ECM MODES OF OPERATION
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.
STARTING MODE
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.
CLEAR FLOOD MODE
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.
RUN MODE – OPEN LOOP
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.
RUN MODE – CLOSED
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.
ACCELERATION ENRICHMENT
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.
DECELERATION LEANOUT
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).
DECELERATION FUEL CUT-OFF
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.