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All EFI systems rely on sensors to report engine conditions to the ECM so it can make intelligent decisions based on the engine’s performance. Each of these sensors reports on a particular engine condition. It is then up to the ECM to gather all this data and condense it to deliver the proper fuel and spark to the engine. The cool thing about this is that current automotive computers can make these decisions thousands of times per second! Try that with your hand calculator.
For example, let’s take a look at a typical late-model TPI- or LT1-style fuel injection system. We’ll deal with only the sensors that have direct input for fuel injection, omitting the emissions-oriented sensors for now. To start with, all EFI systems need an ignition trigger. This is fairly simple since it’s virtually the same as the hookup for a normal analog tachometer.
TPS: The throttle position sensor (TPS) hooks directly to the throttle blades to tell the ECM via a voltage signal not only how far the throttle is open or closed but also the rate at which the throttle opening is changing.
MAP: The manifold absolute pressure (MAP) sensor reads the intake manifold pressure (or vacuum), converting this pressure reading into a voltage signal that the ECM can use. In speed-density systems, this is a critical sensor. If the MAP sensor fails, the system usually reverts to a limp-home mode.
MAF: Certain TPI, LT1, and LS1 Chevy production engines use a mass airflow (MAF) sensor to measure the amount of air the engine is using. This sensor is located upstream of the throttle body and uses what is called a hot-wire sensor. A very small wire strung across the airstream is electrically heated to a certain temperature. As air passes over the wire, it cools. The amount of current required to maintain the wire’s temperature then represents the amount of mass airflow.
O2: Oxygen sensors (O2) are often employed in EFI systems to report the amount of free oxygen in the exhaust. This is a convenient way to measure rich or lean air/fuel ratio conditions in an engine at part-throttle. EFI systems employing an O2 sensor are generally referred to as operating in closed loop. If the engine does not use the sensor, the system is said to be operating in open loop. Closed loop refers to the fact that the O2 sensor readings allow the computer to measure the result of combustion. Low oxygen readings indicate a rich condition, and higher oxygen content in the exhaust is a result of lean air/fuel conditions. Even MAF-equipped engines operate in closed loop, since the O2 sensor’s feedback allows for very tight control over part-throttle air/fuel ratios.
Unfortunately, these factory-type O2 sensors only operate accurately within a very narrow range of the air/fuel ratio scale. They are most accurate when reading ratios hovering around 14.7:1, which is the “ideal” ratio for emissions where hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) are at their combined lowest levels.
Coolant: This is a fairly simple but important sensor. For the engine to run properly at low temperatures, it’s critical that the ECM be aware of the coolant temperature. Low-temperature input, for example, will trigger fuel enrichment outputs from the ECM to act like a choke for cold-start and warmup situations.
MAT: Inlet air temperature affects the air/fuel ratio, so a manifold air temperature (MAT) sensor is needed to help the computer compensate for changes in air density based on temperature. All EFI systems offer compensation tables for inlet air-temperature changes.
Knock Sensor: All late-model GM engines are also equipped with a knock sensor that identifies engine knock or ping. When the knock sensor informs the ECM that it senses knock, the ECM can retard timing to prevent engine damage. Several aftermarket EFI systems also employ a similar knock sensor system.
Cam Position Sensor: Sequential fuel injection systems require a sensor to tell the ECM where cylinder No. 1 is in relation to top dead center in order to trigger the fuel injectors to deliver fuel at the proper time. This sensor is usually some type of magnetic pickup located either on the camshaft drive or on the damper (similar to a racing-style crank-trigger sensor
so please help me understand a few more basic stuff here folks...
