Electronic Control Module – ECM Explained

Jeep 2.5 Liter, four-cylinder engine, chromed....

Jeep 2.5 Liter, four-cylinder engine, chromed. This picture of the display engine shows part of the fuel injection system (with MPFI). The fuel rail is connected to the injectors that are mounted just above the intake manifold. This engine was developed by American Motors Corporation (AMC) and continued to be manufactured by Chrysler. All were built in Kenosha, Wisconsin. (Photo credit: Wikipedia)

The electronic control module, or ECM, is the “brain” of a vehicle. The vehicle uses sensors to tell the ECM what is going on in the engine. It also uses some sensors to tell the engine what to do. These sensors get their instructions from the ECM.

What Does the ECM Control?The ECM controls only things related to fuel injection. A fuel injected vehicle generally runs on 14:1 — 14 parts air to one part fuel. The mixture must be perfect or the vehicle will not run properly. If the mixture is too far off, the vehicle may not run at all.

The ignition system is related to fuel injection and is controlled by the ECM. The ignition must send the spark at the proper time. A four-stroke engine has four strokes. The first stroke is the intake stroke. On the intake stroke, the pistons go down in the bore, which his then filled with the air and fuel mixture. When the pistons come back up, it is compressing the air and fuel mixture to make it ready to fire. This is the second stroke, or the compression stroke.

The third stroke is the combustion stoke. At this point, the pistons are coming back up. The ignition system fires the spark plugs, which light the compressed air and fuel mixture in the cylinders. This forces the pistons down in the bore. The camshaft opens the exhaust valves and lets the resulting gas or exhaust out. This is called the exhaust stroke.

Sensors: In order for the engine to work properly, everything must be “just so.” Sensors sense the barometric pressure outside the engine. Temperature sensors measure how hot the engine is running. There are several more sensors that sense various variables. All of this information is forwarded tot he ECM. The ECM translates the information, then tells the fuel injectors how long to stay open. The longer a fuel injector remains open, the more fuel that goes into the engine. If the air is thin, meaning less oxygen in the air, the engine needs less fuel to make up the 14:1 ratio. If the air is thicker, such as at lower sea levels, the vehicle needs more fuel to make up the 14:1 ratio.

Other sensors, such as the oxygen sensor, sense how much oxygen is in the exhaust. Much of the unburned exhaust is routed through the EGR and back into the cylinders to cool the cylinders. The ECM also senses this and uses information from the EGR to determine the proper air to fuel mixture.

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