Sensors

TPS- Throttle Position Sensor

The TPS is a sensor used to monitor the position of the throttle in an internal combustion engine. The ignition timing and fuel injection timing are altered depending on the position of the throttle.

Potentiometer type sensors are composed of variable resistors that have a slide contact, which changes its position as the butterfly moves.

A voltage is supplied to the sensor by the E.C.U. As the throttle position changes, the output voltage from the sensor changes, this in turn is recognised and a comparison made by the E.C.U to its memory. These sensors are adjusted by loosening the sensor mounting screws.

This sensor is a potentiometer type TPS. As you can see from the video below, the wiper rotates about 0-110 degrees.

There are 3 pins on the TPS. Positive, Negative and Output. There is a resistor connected between the Positve and Output, which alters the resistance and voltage when the wiper is rotated. 

Throttle Position Switch

The throttle position switch detects throttle position at idle or full throttle by using switch contacts that are connected and disconnected as the throttle posistion changes.

The Throttle Posistion switch has 3 terminals. E or B+, IDL and PSW. The E or B+ terminal is either connected to Earth, or the input of the ECU, while the IDL and PSW, or WOT are connected to this wire by a switch. As I slowly rotate the throttle to 10 degrees, the switch becomes an open circuit from the IDL pin, and enters a stage where both IDL and PSW have no continuity. Then after 20 degrees of rotation, the switch then makes contact with the PSW circuit. 

MAP Sensor 

The MAP sensor is a sensor that sends information about the current air pressure to the ECU. This is information is used by the ECU to calculate the most efficient combustion ratio with the fuel. 

Wiring a MAP sensor to a 5v supply and Earth, I measured the output voltage from the return wire as I increased vacuum from a mity vac and got the following results.

         
Because I had the specifications for a Toyota 4A-FE MAP sensor, which is the current car I drive. Instead of bench testing this part I decided to test mine on car. As you can see from the graph I recorded 3.6V with no vacuum. Then after 10 inches of vacuum, the output voltage from the return wire decreased as more vacuum was introduced. After 60 inches of vacuum I was able to reduce the output voltage from 3.6V to 1.25V indicating that as vacuum is increased, the output voltage will decrease.

This voltage changes because as the car accelerates, there is more load put on the engine due to the increase of pressure. Because the engine is spinning faster, the distributor will also be moving faster. The ECU can now adjust the advancement of when the spark will occour which will help in the acceleration of the vehicle. If no advancement was made by the ECU, there will be a minimal charging time of the coil.

MAF sensor

A Mass Air Flow sensor is used to find out the mass of air entering the engine. This information is vital for the ECU to balance and deliver the correct amount to the engine. Air changes its density as it expands and contracts with temperature and pressure, so this sensor helps determine the airflow depending on the situation. Using a BOSCH 0230 218 032 MAF sensor and datasheet, I could determine what pins were connected to the battery, and what pins were connected to earth.

I wired up my sensor using a 12V supply, and a 5v supply, and measured the output voltage as I blew air into the sensor. I recorded 1.072V with no air going through the sensor, and recorded 3V when I blew air into the sensor.

The MAF sensor measures the amount of air flowing into the inlet manifold. The higher the vacuum, the lower the manifold pressure, inturn the lower the vacuum, the higher the pressure. The changes in voltage is then picked up by the ECU which determines the amount of fuel injected into the cylinder to mix with the air.

 Vane Airflow Sensor

The Vane Airflow sensor is a earlier type of Airflow sensor, and has a number of different components inside. Most distinquedly, it has a flap that is connected to a potentiometer which is used to measure the amount of air that flows through. As air approaches the vane air sensor. The air pushes the flap back, which will change the output voltage due to the potentiometer being controlled by the flap. The air flow sensor is the point of entry for air into the fuel injection system. Any air leaks between this sensor and the engine will cause problems with the fuel injection system. The sensor sends air volume information and air tempertaure information to the ECU.

I measured the voltage output as I pushed the flap up on the Vane Airflow Meter. At idle where there is no air flow, I got a voltage reading of 0.8V. The voltage should increase as the flap is opened up more and more. With the flap fully open, the ECU would register the car as moving very quick, and recieving alot of air into the Inlet Manifold, so will adjust the fuel accordingly. 

Temperature Sensors

The ECU needs to adjust a variety of systems based on temperature. It is critical for proper operation of these systems, that the engine reaches operating temperatures, and the temperature is accuratly adjusted by the ECU. For example, for the proper amount of fuel to injected, the ECU must know the correct engine temperature. Temperature sensors measure Engine Coolant Temperature (ECT), Intake Air Temperature (IAT), and Exhaust Emissions (Oxygen Sensor)  

ECT (Engine Coolant Temperature)

  The ECT temperature responds to change in the Engine Coolant. By measuring the engine coolant temperature, the ECU knows whether or not the vehicle is running too hot or too cold. The ECU can either vary the fuel injection, ignition timing or switch the electric fan on depending on the information from the ECT sensor.

Testing this component with a gas stove, thermometer and ohmmeter, I could check the resistance increase, or decrease as the temperature increased.

As you can see from the graph that as the temperature in the stove increased, the resistance increased. This makes this sensor a NTC sensor. If the resistance was increasing with the increasing temperature, the sensor would be PTC.

               

                  Toyota ETC Specifications

Temperature (Degrees)      Resistance (Ohms)

             20                              2000-3000 

             50                              700-1000

             80                              200-400

Looking at the specs for a Toyota ECT sensor I can see that my ECT temperature is within my resistance specs. Indicating that this ECT is good.

Early cars can use a thermistor for the engine gamma enrichment, one for the dash board gauge and a switch for the fan relay, while late model vehicles can use just one thermistor to signal the ECU which then controls the fan/dashboard gauge and the gamma enrichment.

Thermo Fan Switch

The thermo fan switch is designed to control operation of the electric thermo fan. The thermo fan switch responds to changes in the engine coolant temperature, providing circuit switching during a predetermined temperature range allowing fan operation.

Doing the same experiment with the ECT sensor, I measured the resistance with increasing temperature and recorded the following...

As you can see from the results, the resistance decreases as the temperature rises. This makes this component a NTC aswell.

As the temperature increases, the ECU monitors the resistance changing from the increase in temperature. When the temperature reaches a predetermined point, the ECU switches the fan on, which would help cool the engine, lower the temperature and increase the resistance. The fan will turn off once the temperature has been decreased to a certain temperature.

Inlet Air Temperature (IAT)

The IAT detects the temperature of the incoming air stream. On vehicles equipped with a MAP sesnor, the IAT is located in an inlet air passage. On Mass Air Flow sensor equipped vehicles, the IAT is part of of the MAF sensor. The IAT is connected to the THA terminal on the ECU. The IAT is used for detecting ambient temperature on a cold start and inlet air temperature as the engine heats up the incoming air.

Specifications of a Toyota IAT sensor

Temperature              Resistance

    20*C                  2000-3000 Ohms

    80*C                    200-300 Ohms

Like the previous experiments, I measured the resistance of the IAT sensor as I heated the temperature with a gas stove and recorded...

 

As you can see from the graph, the IAT is within it's specifications.




The most common results with these results on the ECT sensor, Thermo Fan Switch and IAT sensor were all NTC. Their resistance decreased when the temperature increased. They were all also in the manufactures specification range, therefore are in good condition. The ECT and IAT were almost identical with their results.

Knock Sensor 

The frequency of knocking (pinking) is approximatly 15KHz. (15,000 times a second) and so the manufacture tunes the knock sensor to be sensitive to this frequency, this reduces the possibility of interference from other engine noise. The pattern is an analogue sign wave and produces both a positive and negative voltage.

Connecting a Knock Sensor to an Oscilloscope, I observed the waveform when I tapped the end and recorded....



A= The oscillioscope is showing the Knock sensor at 0V.  



 B= I tap the end off the knock sensor. The voltage quickly rises

C=The amplitude and frequency have decreased, due to the decreasing oscilliations

D= The Knock sensor has returned back to 0V.

The knock sensor does not need a supply voltage because this component is passive. It creates it's own voltage when a vibration goes through it. The knock sensor would then send this information to the ECU. The knock sensors are located in the engine block, cylinder head, or inlet manifold.