Electronic Control Unit

MPI Electronic Control Unit (Early Production w/2 Connectors):

MPI Electronic Control Unit (Late Production w/4 Connectors):

The MPI engine management system is new for the 1990 model year and, according to information at the time of this release, is available only on the 20 valve 2.3 liter DOHC engine (also new for 1990 in the U.S.A.). The system is produced in two basic versions during its introductory year. In each version, a single ECU performs both fuel management and ignition control. The early and late versions can be identified by the connectors on the ECU. The early version (produced before March 1990) has two ECU harness connectors. The late version (produced after March 1990) has four ECU harness connectors. Aside from physical differences in the wiring harness and some software changes, the sensor inputs and control output functions of the ECU are the same for both versions. The ECU monitors various engine sensors and controls the air/fuel ratio, ignition timing, and ignition dwell, through output devices and signals.

SEQUENTIAL FUEL INJECTION
The MPI system controls fuel injection sequentially, that is, the injectors (one for each cylinder) are controlled individually so that each injector sprays a mist of fuel directly into the cylinder head intake port at the same time as the intake valve for that cylinder opens. This method of injection eliminates fuel condensation in the intake manifold and intake port during cold starts. This reduces the necessary amount of cold start enrichment of the air/fuel ratio, and improves fuel atomization at all engine temperatures and speeds for better performance and mileage.

MPI Start-Up Reference Signals:

When the ignition is turned on, all the injectors receive power at one of their terminals through an array of series resistors (one for each injector). Each injector has its own ground circuit through the ECU. Each injector is energized individually when the ECU completes that circuit to ground. The ECU "knows" when to fire each injector by comparing the signals from the hall sensor and the crankshaft position sensor (reference sensor). When the hall sensor signal occurs at the same time as the reference sensor signal, cylinder #1 is at TDC of its power or firing stroke (reference point). The intake valve for cylinder #5 will open approximately 72~ of crankshaft rotation later. When the reference point is sensed, the ECU counts 27 impulses from the engine speed sensor and begins injection with cylinder #5. Once the reference point is established during cranking, and the #5 cylinder injector is fired, the ECU counts 54 flywheel teeth (144~ of crankshaft rotation) and then fires the next injector, and so on. Injectors are energized in the same order as the ignition firing order, so that injection for each cylinder occurs one crankshaft revolution after TDC of the power stroke for that cylinder, corresponding to the opening of the intake valve.

BASIC FUEL CONTROL
The basic fuel quantity (injector pulse width) is controlled by the ECU depending on the calculated engine speed and load. The engine speed is determined by signals from the engine speed sensor on the rear of the engine. This information is then compared with signals from the air mass sensor and throttle valve potentiometer to determine the load on the engine. Once the engine speed and load are determined, the ECU calculates the specific air/fuel ratio required to meet the demands of the operating conditions, and determines the injector pulse width that will deliver the correct amount of fuel to achieve the desired mix. At low engine speeds and high loads or during wide open throttle operation, the air/fuel ratio is slightly rich to maximize torque. At moderate engine speeds and loads (cruise), the air/fuel ratio is maintained as close as possible to the ideal stoichiometric ratio, for low emissions and fuel economy. During deceleration, fuel delivery is reduced to nearly zero for low emissions, increased catalyst life, and increased fuel economy.

Air/Fuel Correction Factors (Cold Engine):

WARM-UP ENRICHMENT
During cold engine operation (coolant temperature less than approx. 150~F/80~C), the ECU will enrich the air/fuel ratio according to a temperature correction factor programmed into the computer memory. The correction factor is different for different engine temperatures (the colder-the richer) and is also dependent on the engine load and speed. At low engine speeds and light loads when the velocity of air through the intake manifold is slow, fuel atomization is poor and a richer mixture is required. As the intake air velocity increases, atomization is improved and a leaner mixture will do. The diagram illustrates one of several pre-programmed enrichment "maps" showing the additional fuel required for different engine speeds and loads at a given engine temperature. Other "maps" determine the "enrichment factor" for other temperature increments. The ECU will switch from one "map" to another, as the engine temperature increases to the normal operating range.

Oxygen Sensor Output Voltage vs. Air/Fuel Ratio:

ELECTRONIC FEEDBACK FUEL CONTROL
In addition to the basic fuel control, the ECU performs calculated adjustments to the air/fuel ratio depending on the "feedback" from the exhaust gas oxygen sensor. When the engine reaches normal operating temperature, the ECU switches to "closed loop" operation, where it begins to regulate the air/fuel ratio depending on the signal from the oxygen sensor. When the air/fuel ratio is near the ideal stoichiometric mixture (14.7:1 by mass), the voltage signal from the oxygen sensor will be between 300mV and 600mV (0.3 and 0.6 volt). A signal of 300mV represents a slightly lean mixture while a signal of 600mV represents a slightly rich mixture. When the ECU receives a low voltage signal from the oxygen sensor, it reacts with a rich "command" to the fuel injectors and more fuel is injected until the oxygen sensor signal shows a rich mixture, at which time the ECU reacts with a lean "command". This process repeats over and over, several times per second, modulating the air/fuel ratio between slightly rich and slightly lean. Overall, the air/fuel ratio is kept near the ideal stoichiometric ratio.

Ignition Maps And Knock Sensor Control:

IGNITION CONTROL
The ECU also controls the ignition timing and dwell, according to complex "maps" programmed into the computer memory.
The ECU monitors engine speed, load, and temperature, then plots these points on a complex 3 dimensional graph (ignition map) to determine the degree of ignition advance. A knock sensor is used to detect spark knock. If a knock is detected the ECU retards the ignition timing for that cylinder, in steps, until the knocking ceases, then gradually steps the timing back to its previous setting. If knocking persists when the timing has been retarded the maximum number of steps, the ECU will switch to a second ignition map programmed for fuels with lower octane ratings. If the knocking still continues, a trouble code will be stored in the computer memory.

Primary Current Vs. Charge Time W/Respect To Voltage:

The ignition dwell is controlled according to a similar map. With a constant dwell angle, the charging time changes depending upon the engine speed. At high engine speeds, the charging time is significantly reduced, and consequently, spark voltage is reduced due to insufficient coil saturation. At low engine speeds coil saturation is reached well before spark occurs, resulting in wasted energy and unnecessary coil heating. Since coil saturation is directly proportional to the amount of current flowing through the primary windings, by controlling the dwell angle and voltage, charge time (length of time required to reach nominal current flow and coil saturation) can be controlled. A low voltage results in a slow charge rate, and a relatively long period of time required to reach nominal current flow. A higher voltage has a faster charge rate and correspondingly shorter time to reach nominal current. The ECU monitors the engine rpm, and charging system voltage, determines the required charge time for optimum spark at that engine speed (according to the "map" programmed into the computers memory), and adjusts the dwell angle and the voltage across the ignition coil, to maintain adequate charging time for optimum sparking voltage at all engine speeds and loads.


ECU LEARNING ABILITY
The ECU monitors its own output signals during normal warm engine operation and stores these values in its memory to use as a reference for basic control during open loop and closed loop operation. The ECU continually re-learns these references to compensate for operating conditions that may change over time, such as engine wear or even intake air leaks that may develop as the vehicle ages. This learning ability helps maintain good cold start up and open loop driveability even if the vehicle is driven from warm low elevation conditions to cold high mountain areas. The information that the ECU "learns" is stored in a volatile memory. If the battery is disconnected or the ECU is disconnected from its wiring harness, the memory is cleared, and the system defaults to the original (pre-programmed at the factory) base control reference settings. This may affect the "feel" of the engine after being serviced while the ECU re-learns, even if that service was not engine related, such as installation of an alarm system or a new car radio, or a simple battery replacement. Normal operation should resume after approx. 10 minutes of driving under moderate load conditions and highway speeds (above 35 m.p.h.).

SELF DIAGNOSTICS/FAULT MEMORY
The ECU monitors sensor inputs and its own output signals, comparing these values with those stored in its memory. If a signal deviates greatly from what the ECU "knows" is the correct value, a fault code (trouble code) is stored in the fault memory, and the ECU may substitute a good signal value so the vehicle can still be operated. Because of this substitution of signals, the operator may not be aware that there is a problem. The ECU also makes a plausibility check of the signals it receives, by comparing signals from one sensor to that of other sensors. For example, the idle switch cannot be checked for open or short circuits because its signal is either on or off. So to determine if the signal is correct, the ECU compares the idle switch signal to that of the air flow sensor and throttle valve potentiometer. If the idle switch signal indicates the idle condition, but the throttle valve potentiometer and air flow sensor signals do not confirm this state, then a fault code is stored for the idle switch. Fault codes are stored in the computer memory until:

1. Fault memory is manually erased, either by following the procedure using the diagnostic connectors, or by disconnecting the battery or ECU.

2. A fault code was stored and the fault was not detected again within 50 engine starts of the last occurence.

For procedures on displaying fault codes and clearing the fault memory, Testing and Inspection.