94-95 Mustangs Electrical Throttle Position Sensor
From Mustang-Tech
Throttle Position Sensor Operation [1]
When an EEC-IV calibration needs to determine the current throttle mode for a particular operating condition, or when the EEC needs to reference the current throttle position for a particular function or table, it will calculate and utilize the current relative throttle position by using the following formula:
Relative TP = Actual TP - Closed Throttle TP
As with many EEC variables and algorithms, the EEC will constantly seek out the lowest possible closed throttle voltage from the TPS and use that value in place of the Closed Throttle TP variable given above. Since almost all EEC-IV calibrations are hard-coded to clip this closed throttle value to a maximum value of 250 A/D counts (or 1.22V), one might experience a high idle or other drivability problems if their closed throttle TPS voltage is set above the hard-coded maximum of 1.22V.
For example, let's say that you just installed a brand-new TPS sensor on your Mustang and the sensor is reporting a closed throttle voltage of 1.43V back to the EEC. Using the standard relative TP equation, and keeping in mind the closed throttle maximum that we just reviewed, this is how a closed throttle voltage of 1.43V would look to the EEC:
Relative TP = 1.43V - 1.22V = 0.23V
where:
Current TP = 1.43V
Closed Throttle TP = 1.22V
Remember, since the EEC cannot use values above 1.22V for the Closed Throttle TP variable, it is forced to use the difference between the current Closed Throttle TP and the Actual TP (or in this case, 0.23V) as the basis for the current Relative TP value. The EEC would then take this new (but incorrect) TP value of 0.23V as an indication that the throttle had been depressed and would enter part throttle mode as a result, despite the fact that the throttle itself hasn't actually been touched.
On the other hand, if our closed throttle voltage was within the typical, expected range of 0.48 - 1.22V, then it wouldn't matter where our closed throttle TPS voltage was set since the EEC would always calculate a relative throttle position value of ZERO:
For example, if the TPS reads 0.99V at closed throttle, the EEC would see this at idle:
Relative TP = 0.99V - 0.99V = 0
where:
Current (idle) TP = 0.99V
Closed Throttle TP = 0.99V
Similarly, if the TPS reads 0.73V at closed throttle, the EEC would see this at idle:
Relative TP = 0.73V - 0.73V = 0
where:
Current (idle) TP = 0.73V
Closed Throttle TP = 0.73V
See where this is going? :)
Other than the fact that many TPS sensors themselves are not adjustable without some sort of modification to the sensor housing (and this is for a reason), the relative TP equation is enough in itself to show why one will not gain anything by adjusting their TPS while it is within the specified 0.48V - 1.22V range.
Note: this entry has been edited by the original author.
