ECM COMPONENTS AND COMPONENT FUNCTION

ECM Modes of Operation

The basic ECM modes of operation are:


When the ignition is first turned to the "on" position, the fuel pump will be energized by the ECM for 2 seconds. This is done to prime the fuel system prior to starting for easy start up.  Unless the ignition key is turned to crank the engine and the ecm is seeing ignition  reference, the fuel pump is de-energized. 

In the short time before the engine is cranked, the ECM has gathered information from the coolant temperature, air temperature, throttle position and manifold pressure to determine the start up fuel ratio. The ECM will deliver one injector pulse for each distributor pulse it receives. The lower the initial start up coolant and air temperature, the richer the fuel to air will be commanded by the ECM by allowing a longer injector pulse. Once the engine starts up and the coolant temperature rises, the leaner the mixture will become as the injector pulse width becomes shorter. 

Start up mode air/fuel ratio may be as rich as 1.5 to 1 at -35F (-33C) to 14.7 to 1 at 200F (93C).

It must be noted that as long as the throttle position is seen by the ECM as 0%, the description above is followed. Opening the throttle, even slightly, will alter the air/fuel ratio.



When the engine is running and the ECM sees the engine RPM between 400-600 rpm, the system is running in open loop. In this mode, the ECM is not seeing the O2 sensor for feedback in regards to air/fuel control. It will calculated by using the TPS, coolant and air temperature sensor, from the manifold pressure (MAP) or Mass Air Flow (MAF) for specific engine load and ignition pulse references for engine speed. 

The system will stay in open loop until the following conditions are met:

The specific values are stored in the ECM prom and is calibrated to the specific engine and vehicle it was designed for. 



When the specific conditions are met by the calibration for closed loop. This means the ecm is using the feedback signal from the O2 sensor and corrects the air/fuel ratio based on the varying signal from the O2 sensor. 

A O2 signal that is below 450mv will have the ECM command the fuel injector pulse to be longer, thus enriching the mixture. When the signal swings above 450mv, the ECM commands the fuel injector pulse to be shorter and this will lean the mixture.  Other sensors in the loop will modify this slightly, but ideally, the air/fuel ratio will be close to 14.7 to 1 (stoichiometric) ratio. This is the best ratio for the catalytic converter to do its job.



When the throttle is suddenly opened to accelerate the vehicle, the opening of the throttle will cause a sudden change in manifold pressure. Without a method to add extra fuel to this situation, the vehicle would stumble or hesitate due to a leaner fuel mixture. . The amount of fuel must be increased to compensate for the extra air being introduced in the manifold. In a TBI (throttle body injection) system, the rapid increase in manifold will cause the fuel to condense on the intake runner walls. 

With the sudden change in throttle position and MAP signals, the ECM will lengthen the injector pulse and add additional pulses timed in between the base or synchronous pulses.  The extra pulses that are not synchronized to crankshaft position, they are known as asynchronous pulses.

On PFI (Port Fuel Injection) systems, the enrichment is handled the same way.



When the engine is decelerating, a leaner air/fuel ratio is required. This is to reduce emissions of hydrocarbons and carbon monoxide (HC and CO) and to prevent backfire. The ECM detects the decrease in the MAP or MAF signal input and decrease in throttle position to calculate a decrease in the injector pulse width. 

The ECM needs to make the distinction if the driver is just backing off the throttle slightly or coming to a stop, allowing the throttle to close completely.  Under light deceleration, the ECM reduces flow by shorten the injector on time (pulse width). When the intent is to return to idle speed, the ECM may cut off the injectors in a mode which is known a DFCO (Deceleration Fuel Cut Off). As the engine approaches desired idle speed, fuel delivery and idle speed control is maintained. This is done so seamless, especially on the newer systems.



The fuel cut-off mode shuts down the injectors during extreme deceleration conditions. The ecm also may be called upon to shut off fuel for safety reasons when a vehicle reaches a predetermined speed and also may cut off fuel when the engine speed (RPM) reaches a predetermined maximum. These values, maximum speed and rpm is different depending on the vehicle, engine, transmission and final gear ratio. 



In case the ECM cannot operate normally, it will switch to fuel backup mode, which will allow the engine to run only with distributor references pulses and throttle position to alter fuel and timing calculations. This is known as "limp-home" mode. 

A ECM will go into back up mode under one or more of the following conditions:



Battery correction compensates for the variations in battery voltage to the fuel pump and the injectors. A programmed in correction factor allows the ECM to increase fuel injector pulses in case the battery voltage drops below a predetermined value. A low voltage to the pump decreases fuel flow and this correction will allow the ECM to maintain an acceptable fuel pulse width. 

The ECM may also compensate for low battery voltage by increasing the idle speed. On direct ignition systems (DIS) the ecm will also correct for low primary ignition current by increasing the dwell or coil saturation time to ensure a hot enough spark to ignite the fuel mixture.





Another capabilty of the ecm is what is called clear flood mode. If the accelerator is pushed all the way to the floor, while cranking, the ecm will see the engine speed below 400 rpm and shut off the injector to clear a fuel flooded engine. This is handy in case you have accidentally pushed the accelerator partially down while cranking the engine and the ecm triggers the injector to provide fuel when it is not needed. You should never have to put your foot on the accelerator pedal when cranking a fuel injected engine. If a normally sound engine will not start within a reasonable amount of time 2-10 seconds, you have problems that needs to be investigated and corrected.





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