See Figures 1, 2, 3 and 4
The Computer Command Control (CCC) System is an electronically controlled exhaust emission system that can monitor and control a large number of interrelated emission control systems. It can monitor various engine/vehicle operating conditions and then use this information to control multiple engine related systems. The CCC system is thereby making constant adjustments to maintain optimum vehicle performance under all normal driving conditions while at the same time allowing the catalytic converter to effectively control the emissions of HC, CO and NO x .
The Electronic Control Module (ECM) is required to maintain the exhaust emissions at acceptable levels. The module is a small, solid state computer which receives signals from many sources and sensors; it uses these data to make judgements about operating conditions and then control output signals to the fuel and emission systems to match the current requirements.
Inputs are received from many sources to form a complete picture of engine operating conditions. Some inputs are simply Yes or No messages, such as that from the Park/Neutral switch; the vehicle is either in gear or in Park/Neutral; there are no other choices. Other data is sent in quantitative input, such as engine rpm or coolant temperature. The ECM is pre-programmed to recognize acceptable ranges or combinations of signals and control the outputs to control emissions while providing good driveability and economy. The ECM also monitors some output circuits, making sure that the components function as commanded. For proper engine operation, it is essential that all input and output components function properly and communicate properly with the ECM.
Since the control module is programmed to recognize the presence and value of electrical inputs, it will also note the lack of a signal or a radical change in values. It will, for example, react to the loss of signal from the vehicle speed sensor or note that engine coolant temperature has risen beyond acceptable (programmed) limits. Once a fault is recognized, a numeric code is assigned and held in memory. The SERVICE ENGINE SOON Malfunction Indicator Lamp (MIL), will illuminate to advise the operator that the system has detected a fault.
More than one code may be stored. Although not every engine uses every code, possible codes range from 12-999. Additionally, the same code may carry different meanings relative to each engine or engine family. For example, on the 3.3L (VIN N) engine, code 46 indicates a fault found in the power steering pressure switch circuit. The same code on the 5.7L (VIN F) engine indicates a fault in the VATS anti-theft system.
In the event of an ECM failure, the system will default to a pre-programmed set of values. These are compromise values which allow the engine to operate, although possibly at reduced efficiency. This is variously known as the default, limp-in/limp-home or back-up mode. Driveability is almost always affected when the ECM enters this mode.
The ECM can compensate for minor variations within the fuel system through the block learn and fuel integrator systems. The fuel integrator monitors the oxygen sensor output voltage, adding or subtracting fuel to drive the mixture rich or lean as needed to reach the ideal air fuel ratio of 14.7:1. The integrator values may be read with a scan tool; the display will range from 0-255 and should center on 128 if the oxygen sensor is indicating a 14.7:1 mixture.
The temporary nature of the integrator's control is expanded by the block learn function. The name is derived from the fact that the entire engine operating range (load vs. rpm) is divided into sections or blocks. Within each memory block is stored the correct fuel delivery value for that combination of load and engine speed. Once the operating range enters a certain block, that stored value controls the fuel delivery unless the integrator steps in to change it. If changes are made by the integrator, the new value is memorized and stored within the block. As the block learn makes the correction, the integrator correction will be reduced until the integrator returns to 128; the block learn then controls the fuel delivery with the new value.
The next time the engine operates within the block's range, the new value will be used. The block learn data can also be read by a scan tool; the range is the same as the integrator and should also center on 128. In this way, the systems can compensate for engine wear, small air or vacuum leaks or reduced combustion.
Any time the battery is disconnected, the block learn values are lost and must be relearned by the ECM. This loss of corrected values may be noticed as a significant change in driveability. To re-teach the system, make certain the engine is fully warmed up. Drive the vehicle at part throttle using moderate acceleration and idle until normal performance is felt.Malfunction Indicator Lamp
The primary function of the MIL is to advise the operator and the technician that a fault is detected, and, in most cases, a code is stored. Under normal conditions, the malfunction indicator lamp will illuminate when the ignition is turned ON . Once the engine is started and running, the ECM will perform a system check and extinguish the lamp if no fault is found.
Additionally, the lamp can be used to retrieve stored codes after the system is placed in the Diagnostic Mode. Codes are transmitted as a series of flashes with short or long pauses. When the system is placed in the Field Service Mode, the dash lamp will indicate open loop or closed loop function to the technician.Intermittents
If a fault occurs intermittently, such as a loose connector pin breaking contact as the vehicle hits a bump, the ECM will note the fault as it occurs and energize the dash warning lamp. If the problem self-corrects, as with the terminal pin again making contact, the dash lamp will extinguish after 10 seconds but a code will remain stored in the ECM memory.
When an unexpected code appears during diagnostics, it may have been set during an intermittent failure that self-corrected; the codes are still useful in diagnosis and should not be discounted.