TPS testing

In engine operations, there are 3 main phases: Off, Idle, Throttle. The throttle butterfly is an inevitable component that decides anywhere the engine is between Idle and Full-open throttle, and that control is mainly in the hand of the driver, and partially the ECU. 

Electronic engine management has become so importantly accurate, therefore the ECU knowing exactly how wide-open the throttle butterfly does is a MUST, so it can manage the Rest of the subsystem to operate in the satisfaction of this throttle position.

There are 2 main sides of the throttle body: Mechanical and Electrical. 
The mechanical part is usually a coiled/spring butterfly vane that is always coiled to be at closed position, and linked to the gas pedal. The throttle is inside the throttle body and is mounted right before all the intake valves of the cylinder head - for best accuracy and air efficiency.
Beside the throttle butterfly, when idling @ closed position, there needs to be an auxiliary air passage called by-pass air, this will be addressed in bi-pass air control system.

There are 2-types of electrical insides the throttle position sensors, based on what type of ECU-feeding signals they give: analog and digital, hence hereby we have Linear(analog) TPS and Switch-type TPS(digital)

    In linear: The electrical part consists of where the mechanism of the butterfly is linked to the variable resistor part of the position sensor circuitry. The variable resistor works on the same principal as the Vane type Mass airflow meter, as a voltage divider where the output signal that simulates the throttle position feeding to the ECU is variable along the side of the rheostat. 
Here is an example:

 (autoshop101.com - Toyota)

In expansion, there are many others modification can be done to the TPS circuit. For example, because @ Idle, the engine needs minimum fuel, therefore a fuel-cut switch can be integrated with the wider slider of the rheostat when it is at closed position so it pushes the fuel switch open - similar to the fuel switch of the vane airflow meter. From this, We can see that the Mass airflow meter and the throttle position sensor are kind of alternatively functional, since the kind of signal when mass air flows through the sensor as well as the signal when the throttle is open are proportionally, and sequentially related. In other word, the ECU can't receive a 4.5V signal of mass air without 4.5V signal from the wide opened TPS, and even if, this has got to be faulty.

Testing an analog TPS: 

We can see that this potentiometer is capable of delivering variable signals. As we move the slider at any various position, we get different voltage output signal from VTA. For all the test procedure to be successful, it is important that we determine the location of the potentiometer on the throttle body - in this case it is the black box on the left of the throttle body as in the pictures. After obtaining the part numbers and determining the TPS type, we have to get the correct wiring diagram to show us what the 4 pins with wires coming out are. This is a fairly simple circuitry to we can generally break them down as this standard wiring diagram, though different manufacturer have different layouts.

 The diagram shows the fuel cut-off switch for IDLE position as the Slider closes it will push the IDL1 terminal closes, this should be an alternative or complement to the fuel pump switch.
Similar principal to the Vane MAF meter, we can also have a resistor check. As we put the Ohms meter between Vc and E2 and move the slider, we will experience a constant resistance, because we check the circuit from Vc, through the whole rheostat NOT the VTA terminal.
Between VTA and E2, we can see that the resistance changes decreasingly from large, as we open the slider. This means that the resistance is largest @ closed position so we get minimum voltage signal, and smallest @ wide open so we can have large (not exceed 5V) voltage signal. Remember this rheostat and VTA is just like a voltage divider, and resistance is inversely proportional to Output voltage signal. Therefore this explains the result obtained:

For the IDLE switch, @ closed position we will have continuity between VTA and IDL1, as we slightly move the slider away that connection disappears.





Switch type signal: 

There are several reasons for the Throttle Position Switch to be developed along side as a complement for the analog signal, and yet they are quite important. The IDLE fuel cut-off switch control mentioned above on the analog TPS is an important part of Switching TPS function as a whole, and yet it is one of the reasons. What about @ full throttle? What would we want the engine to run? We want full-power, and yet all the fuel pump switch is on, ignition timing is advanced and injector frequency get higher - another reason for this switch type. Those 2 are the main visible reason. But the truth is in order for the ECU to recognize clearly and reliably fuel cut off control and ignition timing corrections, the feed should be the solid and reliable digital signal, not the precise varying analog signal.
Here is a casual layout for any switch typre TPS: 

 

The output signal doesn't care how many degree the throttle is at, it only cares when the slider is in a certain range where either the IDLE switch or the PSW switch is STILL in contact, in this case 0-1.5 degree and 70-wide open, anywhere in between neither the IDLE fuel cut off nor the Full throttle fuel pump is activated. 

Therefore, continuity check is the only thing that maters since there are normally only 3-4 pins coming out and we know is PSW, Common(E), IDLE and maybe a 5V Vc feed.

What happen when the slider is @:

 IDLE: this is where we do nothing to the slider, it's already @ IDLE position. So 5V from ECU is grounded, therefore all the voltage drop will be across a resistor inside, hence we will have near zero volt coming out. There is continuity and the resistance over this is readable(1k Ohms)

Anywhere in between: We tilt the slider, and our V meter reads "5V", @ both PSW and IDLE we get 5V signal this means the 5V circuit is not completed, the resistor will not consume any of the available voltage hence we have 5V out of the pins.

PSW: similar to IDLE when the slide is in range.

For the record, position sensor is the biggest and most important "Commander" for the ECU while it's acting like a CEO. Position sensors represents the driver's demand of operating modes/loads for the engine, so it is very important that the ECU receive both analog and digital signals.