This 1995 Ford F150 pickup came in with the complaint that the idle was very high. The customer stated that the original problem was that the idle was too low and the engine would stall. His neighbor was familiar with Fords and had replaced the IAC valve and now it idles very high. They had disconnected the harness to the IAC valve in order to drive it in. The first thing I did was to plug the harness back into the IAC to see what happens. Idle went very high, close to 2000 rpms. Next I checked codes and found a code 121 stored for a throttle position range fault.
A quick look at data stream and I found the tps voltage at 3.87 volts. Normal on a Ford with a closed throttle is about 1.0 volts. Wide open throttle should be about 4.5 volts so you can see the computer thinks the throttle is much closer to wide open than to closed.
The throttle position sensor is located under the throttle body assembly on this vehicle. It is the part in the following picture with the orange and green wires.
I pulled the connector up to be able to access it and backprobed the orange wire with a sharp pick slid in parallel with the wire through the hole where the wire goes through the seal.
Key on, engine off there was a reading of 5.02 volts. Meaning that the five volt reference voltage to the sensor is okay.
Next I checked the ground in the same manner except that I was checking the black wire. The reading of .024 volts with the other lead connected to battery ground is good. Any reading less that .05 volts is considered okay.
Another way to test the ground is to connect the other lead to battery positive. A reading of a nominal 12 volts would indicate a good ground.
My meter is auto ranging so I do not have to be concerned with polarity while checking voltage. The reversed polarity is indicated by the minus sign to the left of the voltage reading. If your meter is not auto ranging you will need to reverse the leads.
Now on to the tps signal wire. The signal on the green wire is sent to the computer and varies as the throttle is opened and closed.
A voltage of 3.99 volts was found at the tps confirming that the tps is indeed faulty. A normal voltage for a Ford is 1.0 volt at closed throttle.
A diagnosis of a faulty throttle position sensor causing an high idle. The reason the idle would go high in this condition is the program within the computer opens the IAC when the computer sees a signal that the throttle plate is significantly open. This gives a cushion to the system when the throttle is released quickly. Without this cushion the engine would stall.
I have started doing some videos along with my regular posts. I have some refining to do and it will improve with time. To see the part 1 video please click here. To see the part 2 video please click here.