GM Caprice 1990-1993 Repair Guide

Computer Command Control (CCC) System


See Figures 1, 2 and 3

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Fig. Fig. 1: Computer Command Control schematic

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Fig. Fig. 2: PROM and ECM assembly

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Fig. Fig. 3: ALDL connector - connect terminals A & B to retrieve any stored ECM codes

The Computer Command Control System (CCC) is an electronically controlled exhaust emission system that can monitor and control a large number of interrelated emission control systems. It can monitor up to 15 various engine/vehicle operating conditions and then use this information to control as many as 9 engine related systems. The system is thereby making constant adjustments to maintain good 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 NOx.

Electronic Control Module

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 dashboard warning lamp - CHECK ENGINE or SERVICE ENGINE SOON - 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 to 999. Additionally, the same code may carry different meanings relative to each engine or engine family. For example, on the 3.3L (VIN N), 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 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 seeing 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 16 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 reteach 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.


The primary function of the dash warning lamp is to advise the operator and the technician that a fault has been detected, and, in most cases, a code stored. Under normal conditions, the dash warning 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 warning lamp if no fault is found.

Additionally, the dash warning 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.


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.

Scan Tools

Although stored codes may be read with only the use of a small jumper wire, the use of a hand-held scan tool such as GM's TECH 1 or equivalent is recommended. There are many manufacturers of these tools; a purchaser must be certain that the tool is proper for the intended use.

The scan tool allows any stored codes to be read from the ECM memory. The tool also allows the operator to view the data being sent to the ECM while the engine is running. This ability has obvious diagnostic advantages; the use of the scan tool is frequently required by the diagnostic charts. Use of the scan tool provides additional data but does not eliminate the need for use of the charts. The scan tool makes collecting information easier; the data must be correctly interpreted by an operator familiar with the system.

An example of the usefulness of the scan tool may be seen in the case of a temperature sensor which has changed its electrical characteristics. The ECM is reacting to an apparently warmer engine (causing a driveability problem), but the sensor's voltage has not changed enough to set a fault code. Connecting the scan tool, the voltage signal being sent to the ECM may be viewed; comparison to either a chart of normal values or a known good vehicle reveals the problem quickly.

The ECM is capable of communicating with a scan tool in 3 modes:


This mode is not applicable to all engines. When engaged, certain engine data can be observed on the scanner without affecting engine operating characteristics. The number of items readable in this mode varies with engine family. Most scan tools are designed to change automatically to the ALDL mode if this mode is not available.


Also referred to as the 10K or SPECIAL mode, the scanner will present all readable data as available. Certain operating characteristics of the engine are changed or controlled when this mode is engaged. The closed loop timers are bypassed, the spark (EST) is advanced and the PARK/NEUTRAL restriction is bypassed. If applicable, the IAC controls the engine speed to 1000 rpm plus or minus 50, and, on some engines, the canister purge solenoid is energized.


Sometimes referred to as BACK-UP mode, this level of communication is primarily used during vehicle assembly and testing. This mode will confirm that the default or limp-in system is working properly within the ECM. Other data obtainable in this mode has little use in diagnosis.

A scan tool that is known to display faulty data should not be used for diagnosis. Although the fault may be believed to be in only one area, it can possibly affect many other areas during diagnosis, leading to errors and incorrect repair.

To properly read system values with a scan tool, the following conditions must be met. All normal values given in the charts will be based on these conditions:

Engine running at idle, throttle closed
Engine warm, upper radiator hose hot
Vehicle in park or neutral
System operating in closed loop
All accessories OFF

Electrical Tools

The most commonly required electrical diagnostic tool is the Digital Multimeter, allowing voltage, ohmage (resistance) and amperage to be read by one instrument. The multimeter must be a high-impedance unit, with 10 megohms of impedance in the voltmeter. This type of meter will not place an additional load on the circuit it is testing; this is extremely important in low voltage circuits. The multimeter must be of high quality in all respects. Replace batteries frequently in the unit.

Other necessary tools include an unpowered test light, a quality tachometer with inductive (clip-on) pick up and the proper tools for releasing GM's Metri-Pack, Weather Pack and Micro-Pack electrical terminals as necessary. The Micro-Pack connectors are used at the ECM connector. A vacuum pump/gauge may also be required for checking sensors, solenoids and valves.