ECM COMPONENTS AND COMPONENT FUNCTION
ECM INPUTS AND SENSORS
The ECM controls many functions related to the operation of the fuel injection system. The early systems were in a sense relatively simple, but as time went along and mandates for stricter emission controls has added to the complexity of the system grew. This went to include not only the ECM, but also transmission controls beyond the TCC (Torque converter clutch) and also the addition of more onboard computers to control everything from the climate control, convenience items such as power door locks, interior lighting and even the radio. All these added to the complexity of the communication protocols to evolve into a communication network know as CAN (Controller Area Network) which was mandated by federal (U.S.) law for 2008 and newer vehicles.
But, were digressing here, I'm going to step back and keep it simple for easier understanding on how all these play together to provide you with a vehicle that starts almost instantaneously, runs smoothly even on the coldest day from a cold start and provides many thousands and even hundreds of thousands of miles of nearly trouble free driving.
In order to make the decisions necessary to run the engine, the ECM depends on a network of sensors and switches to retrieve information from. These are considered input devices. The fuel injectors and cooling fan relays and solenoids that control torque converter lock up and considered output devices.
Not all sensors are used in any one system. O2 sensors and throttle position sensors is an example of what is used in all systems, while a cam position or cruise control may not be.
Basic Information and Operation of the Sensors
The operation begins with a signal to and/or from the sensor. The input signal is either or 5 or 12 volt reference signal. The internal circuitry within the ECM monitors this input signal and this signal is used to make the various calculations for speed, fuel control and to control the various outputs such as fan relays.
Depending on their method of operation, sensors will fall into two categories:
The simplest circuit that the ECM uses for an input is the "switch" input. Basically a "yes" or "no" answer. The ECM , however, can receive two types of switch inputs, depending on the application and the source of the power to the input.
A pull up circuit (shown above) has a power source outside the ECM and does not provide the reference signal. When the switch is closed, the voltage will go "high" to the input of the ECM. An open switch will cause the signal input to go "low". A good example of this is the A/C request switch.
A pull down circuit (shown above) has a power source internal to the ecm. When the switch is closed, the source voltage is pulled to "low" to an external ground. The ECM will see this as a low value input. When the switch is opened, again the ECM will see it as a high reference signal.
A thermistor is a variable resistor sensor whose electrical resistance varies with it temperature. A low temperature will result in a high resistance and a high voltage voltage value being interpreted by the ECM. As the temperature rises, the resistance gradually decreases and the voltage drops. Good examples is the engine coolant temperature and intake air temperature sensors.
ONE WIRE SENSORS
A one wire sensor carries the reference signal to the ECM and the body of the sensor completes the circuit to a ground reference. Two examples are the early one wire O2 sensors and the knock sensor. Each one of these devices provide its own signal and the ECM interprets this signal.
TWO WIRE SENSORS
In a two wire sensor, one wire is the reference signal to the ECM, the other wire is the ground. A variable signal, such as that from a IAT is used by the ECM for interpretation. Another example of a two wire sensor is a heated O2 sensor, where the sensor input and low reference is provided to the ECM. The other two wires is for the heater to bring the O2 sensor up to operating temperature quickly.
THREE WIRE SENSORS
The TPS, MAP and MAF sensors are examples of a three wire sensor. There is a reference voltage (5 or 12V), a signal input to the ECM and a ground circuit.
The signal voltage input to the ECM is connected to the wiper arm on a three wire resistance called a potentiometer. The other side of this sensor is connected to a reference voltage source and the other is connected to ground. The amount of voltage on the wiper arm depends on where it touches the resistor. If the wiper is close to the source voltage, it will read close to that voltage, if the wiper is closer to the ground, it will sense a lower voltage, while in between it will sense variable voltage.
It must be noted here there is a couple of sensors that will have two or more three wire sensors. These will be used on (DBW) Drive By Wire vehicles. The early Corvette LS1's starting in 1997, had three such three wire sensors for the APP (Accelerator Pedal Position). The electronic throttle will have 2 TPS's in it. The reason for the "extra" sensors is to provide additional feedback and cross checking across the sensors to insure safe operation of the drive by wire system. If one or more of these sensors values are out of range, it will set a trouble code and may reduce engine power for safe operation, or could completely shut down the engine.
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