wire. The computer uses this information, along with other sensor input, to determine engine load and calculate fuel and ignition system requirements.
Turn the ignition key to the run position. Turn the digital volt/ohm meter on and set the controls for d/c volts. Verify that the 5-volt reference signal is present by touching the black meter lead to the engine block and piercing the orange wire on the TPS sensor wiring with the red meter lead. The meter should indicate approximately 5 volts. If there is no voltage, check and replace the fuses as needed.
Verify the sensor ground by piercing the black TPS wire with the red meter lead while touching the black meter lead to the engine. The voltage indicated by the meter should be less than .3 volts. This is a "voltage drop test." If the voltage is above .3 volts, clean the chassis ground connection on the fender well.
Verify that the signal return operates smoothly with no drop-outs as the throttle is opened slowly. Do this by piercing the green TPS wire with the red meter lead and touching the black meter lead to the engine. Have an assistant push the gas pedal slowly to the floor and slowly release it. The voltage indicated on the meter should sweep smoothly from .5 volts to 4.9 volts and back to .5 volts, without dropping to 0 volts or sticking at one voltage. Replace the sensor if the voltage drops to 0 or sticks during the test.
