I found my old writings on someone elses website
The pulsing of the injectors is controlled by the ECM this is known as Pulse Width Modulation (PWM). The ECM (
Electronic Control Module) is a computer, wich is mounted in the drivers side storage compartment in front of
the battery.
The ECM is the "brain" of the fuel injection system.
The '82 ECM is the 1225550, the 84 manual is the 1226430 and the 84 auto is the 1226026.
The ECM is devided into three major sections:
The ROM (READ ONLY MEMORY.)
The PROM (PROGRAMMABLE READ ONLY MEMORY) also known simply "the chip".
There's also a CALPAK, wich is the calibration for the type of engine. Some ECM's have been used in bothe V8 and V6 engines, the '7747 is such an ECM. These have different calpaks (can be swapped out).
The RAM (RANDOM ACCESS MEMORY).
The ROM contains the basic set of instructions for the ECM to follow. The programming in this chip cannot be
erased or changed. The ROM is NON Volatile.
The PROM works along with the ROM to fine-tune the functions of fuel and timing control. The PROM can be
replaced by a "power" chip (also a prom) or an EPROM (or even EEPROM) from wich the data can be erased and
changed. The PROM is NON Volatile.
The RAM has three primary functions:
To act as scratchpad (temporary memory) in case of a mathematical calculation.
To store BLM (Block Learn Multiplier) information when the engine is off or the ECM is in open loop.
To store fault (trouble) codes. These codes are stored for 50 starts or whenever the power (orange wire on
juction block for '82 vette) is disconnected. Unlike the ROM and PROM the RAM is volatile.
The ECM controls the injection system, it fuctions in either one of 3 different modes:
* Shutdown Mode..
When the ignition is off for 2 or more seconds. Only the RAM (trouble codes) and the BLM remain powered.
* Startup Mode.
When engine is cranked. Timing is locked at initial starup and air/fuel ratio is enriched. This mode is on for
about two seconds after starting to ensure the engine will continue to run.
* Running mode
devided in:
- CLOSED LOOP.
The red arrow doesn't indicate a signal, it's an event. Due to the more/less pulsing of the injectors a
rich/lean condition will be observed and the ECM will compensate for this.
In closed loop mode the ECM has 2 differend modes;
- Enleanment Mode
When the throttle is closed.
- Enrichment Mode.
At wide open throttle.
High engine load.
High engine RPM.
Overheating engine. (the cold fuel is a great contributor to engine cooling)
The ECM is normally in enleanment mode, it constantly lowers fuel addition. When the oxygen sensor detects a
lean condition enrichment mdode is entered untill the oxygen sensor doesn't sense a lean condition anymore.
Than enleanment is entered again. This (theoretic) gaurantees an "ideal" air/fuel ratio.
* OPEN LOOP.
- Fuel cutoff Mode.
When the throttle is closed and the MAP sensor notices a negative load (deceleration) untill about 1500 RPM..
- Clear flood Mode.
When throttle is depressed more than 80% while cranking. Injectors are turned off or air/fuel ratio is lowered
to 20:1.
- Limp Home Mode.
When there is major trouble with any of the critical sensors/actuators.
The ECM monitors different sensors and controls the injectors and actuators to respond to changes/demands.
The different sensors and actuators are:
The Oxygen sensor
The oxygen sensor is the only sensor on the car that creates it's own voltage and has therefore only one wire
coming from it. It's actually a chemical voltage generator, based on the potential difference between the
oxygen rich condition in outside air and the lean condition in exhaust gas.. When it's at it's operating
temperature of about 600ºFahrenheit it will produce a Voltage ranging from 100 to 900 mV. At operating
temperatures the sensor will respond to changes in the oxygen content in the exhaust gasses. It creates a
potential difference between the "clean" oxygen rich outside air and the oxygen poor exhaust gas. The voltage
potential is developed due to chemical imbalance between the two oxygen concentrations. When the voltage is
low (oxygen levels in exhaust high) the ECM will assume the engine is running lean and thus enter the enrichment
mode. Lean conditions usually produce voltages lower than 450mV, rich will be over 450mV. As you can see there
is a very critical value for "ideal" air-fuel ratio. This is done to ensure that the engine never runs very
lean/rich. This way a nearly ideal air fuel ratio is always obtained, the engine never gets the chance to
run off this point by far without the ECM noticing it.
Due to the nature of the oxygen sensor it has a hard time distinguishing between a perfect and a rich running
engine, when all oxygen is burnt the mixture can be ideal or rich. The oxygen sensor doesn't recognise surplus
(unburnt) fuel in the exhaust. Therefore the ECM will always assume the engine is running rich unless it
notices a lean condition.
The Tach Signal
The Tach signal is used to synchronise and sometimes (WOT) sequence the injectors. At very high engine RPM the
ECM always enters enrichment mode.
The TPS is actually a three wire potentiometer. It's got a 5V reference sent to it
by the ECM. The TPS is grounded through the ECM. The 5V is lowered to a different voltage through the
potentiometer (read variable resistor) When at no throttle the resistance is lowest and the voltage is also
lowest (normalle set to 0.525V ±0.025) When at full throttle the resistance between the 5V and the ground is
highest (theoretical infinity) and the potential difference (voltage) will be 5V.
The Idle Air Control valve or IAC (stepper motor)
The IAC is a motorised plunger that regulates the idle speed of the
engine by regulating the incoming airflow. The IAC has 256 positions. At normal idle it's somwhere around
position 20. When full throttle is applied the IAC valve completely opens.
The EST retards timing when the knock sensor (a sort of microphone) notices
detonation. Normal spark timing is controlled by the EST, commanded by the ECM.
The EGR or Exhaust Gas Recirculation
EGR is a method to decrease emissions by venting exhaust gas into the intake
(about 7% of the intake volume is rendered "useless" due to this). It also decreases detonation due to the
uncombustable (inert) exhaust gas. The EGR position is ECM controlled via the EGR position control solenoid.
This is a solenoid that regulates vacuum to the EGR valve thus opening/closing it.
The AIR system or Air Injection Reactor
The A.I.R system consists of an AIR pump or just SMOG pump, that pumps fresh air into the headers/catalytic converter.
When the engine is in closed loop air flows into the headers (and cylinder head exhaust ports) to speed up O2
sensor warmup. When the engine is running in closed loop air is pumped into the catalytic converter to speed up
the chemical reactions taking place there. Also during deceleration the air is vented to the atmosphere to reduce
detonation in the exhaust. The route the air travels is regulated by a control solenoid (that is ECM controlled)
The TCC or Torque Converter Clutch (lockup converter)
The TCC is engaged when the car is travelling at about 30mph at
relatively light load and in a high gear and when a relatively steady signal is observed from the TPS (steady
throttle) This is done to maximize fuel efficiency.
AC compressor clutch
When the Air Conditioning compressor clutch is on the ECM increases the RPM at idle to account for the increased
load.
The Auxiliary cooling fan
The Cooling Fan is engaged when the ECM detects a high signal from the coolant temperature switch. On the Cross-
Fire system the auxiliary fan is NOT ECM controlled
CTS or Coolant Temperature sender
The Coolant Temperature sensor on the manifold sends a signal to the ECM. The one on the drivers side cylinder
head goes to the temperature gauge, the passenger side one controls the auxiliary fan.
The MAP sensor or Manifold Absolute Pressure sensor
The MAP monitors the manifold pressure and is an indicator for engine
oad. Also the ECM calculates the quantity of incoming air from the signal of this sensor.