Mustang throttle position sensor

12/15/2009 small revisions in wording

7/15/2011 expanded slightly. Added geometric mean of idle voltage

Meters

The first thing everyone should understand is just because a meter says xx.xx volts, it does not mean the voltage is absolutely that voltage. Most cheap meters I've looked at do NOT hold the implied specs. They are closer to 1/2% of actual scale, not the reading. So a meter on a 20 volt scale is generally within .1 volts of the reading, whatever it is. This is a price we pay for cheap imports. A good meter costs around $200 or more. It's like a torque wrench for voltage. We shouldn't expect perfection from a cheap $30 torque wrench. 

Always measure throttle position sensor voltage by connecting your meter between the black and green leads. Never measure from the green lead to an engine component or bolt. This is because the computer reference is to the car chassis inside the passenger compartment, and the black throttle position sensor wire is referenced to that ground point.

Voltages you should see at the TPS (throttle position sensor) with ignition key on and engine off are:

Orange to black.....5.0 volts nominal

Green to black without TPS connected....anything from 0 to 5 volts. It is unpredictable because this is a computer input voltage, not output. It is very sensitive to leakage when the TPS is unplugged.

Black to vehicle chassis, timing chain cover, bell housing, or engine block....less than 0.5 volts under any operating condition

With the TPS connected idle position voltages should be....

Orange to black......5.0 volts NOMINAL depending on your meter

Green to black........0.6 volts to 1.1 volts, 0.8 to 0.9 volts is very good at idle

Green to black with accelerator floored....at least 3 volts higher than the green to black idle voltage!

The Real Ideal Idle Voltage

When we want to center a voltage between two points the ideal point is not the average, it is the geometric mean voltage. This means we have the same percentage of voltage tolerance above and below the ideal point, not the same number spread. The geometric mean of .6 and 1.1 is the square root of .6*1.1. This is .812 volts.     0.812 volts is the same percentage below 1.1 as it is above .6 volts, it is the geometric middle of the numbers when we look at a percentage of error.   

The stock computer is programmed internally to "think" WOT is 2.71 volts higher than the idle TPS voltage. This means if your idle voltage is .75 volts, the computer assumes a total voltage between the green wire and black wire of 3.46 volts or more is wide open throttle. This is 2.71+.75 = 3.46 volts in this example of .75 volts idle voltage.

Let's say this again, because this is the most critical voltage for racing. Your car's idle voltage + 2.71 is what your computer thinks is WOT voltage. This is where the computer transfers to open loop operation and ignores hunting for mixture using the narrow band factory sensors. The TPS at wide open throttle must be .25 volts above the voltage by adding 2.71 to the idle voltage for reliable WOT operation.

WOT has to be about 3 volts (2.71 volts to be exact) more than idle voltage or the computer won't go into wide open throttle with the accelerator floored or nearly floored!

One additional important test:

As the accelerator is slowly moved there should be a smooth steady change on the green without any sudden jumps in voltage. This is best seen on an analog meter (one with a moving needle)! This is important for the acceleration enrichment to work, and for functions that depend on the relative throttle position.

It is important to test everything above. It is foolish to drill the holes out and set the TPS for exactly .xxx volts and call it done without checking the things that actually make a difference. The computer doesn't care and does not change anything so long as idle voltage is anywhere between .6 volts and 1.1 volts. Any voltage within the idle range of .6 to 1.1 volts will be treated the same. Contrary to myths or rumors you do not need to have .97 volts to eliminate stalling, idle surge, or other problems! 

My Background

I built my first tachometer from scratch when I was 13 years old. This was about 1964. I put this in my dad's 57 Ford. It used brand new devices called transistors and a meter from a WW II surplus radio set. At that time I was fixing car radio's for the teenagers next door to me, including hand making solid state tubes to replace troublesome tubes in car radios.

The older teenagers next door, especially Tom Hilding, got me interested in cars. Tom had a 62 Chevy II with six cylinder flags, but it had a dual quad small block and four speed. It would go sideways in the first three gears (mostly due to the bad tires back then). This made me think sleepers are good. 

I primarily design analog systems and have for the past 35 years. I have designed commercially manufactured solid state ignition systems, as well as metering, instrumentation, and automotive test equipment. I managed engineering in the precision meter manufacturering division of a large aftermarket supplier. We supplied equipment to Sun, Sears, Mac Tools, and others.