Computers and Control Systems: Description and Operation
ECM And Connector:
Purpose:
The ECM is the control center of the fuel injection system and constantly monitors the information from various sensors and controls the systems that affect vehicle performance. The ECM also performs the diagnostic function of the system. It can recognize operational problems, alert the driver through the Malfunction Indicator Lamp (MIL) and store a Diagnostic Trouble Code (DTC) which identifies the problem areas to aid the technician in making repairs.
Operation:
The ECM contains a power supply which regulates the 12 volt vehicle supply input to 5 and 12 volts, and these voltages are used for various internal and external functions.
The ECM supplies a buffered 5 or 12 volts to power various sensors or switches. This is done through resistances in the ECM which are so high in value that a test light will not light when connected to the circuit. In some cases, even an analog shop voltmeter will not give an accurate reading because its resistance is too low. Therefore, the use of a 10 megohm input impedance digital voltmeter or a Tech 1 scan tool is necessary to assure accurate voltage readings.
The input/output (I/O) devices in the ECM include analog to digital converters, signal buffers, counters and special drivers. The ECM controls output circuits such as the fuel injectors, IAC, cooling fan relay, etc. by controlling the ground circuit through transistors or a device called a "quad-driver" in the ECM.
Memory:
EEPROM
Electronically Erasable Programmable Read Only Memory (EEPROM) is a permanent memory that is physically soldered to the circuit boards within the ECM. The EEPROM can be reprogrammed by using the Tech 1 scan tool or other Techline terminal equipment.
RAM
Random Access Memory (RAM) is the microprocessor "scratch pad". The processor can write into or read from this memory as needed. This memory is volatile and needs a constant supply of voltage to be retained. If the voltage is lost, the memory is lost and the ECM logs a DTC indicating this loss.
Fuel Trim:
Short Term
Short term fuel trim is a ECM volatile memory register that will contain a number between 0 and 255. The neutral value for the short term fuel trim is 128; any deviation from this value indicates the short term fuel trim is changing the injector pulse width. The amount of pulse width change depends on how far the short term fuel trim value is from 128. The short term fuel trim changes the pulse width by varying the "Closed Loop" factor of the base pulse width equation.
As the ECM monitors the oxygen sensor inputs, it is constantly varying the short term fuel trim value. The value is updated very quickly, therefore, the short term fuel trim only corrects for short term fuel trim trends. The correction of long term fuel trim trends is the function is the long term fuel trim.
Long Term:
The long term fuel trim is a matrix of cells arranged by RPM and MAP. Each cell of the long term fuel trim is a register like the short term fuel trim. As the engine operating conditions change, the ECM will switch from cell to cell to determine what "long term fuel trim" factor to use in the base pulse width equation.
While in any given cell, the ECM also monitors the short term fuel trim. If the short term fuel trim is far enough from 128, the ECM will change the long term fuel trim value. Once the long term fuel trim value is changed it should force the short term fuel trim back toward 128.
If the mixture is still not correct (as judged by the oxygen sensor), the short term fuel trim will continue to have a large deviation from the ideal 128. In this case, the long term fuel trim value will continue to change until the short term fuel trim becomes balanced.
Terms for Power Enrichment - Both the short and long term fuel trims are limited by calibrated values. If the mixture is off enough so that long term fuel trim reaches the limit of its control and still cannot correct the condition, the short term fuel trim would also go to its limit of control in the same direction. If the mixture is still not corrected by both short term fuel trim and long term fuel trim at their extreme values, a DTC 44, 45, 64 or 65 will likely result. Under the conditions of power enrichment, the ECM sets the sort term fuel trim to 128 and freezes it there until power enrichment is no longer in effect. This is done so the "closed loop" factor and the long term fuel trim will not try to correct for the commanded richness of power enrichment.
Speed Density:
Speed
The engine speed signal from the 3X crankshaft position sensor is conditioned by the electronic ignition control module. This signal is input to the ECM on the crank reference high CKT 647. The ECM uses this information to determine the "speed" or RPM factor for fuel and ignition management.
Density
Three sensors contribute to the density factor, the Intake Air Temperature (IAT), the Manifold Absolute Pressure (MAP) and the Engine Coolant Temperature (ECT) sensors.
IAT and ECT sensors - work together to assure proper temperature information reaches the ECM.
MAP sensor - As intake manifold pressure increases, the air density in the intake manifold also increases and additional fuel is required. The MAP sensor sends this pressure information to the ECM and the ECM increases the amount of fuel injected by increasing the injector pulse width. Conversely, as manifold pressure decreases, the amount of fuel is decreased.
These four inputs MAP, IAT, ECT and RPM are the major determinants of the air fuel mixture delivered by the fuel injection system.
The remaining sensors and switches provide electrical inputs to the ECM which are used for modification of the air fuel mixture, as well as for other ECM control functions.