This article explains the core functions of the ECM.  The ECM is a simple computer that runs software programs.  Stored in the main memory chip (PROM) is the software (values programmed by GM or your custom tuner) that tell the computer, among other things, how much fuel and spark to deliver to the engine for a given set of operating conditions.  In addition to these core functions, there are modes of operation the ECM operates within determined by time, temperature, and engine operating load.



When you first key on the ignition, the ECM loads the information off the PROM chip and performs a quick, self-diagnostic test.  Some later ECMs also pulsed the injectors during this period to aid in what is known as “quick start” (obviously helping the engine startup more quickly).  All of this happens in a very short amount of time, usually much less than 1 second after the key is turned on.  Also during this period the fuel pump is activated for 2 seconds to “prime” the system.  Next, the ECM waits until it receives reference pulses from the distributor (or crank sensor).  Once it receives reference pulses (indicating the engine is turning over) it will use air/fuel ratio values stored in the cranking fuel pulse tables (in the PROM programming) which are based on the coolant temperature to determine how much pulse width (amount of injector ON time) it needs to command the fuel injectors open.  The lower the coolant temperature, the richer the Air/Fuel ratio.  The higher the coolant temperature, the leaner the Air/Fuel ratio.  Ignition spark advance and IAC valve position output functions use similar tables.



If for some reason the engine should become flooded, provisions have been built into the computer to help clear this out.  If during cranking of the engine you depress the throttle more than 80%, the ECM will enter what is called “clear flood mode”.  In this mode the ECM commands a delivered Air/Fuel ratio to very lean (usually 20:1).  The ECM will stay in clear flood mode as long as the throttle is 80% or higher and the RPMs are less than about 600.  As soon as throttle position falls below the 80% threshold or RPMs go above 600, the ECM disables clear flood mode and calculates fuel delivery based on coolant temperature and other factors it normally uses.



In run mode, the ECM operates in two conditions commonly referred to as OPEN and CLOSED LOOP.  When the engine is first started and the RPMs go above 400rpm (and is not in clear flood mode), the ECM enters OPEN LOOP fuel control operation.  During open loop operation, the ECM ignores the O2 sensor input when making fuel delivery calculations and relies on the other sensors to determine fuel delivery to the engine.  There are specific tables stored in the PROM programming that contain the instructions the ECM uses to determine how much fuel to give the engine in this operating mode.  Also during this period the ECM still constantly monitors the signal coming from the O2 sensor to see whether or not it is ready (hot enough) for closed loop mode.



Programmed into the PROM memory are set qualifiers the ECM uses to determine when to enable closed loop fuel control operation.  Besides O2 sensor output voltage change, these qualifiers are coolant temperature and time from start.  The amount of coolant temperature and elapsed time from startup qualifiers differ from engine to engine, and differ a great deal depending on whether the ECM is working with a heated or non-heated oxygen sensor.  For most non-heated O2 sensor applications, the general value for coolant temp is 150°F and running time is 2.5 minutes.  For systems using heated O2 sensors, these qualifiers are usually much less.  When the system enters closed loop, the ECM still uses all other sensors/inputs to determine fuel delivery to the engine, but now it uses the O2 sensor inputs to make adjustments to the fuel delivery based on what it sees in the exhaust.



If this were a carbureted engine, this mode would be known as the acceleration pump action of the carburetor.  But even with fuel injection, this function is still required for the engine to run properly.  The reason why is because when the throttle is opened, a rapid increase in manifold pressure causes fuel to condense on the manifold and intake port walls.  This fuel gets briefly trapped on these surfaces and doesn’t make it into the cylinder until it vaporizes again.  If an extra amount of fuel was not delivered to the engine at this moment, the engine would stumble due to a brief lean condition.  On speed density fuel injection systems, the ECM looks at Manifold Absolute Pressure (MAP) and Throttle Position (TP) sensors to determine when to initiate an “Accel Enrich” event, and how much enrichment and how long to sustain that enrichment is to last.  On systems utilizing a Mass Air Flow (MAF) sensor, the computer looks at the MAF and TP sensors to determine accel enrich mode operation.



When the engine is decelerating, such as what would occur when the car is at city driving speeds and you let off the gas, a leaner air/fuel mixture is required in order to reduce the emission of excess hydrocarbons (HC) and carbon monoxide (CO).  The ECM looks at MAP and TP (or MAF and TP) sensors to determine when to enter the deceleration leanout event, the amount of leanout required, as well as how long to stay in decel leanout mode.  If this mode wasn’t operating, not only would there be an increase in emissions, but you could also get a backfiring in the exhaust due to excess fuel buildup (HC’s).



During conditions of extreme deceleration, such as what would happen if you let off the gas when the vehicle is traveling at highway speeds, a complete fuel cut-off of the engine can be commanded by the ECM to cut all emissions output.  This also has the benefits of allowing the engine’s compression to slow down the vehicle (called engine compression braking).  The ECM looks at MAP or MAF, TP, and vehicle speed to determine when to enter decel fuel cut-off mode and how long to stay in this mode.  This mode overrides decel leanout mode.  Instructions in the ECM programming determine when this mode is to be exited to prevent engine stalling.



The ECM looks at battery voltage and uses this information to compensate for variations in fuel pump output and injector response.  This is needed because lower battery voltages cause the fuel pump to produce less fuel flow and also causes the injectors to respond slower compared to what they would do at higher battery voltages.  The ECM compensates for lower battery voltage by increasing the amount of injector on-time (pulse width).  This correction takes place in ALL operating modes.


BACK-UP MODE (also known as limp-home mode)

The back-up mode is used any time the ECM cannot operate normally.  In this mode, the ECM looks to the CALPAC chip to determine engine operation.  The CALPAC is a chip that contains minimal information the ECM can use to allow the engine to run using only distributor reference pulses, throttle position, and coolant temperature inputs to change fuel and ignition timing calculations.  Back-up mode was designed to allow the vehicle to “limp-home” and not leave the customer stranded should a major problem occur.  The ECM will implement back-up mode if any one or a combination of the following conditions exist:



In most OBD1 applications, the back-up mode can also be commanded by a scan tool or by placing a 3.9K ohm resistor across terminals A and B of the ALDL connector.  The engine will run erratically in back-up mode.


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