The position of the throttle is very important...

The TPS or throttle position sensor feeds the ECM with throttle opening and rate of change. The angle of the throttle opening is instrumental in calculating fuel delivery and ignition timing. Newer engines even have control of the valve timing, for which the TPS value is also used. The position of the throttle valve is very important.

Theory of Operation

The TPS is a three wire sensor that measures the throttle plate opening as well as 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. This sensor is also one of the main inputs to the transmission computer (TCM). The TCM uses the TPS input signal to control the transmission shift points and the torque converter (TCC) solenoid lock-up. The TPS together with MAP or MAF sensor are the main ECM indicators of acceleration and load. In other words, the ECM looks at these sensors to calculate engine operation upon acceleration.

Some manufacturers use the TPS signal as sole indicator of engine load when there is a faulty MAP/MAF sensor. In such cases, the MAP/MAF sensor values are calculated from the TPS signal. This means that the ECM substitutes the faulty MAP/MAF value from a look-up-table stored in its ROM memory, so the vehicle can continue to operate until the driver reaches a repair facility.

The TPS sensors usually tends to fail at the lower range of its movement. This is where the driver is usually at, most of the time (as in cruising speed). This sensor usually works with a 5.00 volt reference voltage and ECM provided sensor ground. The signal is output through the signal wire, where all measurements should be made. Problems to any of the ground or power (5 volt) feed lines will cause an incorrect reading from the TPS.

NOTE:  TPS signal problems can greatly affect transmission operation. On a vehicle with an incorrectly shifting transmission, a careful analysis of the TPS signal should be made. A high TPS voltage reading during KOEO could also signal the ECM to go into FUEL CUT-OFF MODE. In which case, the ECM reacts as if the engine is at WOT and cuts pulsation to the injectors and preventing the engine from starting. Again, this is what is called fuel cut-off mode.

Conditions that Affect Operation

Most TPS sensors reset when the ignition key is cycled. This means that whenever the vehicle is shut off and turned back on again, whatever voltage signal the ECM sees (TPS base voltage) it will take as 0 degrees of throttle opening. This ignition key resetting feature also means that as the throttle bore gets dirtier the idle speed/IAC operation will also be affected. Another curious drawback of a re-zeroing or resetting TPS is in the event of a momentary signal drop-out glitch. In this case, if the TPS signal momentarily drops to very low levels, the ECM will take the low reading as the 0.00 % or closed throttle point. Then, as soon as the output signal snaps back to normal, the ECM will perceive the new signal as a wider than normal throttle opening. The result is an increase in injector pulse-width and wrong transmission shift points. TPS sensors are directly linked to the throttle plate shaft. Any binding of the throttle plates can greatly influence the TPS signal. A carbonized throttle bore may also influence this signal. Some TPS sensors are combined with a throttle switch. This is a simple on-off switch that closes whenever the throttle is fully closed. This way the ECM knows that the throttle plates are closed. This combo TPS/Idle switch is mostly found on Euro and Asian imports. It is important to remember that a shorted sensor or actuator that shares the same power feed and ground with the TPS will also have an adverse effect on its signal. If a shared sensor shorts out it may also short the TPS reference voltage or ground line.

Component Testing

TPS sensors are of the three wire type. The sensor ground, reference (usually 5 volt) voltage, and the signal wire. The sensor ground is provided by the ECM, as well as for all its other sensors. A voltage drop test should be performed across it and battery post ground to verify no more that 100 mV voltage drop during KOER. The reference voltage is also provided by the ECM and it is a 5.00 volts regulated feed line. It provides the TPS sensor with its working voltage. A shorted 5.00 volt reference line, either at the wire or another sensor that is shorted, will directly affect the TPS sensor reading and the entire engine. The signal line is the signal return to the ECM. It is the one providing the ECM with the actual TPS sensor reading. This is the line to tap when performing actual tests. A shorted TPS signal line will also skew its reading.

• The first analysis of the TPS signal voltage is done with the KOEO. This base voltage signal is taken by the ECM tobe 0.00 % or degrees of throttle opening. It is very important that this signal be within exact specifications.

• Second, using a DSO sweep the TPS while measuring the voltage output. There should be no glitches or sudden drops in its voltage reading. Refer to Fig 3. Beware of the flat spots. Most TPS failures occur at the lower travel of the throttle plate. The TPS usually develops flat spots in this region and are not easy to detect. A faster frequency setting should be selected so as to be able to detect these flat spots.

• Third, make sure that the signal reaches close to the WOT voltage level. There is always the possibility that the throttle is binding or the TPS is defective at the high range of its travel. This would cause problems with vehicle hesitating at higher cruise speeds or upon heavy acceleration.

• Always make sure that proper idle speed is attained with the TPS fully closed. If idle speed stays high with TPS at base voltage the ECM will assume that it can’t control idle speed and set a code.

NOTE: A minimum air-rate out of adjustment with TPS voltage too high or too low will, in many cases, cause an IAC (Idle Air Control) valve code. In such cases the scanner DESIRED idle speed can not be maintained because the throttle plates are over adjusted. When this happens, a reading of 0 steps on the IAC is seen on the scan tool and the idle speed will remain high. Properly adjusting the minimum air rate will correct this problem.


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