Electronic Control Unit Functions
GENERAL DESCRIPTIONThe V8-MOTRONIC engine management system is a fully electronic sequential fuel injection and ignition control system which incorporates both fuel and ignition control functions into a single Engine Control Module (ECM). The ECM monitors various engine sensors and controls the air/fuel ratio, ignition timing, and ignition dwell.
SEQUENTIAL FUEL INJECTION
The V8 Motronic system controls fuel injection sequentially, that is, the injectors (one for each cylinder) are controlled in pairs, 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.
When the ignition is turned on, all the injectors receive power from the fuel pump relay and fuse #23. Each injector pair has a ground circuit through the ECM, and the injectors are energized when the ECM completes that circuit to ground. The ECM "knows" when to fire each injector pair 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 that cylinder will open approximately 360~ of crankshaft rotation later. When the reference point is sensed, the ECM counts 135 impulses from the engine speed sensor (the number of teeth on the flywheel) and begins injection with cylinders 1 & 5 (the first pair). Once the reference point is established during cranking, and the first pair of injectors are fired, the ECM counts 34 flywheel teeth (90~ of crankshaft rotation) and then fires the next pair, and so on. Injectors are energized in the same order as the ignition firing order, so that injection for each cylinder corresponds to the opening of the intake valves.
BASIC FUEL CONTROL
The basic fuel quantity (injector pulse width) is controlled by the ECM 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 ECM 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.
WARM-UP ENRICHMENT
Air/Fuel Correction Factors (Cold Engine):
During cold engine operation (coolant temperature less than approx. 150~F/80~C), the ECM 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 ECM will switch from one "map" to another, as the engine temperature increases to the normal operating range.
ELECTRONIC FEEDBACK FUEL CONTROL
Oxygen Sensor Output Voltage vs. Air/Fuel Ratio:
In addition to the basic fuel control, the ECM 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 ECM 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 ECM 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 ECM 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 CONTROL
Ignition Maps And Knock Sensor Control:
The ECM also controls the ignition timing and dwell, according to complex "maps" programmed into the computer memory.
The ECM 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 ECM 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 ECM will switch to a second ignition map programmed for fuels with lower octane ratings. If the knocking still continues, and the ECM has retarded the timing the maximum number of degrees for this ignition "map" 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 ECM 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 ECM 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 ECM 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 ECM "learns" is stored in a volatile memory. If the battery is disconnected or the ECM 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 ECM 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 ECM 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 ECM "knows" is the correct value, a fault code (trouble code) is stored in the fault memory, and the ECM 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 ECM 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 ECM 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 ECM.
2. A fault code was stored and the fault was not detected again within 10 engine starts of the last occurence.
For procedures on displaying fault codes and clearing the fault memory, Refer to Computers and Control Systems, Testing and Inspection, Procedures Testing and Inspection.
ENGINE SPEED SIGNAL
The Engine Speed Signal is connected to pin 40 of the Motronic control module, the signal is directional OUT only (PIN 40 - OUTPUT ONLY).
The engine speed signal is produced from information supplied to the control module by the engine speed sensor. This signal is used by the tachometer for display of engine speed in rpm's.
NOTE: This signal is simultaneously used as engine speed information by the automatic transmission control module (if equipped).
DECELERATION FUEL CUT-OFF
Deceleration fuel cut-off is a function of the ECM. When the throttle is closed, the reduced air flow through the air mass sensor causes the ECM the reduce the duty cycle to the fuel injectors, which reduces the amount of fuel injected. If the idle switch is closed and the engine speed is greater than a specified rpm, the ECM cuts the duty cycle to the injectors until either the idle switch is opened again, or the engine speed drops below the specified rpm, then fuel injection is resumed.
ENGINE OVERSPEED FUEL CUT-OFF
Engine Overspeed Fuel Cut-Off:
Engine overspeed protection is an ECM controlled function of the fuel injectors, and is determined by the engine speed sensor signal. When the engine reaches a critical rpm (just before the engines "red-line" rpm), the ECM cuts off the fuel by cutting the duty cycle to the fuel injectors. As soon as the engine speed drops below the critical rpm, the duty cycle is restored and fuel delivery returns to normal. The fuel is alternately cut and restored many times each second, at a critical rpm that is just below the engines "redline" limit. This prevents engine overspeed while maintaining torque and horsepower at the critical rpm.
Exciter Wire Signal
The Exciter Wire Signal is connected to pin 13 of the Motronic control module, the signal is directional IN only (PIN 13 - INPUT ONLY).
The exciter wire signal is used for activation of rapid data transfer (Diagnostic Trouble code extraction) via the white data link connector (L-wire).
Signal Failure
^ When there is a break in the signal wiring (L-wire), the following text appears on the V.A.G. 1551: "control unit does not answer".
Holding Relay
The ECM has a holding relay integrated into it. The holding relay provides power during start-up, Hot Wire "Burn Off", and Output Diagnostic Test Mode.