Electronic Control Unit

CIS-Turbo ECU:

The CIS-TURBO engine control system, like the CIS-MOTRONIC system, uses a single electronic control unit (ECU), located behind the A/C evaporator, to control the fuel delivery and ignition systems. The ECU monitors information from various engine sensors and uses this information, compared with information stored in its memory, to precisely control the fuel and ignition systems for lower exhaust emissions, better fuel economy and better engine performance, under all operating conditions.




BASIC FUEL CONTROL
A mechanical/hydraulic fuel distributor provides the basic fuel delivery functions. Fuel metering is accomplished by varying the cross-sectional area through which fuel can flow between two chambers in the fuel distributor which are maintained at a constant pressure differential. The larger the cross sectional area of the opening between chambers, the larger the quantity of fuel that will flow. Air is drawn into the engine through an air flow sensor, where it deflects a sensor plate. The sensor plate is attached to a lever arm which operates a valve in the fuel distributor, controlling the quantity of fuel delivered. When the throttle is opened, more air is drawn into the engine causing greater deflection of the sensor plate. The greater the deflection of the sensor plate and arm, the greater the quantity of fuel allowed to the injectors. The injectors are normally closed, and open only when fuel pressure in the lines exceeds a specified minimum. The mechanical system is calibrated so that the quantity of fuel delivered is close to the ideal amount under most conditions.


ELECTRONIC FEEDBACK FUEL CONTROL
The use of a three way catalytic converter requires that the air/fuel ratio be regulated more accurately than the mechanical injection alone is capable of. To regulate the air/fuel ratio more precicely, a computer monitors different operating conditions such as engine temperature, load, rpm, and exhaust gas oxygen content, and then modifies the basic injection quantity by making small adjustments to the pressure differential (control pressure) in the fuel distributor, increasing or decreasing the amount of fuel delivered. When the control pressure is reduced, fuel quantity is increased. Correspondingly, when control pressure is increased, fuel quantity is reduced. The ECU regulates control pressure by turning a valve (frequency valve) on and off several times each second. The ratio of "on" time vs. "off time" (dwell or duty cycle) determines the control pressure.


IGNITION CONTROL

Ignition Maps - Knock Sensor 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 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 low elevations to 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. 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 from its other memory to provide good driveability. Because of this substitution of signals, the operator may not be aware that there is a problem. If the fault is emission related, the ECU will store the fault code until the memory is erased. For non-emission related faults, the memory is cleared each time the engine is started, so if a fault is suspected, codes should be displayed and recorded after the fault has been observed and before the engine is re-started. If fault codes have been erased but a fault is still present, codes can be restored by operating the vehicle for 5 minutes (at min. of 3,000 rpm, and at least once at wide open throttle), or by cranking the engine for 6 seconds (if the engine will not start). For procedures on displaying fault codes, Testing and Inspection.