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The TPS is a three wire sensor that measures the throttle plate opening and its rate of change. This sensor is a variable resistor, also called a potentiometer, that is directly linked to the throttle plate shaft. The TPS outputs a voltage directly proportional to the throttle opening. As the accelerator is depressed the throttle plate opens and the TPS voltage increases. The TPS assists the PCM in determining transmission shift points, converter lock-up and along with the MAP and MAF sensor are main PCM indicators of acceleration and load. In other words, the PCM looks at these sensors to calculate engine operation upon acceleration.

Let me preface by saying I do not believe that most Ford TPS Sensors require adjustment unless you have played with your Throttle Plate Set Screw or have a performance application. The procedure is not mentioned in any late model Ford Factory or Engine/Emissions-Diagnosis Manual that I have seen. In other words, Adjustable TPS Sensors may be applicable to older vehicle applications. In late model vehicles, the TPS Sensor is "self calibrating".

If you require an Adjustable TPS you must set it just below 1v, ideally between 0.96v-0.98v. This is because the PCM is programmed to observe the TPS Signal Voltage in three modes:

TPS Modes
Idle Mode - Less than 1v
Part Throttle Mode - Greater than 1v
WOT Mode - Greater than 3.7v (Breakpoint is +2.7v)

The 0.96v-0.98v. setting has been determined to be optimal as it minimizes time delay between Closed Throttle and Part Throttle which increases performance. This is where performance TPS Adjustment Settings Instructions are directed and many mistakenly think it is all inclusive. The shop modification of drilling out the screw holes for additional adjustment is directed at people that use the incorrect TPS or cannot achieve the setting parameters due to intentional Throttle Body variences.

On the other hand, EEC-IV Broncos use a Non-Adjustable TPS. Each time the key is turned on the PCM reads the Closed Throttle TPS voltage and places it in KAM memory as TPREL PID (or Ratch) Voltage. Ratch Voltage then becomes the baseline for Idle where the PCM assumes 0% Throttle Opening. The PCM is programmed for a voltage stairstep, (or Ratch), to Part Throttle and Wide Open Throttle (WOT) values. These stairstep values are based on the initial Closed Throttle readings and are determined each time the ignition key activates the PCM and TPS.

Ratch Values
Closed Throttle - Initial Voltage Setting
Part Throttle - +0.02v above Closed Throttle
Wide Open Throttle - +2.71v above Closed Throttle

Even though the PCM determines Ratch Voltage with key on, there are different initial vehicle specifications for TPS Settings. This is to ensure that your TPS can operate within the full range of it's mechanical/electrical capability (In the case of my 1990 5.8L Bronco, this range is 0.34v-4.84v between 0-85 degrees rotation). As long as your initial TPS settings are within the factory specified range, such as listed below, your TPS will be correct.

TPS Settings
1990 Bronco 4.9L - 0.73v to 1.22v
1990 Bronco 5.0L - 0.73v to 1.22v
1990 Bronco 5.8L - 0.73v to 1.22v
1995 Bronco 4.9L - 0.65v to 1.28v
1995 Bronco 5.0L - 0.65v to 1.28v
1995 Bronco 5.8L - 0.65v to 1.28v

In either case, the PCM generally operates in Open loop on cold Start-up, Closed-Loop on warm idle/low-load cruising and Open-Loop during WOT. Open-Loop refers to shutting down the EGR, ignoring O2, ECT, ACT Sensor Input and relying upon programmed fuel maps. The TPS is advising the PCM through throttle modes when to go into Open or Close Loop Operations. And, as the TPS is an electrical/mechanical device, the TPS Sensor can wear out by developing dead spots which cause idle problems or hesitation usually in the most used lower range.

Finally, Curb Idle and Fast Idle are controlled by the PCM and IAC and are not adjustable. This means adjusting your Throttle Plate Set Screw from the factory calibration will not allow the IAC to effectively control the rpm. This also changes the Throttle Plate position potentially causing it to stick in the bore and it alters the TPS settings by either telling the PCM that you are always at Part Throttle or pushing it outside of setting specification. The Throttle Body Return Screw adjusts the Throttle Plate for Minimum Air Rate, which is the minimum amount of air required to maintain idle with the IAC unplugged. Plug the IAC in and the PCM is in control of your Idle within it's programmed parameters. This means that you should not have to touch the Throttle Plate Set Screw unless someone has played with it or if you have a performance application which is tuned for a higher Idle air flow. That is where you must adjust your TPS back to factory settings to compensate for the change in Throttle Plate position. As the Minimum Air Rate, Engine Load, TPS Sensor and the IAC are very closely linked, they must all work together to maintain a good idle.

Idle problems can be caused by a myriad of other issues which should always be looked at first. These include ignition, fuel, spark, EGR, vacuum and PCV to name a few. After these are ruled out, the KOEO TPS Harness may be tested for Reference Voltage, the KOEO TPS Voltage may be back probed for baseline setting and a smooth increase thoughout the Throttle rotation and the unplugged TPS Sensor may be tested for Resistance also throughout the Throttle rotation.

KOEO TPS Testing
KOEO TPS Reference Voltage - 5v (VREF/SIG RTN)
KOEO TPS Signal Voltage - <1v to <4.8v (TP SIG/SIG RTN while rotating Throttle)
Unplugged TPS Resistance - <4k ohms/>350 ohms (TP SIG/VREF while rotating Throttle)

1988-1991 TPS Wire Colors
VREF - Orange/White
TP SIG - Dark Green/Light Green
SIG RTN - Black/White (My 1990 is actually Red/Gray)

1992-1996 TPS Wire Colors
VREF - Brown/White
TP SIG - Gray/White
SIG RTN -Gray/Red

Motorcraft TPS Wire Colors
VREF - Orange
TP SIG - Green
SIG RTN - Black

For more information, see this link: Throttle Position Sensor Testing, Replacement and Adjustment
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