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Join Date: Jan 2000
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| How does the ECM mode affect engine operation?
The 1985½-1988 TPI system utilizes the following sensors and devices to control the engine: Mass Air Flow Sensor, Manifold Air Temperature, Coolant Temperature, Oxygen Sensor, Throttle Position Sensor, Cold Start Switch, Cold Start Injector, Fuel Injectors, Idle Air Control Valve, Exhaust Gas Recirculation valves, Distributor Electric Spark Timing, (Module in distributor TPI) Electric Spark Control, Module and Knock Sensor.
When the ignition power is first applied to etheECM, the ECM performs a Power-On Self Test (P.O.S.T.) routine.
ECM P.O.S.T. Routine
* RAM Error Detection and Corrective Action;
* NV-RAM Diagnostic Checksum;
* High Voltage Disable of ECM Outputs;
* All A/D inputs read;
* All discrete inputs read;
* All pulse accumulator/pulse period/pulse integrator inputs read;
* PWM outputs energized individually;
* Discrete outputs energized individually each 50-100 mSec;
* Check engine light turned off;
* EST mode disabled;
* Synchronous fuel delivery disabled;
* All discrete outputs de-energized;
* All PWM outputs de-energized (0% duty cycle);
* Asynchronous fuel output fixed at 3 msec every 6.25 msec;
* Checksum of nonvolatile RAM calculated;
* One second ECM turn off delay;
* IAC outputs on, but not changing state;
* EPROM Checksum;
* Check engine light turned on.
That all takes about 1.0 seconds.
When the starter is engaged and the coolant temperature is less than 100 deg F. The cold start injector provides a spray of fuel, of 8 seconds duration max, to each cylinder via a air distribution system built into the intake manifold. If the engine temperature is greater than 100 deg F, the cold start injector is disabled by the cold start switch. Upon startup the ECM utilizes information in the calibrator to establish the initial pulse rate for the injectors and the engine starts. At this time the engine is operating in open loop mode and will continue to do so until the engine warms up. After the warm up period the ECM scans the sensors, if all sensors are operating and within their proper range, the engine then goes into closed loop operation. This means that the sensors are dynamically controlling the engine. In the event the information received is higher or lower than the normal range, a code will set in the ECM, and the Check Engine or Service Engine Soon light will come "on". The ECM receives information on air flow, engine temperature, air temperature, exhaust gas oxygen content and throttle position. This information is used to calculate the proper pulse width for the injectors and fires the injectors for the calculated period. This procedure is repeated continuously in very rapid sequence to maintain the optimum fuel air ratio. The electronic spark control components provide maximum advance, if engine knock is detected the spark is automatically retarded. This too, is a continuous process. It should be noted that the following components are MATCHED for optimum performance; Distributor - EST module, ESC module, knock sensor and ECM calibrator. These components are not interchangeable between 5.0L - 5.7L engines.
GM OBD I System Operation
This generation of engine control systems is a refinement of the first engine control computers and systems. First generation engine controls have some control over spark timing, a few output devices like the EFE system and EGR, and a minor role in fuel mixture adjustment. The OBD I system is designed to control and sense a wider range of functions than the earlier systems, including complete control of fuel delivery, and has proven to be much more reliable and effective.
The system has two basic modes of operation when the engine is running - Open Loop Mode and Closed Loop Mode. In a nutshell, Open Loop mode means the ECM is controlling the engine with essentially no feedback from the engine sensors - the control loop is said to be "open". Closed Loop mode means that the ECM sends control signals to the engine, scans the sensors for feedback to determine further corrections, then adjusts accordingly. The process is continuous, and the control loop is said to be "closed" since the sensors effectively communicate back to the ECM regarding the effectiveness of any adjustments made.
Open Loop Mode
When the starter is engaged, if the coolant temperature indicates less than 100°F and the intake air temperature is within 20°F of the coolant temperature, the cold start injector provides a spray of fuel or the cold enrichment subroutine allows longer fuel injector pulses on the service injectors. The cold start or cold enrichment subroutine is of eight seconds duration at a maximum. On a cold-start injector system the extra fuel is distributed to each cylinder via a air distribution system built into the bottom of the intake manifold. If the engine temperature is greater than 100°F, the cold start injector/subroutine is disabled by the cold start switch or ECM. Upon startup the ECM utilizes information in the PROM to establish the initial pulse rate for the injectors and the engine starts. At this time the engine is operating in Open Loop mode and will continue to do so until the engine warms up sufficiently to enable the oxygen sensor.
Closed Loop Mode
After a warm up period the ECM scans the sensors. If all sensors are operating and within their proper ranges, the ECM will enter Closed Loop operation. This means that the sensors are dynamically controlling the engine fuel and spark. The ECM receives information on intake air flow, coolant temperature, intake air temperature, exhaust gas oxygen content, barometric pressure, manifold pressure or vacuum, air conditioner status, power steering pump load, EGR operation, evaporative canister valve operation, engine RPM, vehicle speed, transmission gear selection, fuel pump status, combustion detonation, and throttle position. This information is used to calculate the proper ignition timing advance and pulse width for the injectors, and fires the injectors for the calculated period. This procedure is repeated continuously in very rapid sequence to maintain the optimum fuel air ratio. The electronic spark control components provide maximum advance, if engine knock is detected the spark is automatically retarded. This too, is a continuous process. It should be noted that the following components are matched for optimum performance - distributor EST module, ESC module, knock sensor and ECM PROM. These components are not interchangeable between 5.0L - 5.7L engines. In the event the information received is higher or lower than the normal range, a code will set in the ECM, and the Check Engine or Service Engine Soon lamp will light.
Block Learn / Integrator
The factory ECM/PCM has a learning capability which allows it to make corrections for minor variations in the fuel system to improve performance and driveability. There are two learning features. The Integrator and Block Learn (INT and BLM) and Block Learn Memory (BLM) cell. The INT and BLM feature is normal with a value of around 128 (the possible range is 0-255). If this value is higher than 128, it indicates that the ECM is adding fuel to the base fuel calculations because the system is running lean. A value lower than 128 indicates that the ECM is taking out fuel because the system is running rich. The integrator is a short term corrective action while the BLM is along term correction. The BLM value will change if the integrator has seen a condition which lasts for a longer period of time. There are from two to sixteen different cells which the ECM modifies, depending on RPM, airflow or manifold air pressure and other conditions such as AC on or off, etc. The ECM learns how much adjustment is required in each cell, retains it in memory, and applies these adjustments when the engine operates in that cell or RPM/Load Range. These features of the ECM allows the system to adjust itself constantly to your engine and assure peak performance for stock and other than stock engines.
NOTE: When the vehicle power is disconnected for repair or to clear diagnostic codes, the learning process has to begin all over again. To reinitialize the ECM, drive the vehicle at operating temperature with moderate acceleration and idle conditions for at least twenty minutes.
Performance PROMs typically change the parameters for fuel flow, fuel cut-off and spark advance-timing and will allow increased fuel flow and modify the spark advance curves during rapid acceleration. They can be programmed to tolerate differences in idle quality and manifold vacuum due to altered cam profiles and fuel systems.
Field Service Mode
On the OBDI ECMs, you can jump 'A' and 'B' terminals on the ALDL while the engine is running.
WARNING! This must be done after the engine is running. This is called "Field Service Mode" and will not harm the ECM.
If the ECM is in Open-Loop mode, the SES light will flash rapidly, about 2½ times per second. If it's in Closed-Loop mode, it will flash about once per second. When in Closed Loop mode, flashing less than once per second indicates the ECM is enriching the mixture above the 128 count base line. Flashing more than once per second indicates the ECM is leaning the mixture below the 128 base line.
Backup Fuel Mode
A 3.9K resistor installed between ALDL terminals 'A' and 'B' forces the ECM into Backup Fuel mode. This mode forces the ECM to use predetermined fuel calculations in the ECM PROM instead of the learned inputs in active RAM. This is more commonly referred to as the "Limp Home" mode of operation. Backup fuel is usually around 12:1 or richer from the factory.
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