Principle Of Operation
Battery Voltage is monitored by the ECM for fluctuations. It will adjust the output functions to compensate for a lower (11.7v) and higher (14v) voltage value. For example, the ECM will
- Modify Pulse Width Duration of Fuel Injection
- Modify Dwell Time of Ignition
When KL15 is switched "on" the ECM is ready for engine management. The ECM will activate ground to energize the Engine Control Module Relay. The Engine Control Module Relay supplies operating voltage to the ECM and the previously mentioned operating components.
When KL15 is switched "off" the ECM operating voltage is removed. The ECM will maintain a ground to the Engine Control Module Relay for a few seconds to hold the Evaporative Purge Valve closed (to prevent engine run on).
Ground is required to complete the current path through the ECM. The ECM also
Checking ECM. Scheme 3
- Internally links a constant ground (1) to the engine sensors.
- Switches ground (2) to activate components
Air flow into the engine is regulated by the Throttle Valve or the Idle Speed Control Valve. Both of these air "passages" are necessary for smooth engine operation from idle to full load. On the M1.7.2 system, the Throttle Valve is mechanically controlled and the Idle Speed Control Valve is electrically controlled . All of the ECM monitoring, processing and output functions are a result of regulated air flow.
Air Flow Principle. Scheme 4
The Throttle Position Sensor is monitored by the ECM for throttle angle position and rate of movement. As the throttle plate is opened, a rising voltage signal (up to 5v) requests acceleration and at what rate. The ECM will increase the volume of fuel injected into the engine, advance the ignition timing and decrease the Idle Speed Valve opening (air is now going by the throttle plate). The "full throttle" position indicates maximum acceleration to the ECM, this will have an effect on the A/C compressor (covered in Performance Controls).
As the throttle plate is closed (integral springs), a decrease in voltage signals the ECM to activate fuel shut off if the RPM is above idle speed (coasting). The Idle Speed Control Valve will then be opened to maintain idle speed.
The ECM monitors the engine idle speed in addition the Throttle Position Sensor voltage. The voltage value is "learned" at the correct idle speed and if the voltage value has changed (mechanical wear of throttle plate or linkage), the ECM will adjust the Idle Speed Control Valve to maintain the correct idle speed based on the "new" voltage. To clear this "learned" value, disconnect the ECM for at least one minute. If the Throttle Position input is defective, a fault code will be set and the "CHECK ENGINE" Light will illuminate. The ECM will maintain engine operation based on the Air Flow Volume Sensor and the Engine RPM Sensor.
The Idle Speed Control Valve is controlled by the ECM modulating the ground signal to the valve, opening it against spring pressure. By varying the duty cycle applied to the winding, the valve can be progressively opened, or held steady to maintain the idle speed. If the Idle Speed Control Valve circuit is defective, a fault code will be set and the "CHECK ENGINE" Light will illuminate. The valve will spring to the fixed opening, allowing the engine to idle.
There are additional factors that influence the ECM in regulating idle speed
- The RPM sensor input allows the ECM to monitor engine speed because of loads that cause idle fluctuations due to drag on the engine: power steering, thick oil (fractional forces), etc.
- Cold engine temperature (coolant NTC) provides higher idle speed to raise temperature sooner.
- Vehicle speed informs ECM when the vehicle is stationary and requires idle maintenance
- A/C on request from the climate control system (arming the ECM) and compressor engage (stabilize idle speed) acknowledgment.
- Range selector provides a Park/Neutral input to the ECM identifying when the vehicle is in a drive gear. This signal allows idle stabilization for the increased load on the engine.
Identifying Idle Speed Control Valve Operation. Scheme 5
The Air Flow Volume Sensor sends a varying voltage (0-5v) to the ECM representing the measured amount of intake air volume. This input is used by the ECM to determine the amount of fuel to be injected. If this input is defective, a fault code will be set and the "CHECK ENGINE" Light will illuminate. The ECM will maintain engine operation based on the Throttle Position Sensor and Engine RPM Sensor.
The Air Temperature signal allows the ECM to make a calculation of air density. The varying voltage input from the NTC sensor indicates the larger proportion of oxygen found in cold air, as compared to less oxygen found in warmer air. The ECM will adjust the amount of injected fuel because the quality of combustion depends on oxygen sensing ratio.
The ignition timing is also affected by air temperature. If the intake air is hot the ECM retards the base ignition timing to reduce the risk of detonation. If the intake air is cooler, the base ignition timing will be advanced. If this input is defective, a fault code will be set and the "CHECK ENGINE" Light will illuminate.
DISA is controlled by the ECM activating the Change Over Solenoid below 4840 RPM.
- When activated the solenoid applies vacuum to the change over valve and the valve closes, providing the long pipe effect.
- Above the RPM, the solenoid is switched off and the Change Over Valve springs opens, providing the short pipe effect.
- On decel, the solenoid will not be activated until 4760 RPM. This over lap prevents repeated opening and closing of the valve while driving at a constant engine speed of 4800 RPM.
If there is a defect in this system, the Changeover Valve will be opened to ensure intake air availability for maximum power (short pipe affect). The Vacuum Motor and valve shaft are both spring loaded to open the Changeover Valve if vacuum is not applied.
Identifying Changeover Valve Operation. Scheme 6
Fuel Management delivers fuel from the tank to the intake ports of the engine. To accomplish this, fuel supply must be available to the fuel injectors. Then the fuel must be injected in the precise amount and at the correct time. The ECM does not directly monitor fuel supply, although it does control fuel supply. The Fuel Pump supplies fuel when it receives operating voltage from the Engine Control Module Relay supplying the Fuel Pump Relay. The ECM controls and monitors fuel injection.
Identifying Fuel Management Delivers Fuel From Tank. Scheme 7
The Fuel Pump will be activated when the ignition (KL15) is switched "on" and the ECM supplies a ground circuit to activate the Fuel Pump Relay. The Fuel Pump Relay supplies operating power to the in-tank mounted fuel pump. This is a momentary activation to "pressurize" (prime) the fuel system.
The ECM then requires an engine RPM signal from the Crankshaft Position/RPM Sensor to maintain continuous Fuel Pump Relay activation.
If the engine RPM signal is not present, the ECM will deactivate the Fuel Pump Relay.
Identifying Fuel Pump Relay. Scheme 8
The Fuel Injectors will be opened by the ECM to inject pressurized fuel into the intake ports. The Fuel Injectors receive voltage from the Engine Control Module Relay. The ECM controls the opening by activating the ground circuit for the Solenoid Windings. The ECM will vary the duration (in milli-seconds) of "opening" time to regulate the air/fuel ratio.
The ECM has two Final Stage output transistors that switch ground to the four injector solenoids. The Injector "triggering" is first established from the Crankshaft Position/RPM Sensor.
The ECM is programmed to activate the Final Stage output transistors once for every revolution of the crankshaft (Parallel Injection). The ECM calculates the total milli-second time to open the injectors and cuts that value in half.
Checking ECM. Scheme 9
The injectors are all opened at the same time (in parallel) for every complete crankshaft revolution. This delivers half of the fuel charge at each injection so that the engine receives the full fuel charge during a complete working cycle. This process enhances fuel atomization during start up.
During start up, the ECM recognizes the Camshaft Position (Cylinder ID) input. It then switches the injection to Semi-Sequential. This process "times" the injection closer to the intake valve opening for increased efficiency.
Identifying Parallel Injector Opening Time Display. Scheme 10
When activated, each group (grouped in pairs) delivers the full fuel charge at separate times for each engine working cycle.
The Camshaft Position input is only checked by the ECM during start up. The camshaft position is referenced to the crankshaft position, and is not monitored until the next engine start up. Therefore, if this input is lost when the engine is already running, there will be no effect. There will only be an effect if this input is missing when the engine is started. For this condition, the ECM will continue operating the injectors in Parallel.
Identifying Semi-Sequential Injector Opening Time Display. Scheme 11
The Injector "open" Time to maintain engine operation after it has been started is determined by the ECM (programming). The ECM will calculate the engine "load" based on a combination of the following inputs
- Battery Voltage
- Throttle Position
- Air Flow Volume
- Air Temperature
- Crankshaft Position/RPM
- Crankshaft Position (Cylinder ID)
- Engine Coolant
- Oxygen Sensor (Detail in Emissions)
Adjusting Injector Opening Time. Scheme 12
The injection ms value will be regulated based on battery voltage. When cranking, the voltage is low and the ECM will increase the ms value to compensate for injector "lag time". When the engine is running and the battery voltage is higher, the ECM will decrease the injection ms value due to faster injector reaction time.
Cold starting requires additional fuel to compensate for poor mixture and the loss of fuel as it condenses onto cold intake ports, valves and cylinder walls. The cold start fuel quantity is determined by the ECM based on the Engine Coolant Temperature Sensor input during start up.
During cranking, additional fuel is injected (in Parallel) for the first few crankshaft revolutions. After the first few crankshaft revolutions, the injected quantity is metered down as the engine comes up to speed. When the engine speed approaches idle RPM, the ECM recognizes the Camshaft Position and switches to Semi-Sequential injection.
When the engine is cold, optimum fuel metering is not possible due to poor air/fuel mixing and an enriched mixture is required. The Coolant Temperature input allows the ECM to adjust the injection ms value to compensate during warm up and minimize the injected fuel at engine operating temperature.
When the engine is at idle, minimum injection is required. Additional fuel will be added if the ECM observes low engine RPM and increasing throttle/air volume inputs (acceleration enrichment). As the throttle is opened, the ECM monitors acceleration and rate of movement. The ECM will increase the volume of fuel injected into the engine by increasing the injection ms value. The "full throttle" position indicates maximum acceleration and the ECM will add more fuel (full load enrichment).
As the throttle is closed, the ECM decreases the injection ms value (fuel shut off) if the RPM is above idle speed (coasting). This feature decreases fuel consumption and lowers emissions. When the engine RPM approaches idle speed, the injection ms value is increased (cut-in) to prevent the engine from stalling. The cut-in RPM is dependent upon the engine temperature and the rate of deceleration.
The Air Flow Volume signal provides the measured amount of intake air volume. This input is used by the ECM to determine the amount of fuel to be injected to "balance" the air/fuel ratio.
The Air Temperature Signal allows the ECM to make a calculation of air density. The varying voltage input from the NTC sensor indicates the larger proportion of oxygen found in cold air, as compared to less oxygen found in warmer air. The ECM will adjust the amount of injected fuel because the quality of combustion depends on oxygen sensing ratio (details in Emissions).
The Crankshaft Position/RPM signals the ECM to start injection as well as providing information about the engine operation. This input is used in combination with other inputs to determine engine load which increases/decreases the injection ms value. Without this input, the ECM will not activate the injectors.
The Camshaft Position (Cylinder ID) affects the injection ms value (half= Parallel Injection or full= Semi-Sequential Injection) and the timing when it is injected to the engine. To accomplish this, the ECM contains two Final Stage output transistors that activate the injectors in two groups. The engine operates sufficiently on Parallel Injection, but more efficiently on Semi-Sequential Injection. If one of the circuits faulted, the engine can still operate on limited power from the remaining circuit.
Injection "Reduction" Time is required to control fuel economy, emissions, engine and vehicle speed limitation. The ECM will "trim" back or deactivate the fuel injection as necessary while maintaining optimum engine operation.
Identifying Injection "Reduction" Time. Scheme 13
As the throttle is closed during deceleration, the ECM decreases the injection ms value (fuel shut off) if the RPM is above idle speed (coasting). This feature decreases fuel consumption and lowers emissions.
When the engine RPM approaches idle speed, the injection ms value is increased (cut-in) to prevent the engine from stalling. The cut-in RPM is dependent upon the engine temperature and the rate of deceleration. This function can be observed as displayed on the Fuel Economy (MPG) gage.
The ECM will deactivate the injectors to control maximum engine RPM (regardless of vehicle speed). When the engine speed reaches 6500 RPM, the injectors will be deactivated to protect the engine from over-rev. As the engine speed drops below 6500 RPM, injector activation will be resumed. This feature does not protect the engine from a forced over-rev such as improperly downshifting a manual transmission equipped vehicle (driver error).
Identifying Speedometer. Scheme 14
Maximum vehicle speed is limited by the ECM reducing the injection ms value (regardless of engine RPM). This limitation is based on the vehicle dimensions, specifications and installed tires (speed rating).
The ECM will also protect the Catalytic Converter by deactivating the injectors.
If the ECM detects a fault in the primary ignition system, it can selectively deactivate the Final Stage output transistor for that cylinder.
The injector will not open, preventing unburned fuel from entering the exhaust system.
On the M1.7.2 system, there are two injectors per circuit resulting in deactivation of both.
This will limit engine power, but protect the Catalytic Converter.
Testing Fault In Primary Ignition System. Scheme 15
Ignition Management provides ignition to the combustion chambers with the required voltage at the correct time. Based on the combination of inputs, the ECM calculates and controls the ignition timing and secondary output voltage by regulating the activation and dwell of the primary ignition circuit. The ECM does not directly monitor secondary ignition output, although it does control and monitor the primary ignition circuit.
Ignition Management Principle. Scheme 16
The ECM has a very "broad" range of ignition timing. This is possible by using a Direct Ignition System, or sometimes referred to as "Static Ignition System". Reliability is also increased by having separate individual ignition circuits.
The Ignition Control is determined by the ECM (load defendant). The ECM will calculate the engine "load" based on a combination of the following inputs
- Battery Voltage
- Air Temperature
- Camshaft Position (Cylinder ID)
- Throttle Position
- Engine Coolant
- Knock Sensors
- Air Flow Volume
- Crankshaft Position/RPM
The dwell time will be regulated based on battery voltage. When cranking, the voltage is low and the ECM will increase the dwell to compensate for saturation "lag time". When the engine is running and the battery voltage is higher, the ECM will decrease the dwell due to faster saturation time.
The Crankshaft Position/RPM signals the ECM to start ignition in firing order (1-3-4-2) as well as providing information about the engine operation. This input is used in combination with other inputs to determine engine load which advances/retards the ignition timing. Without this input, the ECM will not activate the ignition.
Cold start is determined by the ECM based on the engine coolant temperature and RPM during start up. A cold engine will crank over slower than a warm engine, the ignition timing will range between top dead center to slightly retarded providing optimum starting.
When starting a warm engine, the RPM is higher which results in slightly advanced timing. If the engine coolant and intake air temperature is hot, the ignition timing will not be advanced reducing starter motor "load".
During cranking, the ECM recognizes the Camshaft Position (compression stroke) and activates a single ignition per cylinder.
If this signal is not recognized, the ECM will activate "Double Ignition". The ignition coils will be activated on both the compression and exhaust strokes to maintain engine operation. The ignition timing will be progressively advanced assisting the engine in coming up to speed.
As the engine speed approaches idle RPM, the timing remains slightly advanced to boost torque. When the engine is at idle speed, minimum timing advance is required. This will allow faster engine and catalyst warm up.
Double Ignition Function. Scheme 17
The timing will be advanced when the ECM observes low engine RPM and increasing throttle/ air volume inputs (acceleration torque). As the throttle is opened, the ECM advances the timing based on engine acceleration and at what rate. The ECM will fully advance timing for the "full throttle" position indicating maximum acceleration (torque).
The Air Flow Volume signal provides the measured amount of intake air volume. This input is used by the ECM to determine the amount of timing advance to properly combust the air/fuel mixture.
The Air Temperature Signal assists the ECM in reducing the risk of detonation (ping). If the intake air is hot the ECM retards the ignition timing. If the intake air is cooler, the ignition timing will be advanced.
As the throttle is closed, the ECM decreases the ignition timing if the RPM is above idle speed (coasting). This feature lowers the engine torque for deceleration. When the engine RPM approaches idle speed, the timing is slightly advanced to prevent the engine from stalling. The amount of advance is dependent upon the engine temperature and the rate of deceleration.
Emissions Management controls evaporative and exhaust emissions. The ECM controls the purging of evaporative fuel. The ECM monitors and controls the exhaust emissions by regulating the combustible mixture . The catalytic converter after-treats by further breaking down remaining combustable exhaust gasses.
Emissions Management Controls Evaporative Principle. Scheme 18
Evaporative Emission Purging is regulated by the ECM controlling the Evaporative Emission Valve. The Evaporative Emission Valve is a solenoid that regulates purge flow from the Active Carbon Canister into the intake manifold. The ECM Relay provides operating voltage and the ECM controls the valve by regulating the ground circuit. The valve is powered closed and opened by an internal spring. The "purging" process takes place when
- Oxygen Sensor Control is active
- Engine Coolant Temperature is <60°C
- Engine Load is present
The Evaporative Emission Valve is opened in stages to moderate the purging.
- Stage 1 opens the valve for 10 ms (milli-seconds) and then closes for 150 ms.
- The Stages continue with increasing opening times (up to 16 stages) until the valve is completely open.
- The Valve now starts to close in 16 stages in reverse order.
- This staged process takes 6 minutes to complete. The function is inactive for 1 minute then starts the process all over again.
- During the purging process the valve is completely opened during full throttle operation and is completely closed during deceleration fuel cutoff.
Evaporative Purge System Flow Check (1996 MY - HC II Emission Compliance) is performed by the ECM when the oxygen sensor control and purging is active. When the Evaporative Emission Valve is open the ECM detects a lean/rich shift as monitored by the oxygen sensors indicating the valve is functioning properly. If the ECM does not detect a lean/rich shift, a second step is performed when the vehicle is stationary and the engine is at idle speed. The ECM opens and close the valve (abruptly) several times and monitors the engine RPM for changes. If there are no changes, a fault code will be set.
Fuel System Monitoring is performed by the ECM which verifies the calculated injection time (ti) in relation to engine speed, load and the oxygen sensor signal as a result of the residual oxygen in the exhaust stream.
The ECM uses the oxygen sensor signal as a correction factor for adjusting and optimizing the mixture pilot control under all engine operating conditions.
Displaying Fuel System Monitoring. Scheme 19
Adaptation Values are stored by the ECM in order to maintain an "ideal" air/fuel ratio. The ECM is capable of adapting to various environmental conditions encountered while the vehicle is in operation (changes in altitude, humidity, ambient temperature, fuel quality, etc.).
The adaptation can only make slight corrections and can not compensate for large changes which may be encountered as a result of incorrect airflow or incorrect fuel supply to the engine.
Within the areas of adjustable adaption, the ECM modifies the injection rate under two areas of engine operation
- During idle and low load mid range engine speeds (Additive Adaptation).
- During operation under a normal to higher load when at higher engine speeds (Multiplicative Adaptation).
These values indicate how the ECM is compensating for a less than ideal initial air/fuel ratio.
Note. If the adaptation value is greater than "0.0 ms" the ECM is trying to richen the mixture. If the adaptation value is less then "0.0 ms" the ECM is trying to lean-out the mixture.
Oxygen Sensor Heating is controlled by the ECM to reduce warm up time and retain heat during low engine RPM when the exhaust temperature is cooler.
Voltage is supplied from the Oxygen Sensor Heater Relay and the ground circuit for the relay is provided by the ECM when engine RPM is present.
During full throttle operation electrical heating is not required and is deactivated by the ECM.
Oxygen Sensor Heater Relay Monitoring is checked separately for electrical integrity and operation. The Heater Relay function is monitored continuously while the vehicle is in closed loop operation, during activation by the ECM.
An improperly/non operating Heater Relay will not allow the sensor signal to reach its predefined maximum and minimum thresholds which can
- Result in delayed closed loop operation causing an impact on emission levels.
- Result in increased emission levels while in closed loop operation.
As part of the monitoring function for Heater Relay current and voltage, the circuit is also checked for an open, short to ground and short to B+ depending on the values of the current or voltage being monitored. If the power of the Heater Relay is not within a specified range, a fault will be set and the "CHECK ENGINE" light will be illuminated.
The "CHECK ENGINE" Light required for OBD is located in the instrument cluster and is activated by the ECM under the following conditions
- Ignition "on" (KL15) and engine not running bulb check function.
- A component malfunction that affects the vehicle emissions.
- An Implausible input signal is generated
- Manufacturer-defined specifications are exceeded.
- ECM fails to enter oxygen sensor closed-loop control within a specified time interval.
Identifying Instrument Cluster. Scheme 20
The ECM illuminates the "CHECK ENGINE" Light by activating a final stage transistor to supply a ground circuit (arrow). The light has voltage supplied whenever KL15 is switched "on".
