Troubleshooting a Throttle Position Sensor

[BodybyFisher]

I received this from Autotap today and thought it provides useful information. Autotap is at http://www.autotap.com , Mike

Mike Fahrion back again with this week's topic for fellow shade-tree mechanics. Today's topic is a wear-out prone mechanical device called the Throttle Position Sensor (TPS).

When I was studying electrical engineering, I took a couple mechanical engineering courses since that field always interested me too. My mechanical engineering professor loved to quip "Electrical devices are inherently unreliable, mechanical devices are inherently reliable." I took a lot of verbal abuse when I tried to argue that the opposite was true (and I don't think I helped my grade any either). I submit today's email as supporting evidence for my opinion!

Even though today's vehicles are fuel injected, they still have a throttle plate that is controlled by your right foot. That's what controls how much air flows into the engine's intake manifold. As you push harder on the accelerator, the throttle plate opens further. The TPS is attached to that throttle plate and its electrical resistance goes up as a contact within the sensor slides over an increased length of wire.

Using three wires (power, ground and output) the vehicle's computer sends a voltage through that variable resistor and measures its output. Using that information (and a bunch of other stuff), the computer knows how heavy your right foot is and decides how much fuel to inject.

As you might guess that sensor can wear out (after all, it's a mechanical device in my book). Or, it can get corroded or dirty. Then what happens? The voltage it puts out either gets "stuck" or it gets jumpy and the voltage bounces up or down as you press the gas pedal. Any of this leaves the computer pretty confused. One symptom is engine hesitation when you try to accelerate. It will often (but not always) set the check engine light. Typical trouble codes are P0120 P0121, P0122 and P0123.

So - how do you tell if it's bad? I love this one - you can make the diagnosis from the drivers seat while listening to your favorite radio station. Plug in AutoTap or another scantool that supports real-time data, and read the Throttle Position Sensor voltage with the key on, engine off. I suggest using the "Graph-It" feature of AutoTap to be sure you don't miss anything (jobslike this one make graphing an invaluable feature!). Now, slowly press the accelerator to the floor. You should see a smooth line on your PC or Palm. It should start down around 0.5 volts and rise to around 4.5 volts (exact values depend on the vehicle). If the line is bouncing down to 0 or up to 5, the sensor's bad. If it's stuck at a constant value, the sensor's bad.

This is an easy sensor to replace. You'll find it attached to the side of the throttle body (conveniently located at the top of the engine!). There's a wiring harness and a couple screws to remove - that's it!

Note that some throttle position sensors require a simple adjustment once they're installed. This usually involves installing it loosely and checking its voltage in the idle position.

There is a newer type of TPS that no longer uses wear-prone mechanical contacts. The new style uses a magnetic field to generate the signal. I don't know what vehicles use the newer type of sensor but it seems like a good idea.

Happy Repairs,

Mike Fahrion

www.autotap.com