It's been quite a while, I haven't seen any new tech doc here on PW. Thought of adding one in archives of PW, might help someone in understanding these things from the scratch, though many of you already know this stuff pretty well.
Here it goes:-
Computerized Engine Control
The brains of a computerized Engine Control system is the Power train Control Module (PCM) / Electronic Control Module (ECM) / Electronic Control Unit (ECU) / microprocessor / Control module or brain box.
PCM basic function is to crunch numbers. It takes in raw data from its various sensors, translates the sensor data into numbers, then process the numbers to determine what needs to be done next.
The number crunching is controlled by a program. A program is nothing more than a set of step-by-step instructions that tells the computer how to manipulate the data that’s fed into it.
End product of all number crunching is a control part. i.e. switching ON or OFF an electronic circuit. Computer does this by grounding the circuit electronics. This completes the circuit allowing current to flow through a solenoid or relay (actuator).
Oxygen Sensor (O2 sensor)
Exhaust gas oxygen sensor (EGO or O2S) or Lambda sensor is the key sensor in engine fuel control feedback loop. The engine computer uses O2 input to balance the fuel mixture. Leaning the mixture when O2 reads rich and richening the mixture when sensor reads lean.
Rich Mixture (A/F over 14.7:1)=High O2 voltage out put (600~1000mV)
Lean Mixture (A/F under 14.7:1)=Low O2 voltage out put (300mV or less)
When A/F ratio is perfectly balanced at 14.7:1, it is called stoichiometric ratio, and sensor’s out put is about 450mV.
Down stream O2 sensors are used to monitor operating efficiency of catalytic converter. If converter is doing its job efficiently, there will be little unburned oxygen left in exhaust. So down stream O2 sensor gives steady out put instead of switching back and forth like up stream O2 sensor.
If down stream O2 sensor is switching like up stream O2 sensor, it means catalytic converter is not working.
Symptoms of failed O2 sensor
1) Failed emissions test (high CO or high HC)
2) Damaged catalytic converter
3) Poor fuel mileage
4) Fouled spark plugs
5) Engine runs rough
6) Sluggish performance
Open Loop
It takes a few minutes for O2 sensors to heat up and start producing signals, so until there is an O2 sensor signal, computer stays in what’s called open loop mode. In this mode, fuel mixture is slightly rich and does not change.
Closed Loop
It is the operating mode where all engine control sensors including the Oxygen sensor are used to get best fuel economy, lowest emissions, and good power.
Crankshaft/ Camshaft Position Sensors (CPS)
It is deployed in distributor less ignition systems. These sensors serve essentially the same purpose as the ignition pick up and trigger wheel in an electronic distributor, the only difference being that the basic timing signals is read off the crankshaft or harmonic balancer instead of the distributor shaft. This eliminates ignition timing variations that can result from wear and backlash in the timing chain and distributor gear. It also does away with timing adjustments.
Faulty CPS will give rise to the loss of all important timing signal resulting in an engine that cranks but won’t start.
When troubleshooting a suspected CPS problem, one must follow the diagnostic flow charts in the manual to isolate the faulty components, e.g. Crankshaft position (NE) sensor/ Camshaft (G-sensor); otherwise there is no way to know if the problem is in the ignition module, computer, wiring harness or sensor.
Temperature Sensors
The proper operation of various systems, controlled by ECM, requires engine work at its optimum temperature.
i ) Engine Coolant Temperature Sensor (ECT)
It responds to change in Engine Coolant Temperature. By measuring engine coolant temperature, ECM knows the average temperature of engine. It is usually located in a coolant passage just before the thermostat.
It is critical to many ECM functions, e.g. fuel injection, ignition timing, variable valve timing, transmission shifting.
ii) Intake Air Temperature Sensor (IAT)
The sensor detects intake air temperature and transmits a signal to the ECM.
Vehicles equipped with MAP (Manifold Absolute Pressure sensor), IAT is located in an intake air passage, whereas vehicles equipped with MAF (Mass Air Flow sensor) it is part of MAF.
iii) Exhaust Gas Recirculation Temperature Sensor (EGR)
It is located in the EGR passage, and measures the temperature of exhaust gases. When EGR valve opens, temperature increases. From the increase in temperature, ECM knows the EGR valve is open and that exhaust gases are flowing.
Vehicle Speed Sensor (VSS)
The vehicle speed sensor is installed in the transaxle. It contains a pulse generator that provides a vehicle speed signal to the speedometer. The speedometer then sends a signal to the ECM.
ABS works by limiting the pressure to any wheel which decelerates too rapidly. This allows maximum stopping force to be applied without brake lockup. In operation, the wheel speed sensors at each wheel send electronic pulse signals to the control unit. If wheel lockup (rapid deceleration) is detected during brake application, the computer signals the valve unit to limit the hydraulic pressure to the wheel cylinder. This is accomplished by diverting some of the fluid into a small reservoir, which is pumped out when the brakes are not being applied.
Knock Sensor
The knock sensor is attached to the cylinder block. It senses engine knocking using a piezoelectric element. A knocking vibration from the cylinder block is sensed as vibrational pressure. This pressure is converted into a voltage signal and sent to the ECM.
Throttle Position Sensor (TPS)
TPS is mounted on the throttle body and converts the throttle valve angle into an electrical signal. As the throttle opens, the signal voltage increases.
The ECM uses throttle valve position information to know
• Engine mode: idle, part throttle, wide open throttle
• Switch off AC and emissions controls at Wide Open Throttle (WOT)
• Air-Fuel ratio correction
• Power increase correction
• Fuel cut control
Accelerator Pedal Position (APP) Sensor
The APP sensor is mounted on the throttle body of ECTS-i. The APP converts the accelerator pedal movement and position into two electrical signals. Electrically, APP is identical in operation to TPS.
Mass Air Flow Sensor (MAF)
The mass airflow sensor is placed in the stream of intake air. It measures the intake flow rate by measuring a part of the entire intake flow. It consists of a hot wire that is supplied with electric current from the ECM. The temperature of the hot wire is controlled by the ECM a certain amount. The heat generated by the hot wire is reduced as the intake air flows around it. The more air, the greater the heat loss. Therefore, the ECM must supply more electric current to maintain the temperature of the hot wire as airflow increases. The ECM detects the airflow by means of this current change.
Manifold Absolute Pressure (MAP)
The Manifold Absolute Pressure sensor measures changes in the intake manifold pressure resulting from engine load and speed changes. The computer sends a 5-volt reference signal to the MAP sensor. As pressure changes in the intake manifold occur, the electrical resistance of the MAP sensor also changes. By monitoring the sensor output voltage, the computer can determine the manifold absolute pressure. The higher the MAP voltage output the lower the engine vacuum, which requires more fuel. The lower the MAP voltage output the higher the engine vacuum, which requires less fuel.
Barometric Pressure Sensor
Also known as High Altitude Compensator (HAC), measures the atmospheric pressure. Atmospheric pressure varies with weather and altitude. At higher elevations, air is less dense; therefore, it has less pressure. In addition, weather changes pressure. This sensor operates the same as MAP sensor except that it measures atmospheric pressure. It is located inside ECM. If it is defective, entire ECM must be replaced.
Crash Sensors
Crash Sensors are the devices that work with the control module to discriminate between crash and non-crash events. These sensors measure the severity of the impact. They are set up so that sudden "negative acceleration" will cause the contacts to close, sending a signal to the control module which checks for a signal from the rear sensor which must arrive first to activate the Airbag(s).
Most car companies say a vehicle has to experience an impact that generates at least 7 G’s of force before it will trigger the air bag. For most vehicles, this would be the equivalent of hitting a solid barrier at 12 to 15 mph, or hitting another vehicle at a speed of about 25 mph. In reality, the air bag may deploy at higher or lower speeds.
It is important to note that at least 2 of these sensors must signal a crash before airbag deployment.
Do not attempt to check or replace any crash sensor unless the air bag module has first been deactivated (or deployed as a result of an accident). This can be done by unplugging the air bag connector at the base of the steering column and waiting 10 minutes or longer depending on the application (always refer to a service manual for the proper deactivation and removal procedure).
Steering Angle Sensors
Steering-angle sensors measure the angle and/ or the speed of the steering-wheel
angle. Steering-torque sensors determine the force required for this steering action.
Some of its features are
• Sensing the position of the telescoping/tilt power wheel.
• Sensing rotation and speed for 'drive-by-wire' cars.
• Sensing rotation to control steer able headlight systems.