THE VOLTAGE TEST

This test was to test the sensors and check the voltage across them. The following is the list of the sensors:-

·      Coolant temperature THW

·      Manifold Absolute Pressure Sensor (MAP)

·      Air Temperature Sensor

·      Throttle Position Switch

·      Fuel Injector

·      Camshaft Sensor (CAM)

·      Crank Angle Sensor

COOLANT TEMPRATURE SENSOR (THW)

THEORY

A Cylinder head temperature sensor or an engine coolant temperature sensor is usually a negative temperature coefficient thermistor. The CTS provides an analogue voltage signal which is read by the ECU and related to engine temperature for accurate changes to fuel injection.

PROCESS

 I measured the voltage across the Coolant Temperature Sensor. When the engine is cold the Coolant temperature Sensor gave a reading 1.4 V. When the engine got warmed up the sensor gave a reading 0.4 V. The decrease in the voltage reading was because when the engine was cold the sensor was sending a signal that the temperature of the engine was less and as the engine got warmer the sensor sends a signal that the engine temperature had raised.

REFLECTIONS

The voltage I got was under spec so the sensor was working efficiently. If I won’t have got the voltage under the spec then the sensor would have a problem. The problem in the sensor would lead into few faults and problems in the working of car.

The ECU would not know the exact temperature of engine which would lead into improper amount of air fuel mixture going into the combustion chamber.

MANIFOLD ABSOLUTE PRESSURE SENSOR (MAP)

THEORY

MAP sensors come in two operating formats. There is the capacitive type which yields a digital frequency load signal and there is the piezo-resistive type which sends and analogue voltage load signal to the ECU. Both are connected directly to an intake manifold vacuum source. The control unit will provide the MAP sensor with a sensor reference voltage and a sensor ground. A signal is sent back to the ECU as a result of the internal MAP sensor circuit being deformed and strained by the manifold vacuum strength, much like a strain gauge used in engineering applications. The load measurement is relative to absolute pressure and sensors come in the three grades. A one-atmosphere MAP sensor is used for forced induction and sometimes a three-atmosphere sensor for higher performance boost engines.

PROCESS

I checked the voltage across the MAP sensor. For MAP sensor we had to switch the voltmeter to DC volt. The Idle vacuum was found to be 1.4V and with no vacuum or key on engine off was 3.6V. 

REFLECTIONS

We got these readings and they were under the spec. The reason of increase in voltage from idle to no vacuum was because when the engine was running at idle then there was pressure in the manifold but when the engine was turned off and the key was on then the pressure in the manifold was less.

AIR TEMPRATURE SENSOR

THEORY

The manifold air temperature sensors are the same NTC construction as a coolant temperature sensor. These sensors are not load sensors but do contribute to the base air-fuel ratio calculation by assisting the ECU to determine the incoming air density. They are named according to their location and can be located in the intake air ducting to sense the air temperature directly behind the air filter or it is screwed into the intake manifold. It measures the manifold air temperature and sends the result to the ECU as an analogue voltage signal relative to ambient air temperature. The intake air temperature is used by the ECU in calculating the final injector pulse width.

PRACTICAL

I measured the voltage across the air temperature. It was also measured in DC volts. The voltage was checked twice once when the engine was cold and another when the engine had warmed up. When the temperature of the engine was low i.e. the engine was cold then the voltage across the Air Temperature Sensor was 2.4V. When the engine had risen to the optimum temperature then the reading was 1.4V.

REFLECTIONS

The readings for the voltage are well under the spec. The voltage reading is high when the engine is running cold because the sensor wants to convey to ECU that the engine is cold and air is not going to get warmed quickly. When the engine gets warmed up the voltage across the sensor decreases because the engine is now running at an optimum temperature so the engine is warm and air is going get warm.

THROTTLE POSITION SWITCH

THEORY

The throttle position switch is operated by the throttle. It has a set of contacts for both idle and full-throttle positions. The switch signals the throttle opening to the ECU, which provides mixture correction for both idle and off idle as well as full throttle conditions. These switches became a variable resistor potentiometer type in later systems. The engine ECU using a potentiometer TPS receives an analogue voltage signal that varies with throttle position. The advantage of this design is the ECU can determine the exact position of the throttle valve at any instant and can also see the rate of the driver’s throttle application.

PROCESS

In this I checked the voltage across the sensor I connected the voltmeter across the sensor the voltage was found to be 0.7V when the throttle was closed and it was 1V when the throttle was open.

REFLECTIONS

The throttle position sensor gives a reading which tells the ECU that how much fuel should be provided. The TPS gives less voltage when the throttle is closed as the throttle opens it increases the voltage. It tells the ECU how much fuel is required to be injected.

FUEL INJECTOR

THEORY

The injector is located in the manifold ahead of the in direct injection systems intake valve port. In indirect injection systems it is fitted in the intake manifold so that the fuel is sprayed directly into the intake port in the cylinder head. The fuel is directed in a finely atomised form with a mean spray angle of about 6 to 12 degrees. Special fuel injectors are used in multi valve engines so the spray pattern suits the dual intake valve positions.

The atomised fuel is maintained in suspension in the air. Wetting the surfaces of the manifold and valve port is somewhat minimised when compared to central fuel injection or carburettors. This is due to residual fuel being quenched on the intake manifold inner surfaces. This fuel would not be fully burnt in the combustion chamber and would contribute to high hydrocarbon emissions in the engine’s exhaust.

The injector is fitted into the manifold in special rubber mouldings which protect it from heat transfer and vibration but they also form a vacuum seal. It has an electrical connection and a fuel connection.

Fuel is supplied through the top of the injector and retained in the injector by the needle valve, which is held on its seat by the spring. At appropriate times, electrical pulses from the ECU energise the solenoid windings and attract the plunger and needle from its seat. As a result, a spray of pressurised fuel is directed into the intake port of the engine.

PROCESS

In this we checked the duty cycle across the injector. When the engine was running at idle the reading was about 4.8% and when the engine was accelerating the reading was coming out to be around 31 %.

REFLCTIONS

The readings were according to the spec. The readings showed that when the engine is running at idle the injectors are not fully open and when the engine is accelerating the injectors open up a bit more and thus supply more fuel to the engine. If the reading was not under the spec the there would have been a problem in the injector. If the reading would have been high the injectors would be supplying more fuel and thus the engine would be running rich. But when the reading would have been lesser then the spec then the injector would have been supplying less fuel and thus the engine would be running lean.

CAMSHAFT SENSOR (CAM)

THEORY

As the engine management systems developed, newer processes like sequential fuel injection and variable camshaft timing was adopted. For these developments to operate the engine ECU had to calculate events on more than just crankshaft position. For sequential fuel injection the ECU had to know not only crank angle, but it had to know which if the four strokes the piston was on. Also, with the advent of variable camshaft and valve timing, the engine ECU had to be able to read the relative position of the camshaft, in relation to the crankshaft, to determine the exact dynamic cam timing. The sensors are mounted on one or more camshafts or in a camshaft-driven distributor. These sensors are usually hall effect devices, optical triggers or inductive pickup.

PROCESS

In this I checked the voltage across the cam sensor. I put the voltmeter on ac volts. Then I measured the voltage at Idle rpm and at 2500 rpm. The voltage would around 0.3V at Idle and 0.5V at 2500 rpm.

REFLECTION

The voltage reading was under the spec hence the cam sensor is working properly.