See Figure 1
The GM designed Computer Controlled Catalytic Converter System (C-4 System), introduced in 1979 and used on GM cars through 1980, is a revised version of the 1978-79 Electronic Fuel Control System (although parts are not interchangeable between the systems). The C-4 System primarily maintains the ideal air/fuel ratio at which the catalytic converter is most effective. Some versions of the system also control ignition timing of the distributor.
The Computer Command Control System (CCC System), introduced on some 1980 California models and used on all 1981 and later carbureted car lines, is an expansion of the C-4 System. The CCC System monitors up to fifteen engine/vehicle operating conditions which it uses to control up to nine engine and emission control systems. In addition to maintaining the ideal air/fuel ratio for the catalytic converter and adjusting ignition timing, the CCC System also controls the Air Management System so that the Catalytic converter can operate at the highest efficiency possible. The system also controls the lockup on the transmission torque converter clutch (certain automatic transmission models only), adjusts idle speed over a wide range of conditions, purges the evaporative emissions charcoal canister, controls the EGR valve operation and operates the early fuel evaporative (EFE) system. Not all engines use all subsystems.
There are two operation modes for both the C-4 System and the CCC System: closed loop and open loop fuel control. Closed loop fuel control means the oxygen sensor is controlling the carburetor's air/fuel mixture ratio. Under open loop fuel control operating conditions (wide open throttle, engine and/or oxygen sensor cold), the oxygen sensor has not effect on the air/fuel mixture.
On some engines, the oxygen sensor will cool off while the engine is idling, putting the system into open loop operation. To restore closed loop operation, run the engine at part throttle and accelerate from idle to part throttle a few times.
Computer Controlled Catalytic Converter (C-4) System
Major components of the system include an Electronic Control Module (ECM), an oxygen sensor, and electronically controlled variable-mixture carburetor, and a three-way oxidation-reduction catalytic converter.
The oxygen sensor generates a voltage which varies with exhaust gas oxygen content. Lean mixtures (more oxygen) reduce voltage; rich mixtures (less oxygen) increase voltage. Voltage output is sent to the ECM.
An engine temperature sensor installed in the engine coolant outlet monitors coolant temperatures. Vacuum control switches and throttle position sensors also monitor engine conditions and supply signals to the ECM.
The Electronic Control Module (ECM) monitors the voltage input of the oxygen sensor along with information from other input signals. It processes these signals and generates a control signal sent to the carburetor. The control signal cycles between ON (lean command) and OFF (rich command). The amount of ON and OFF time is a function of the input voltage sent to the ECM by the oxygen sensor. The ECM has a calibration unit called a Programmable Read Only Memory (PROM) which contains the specific instructions for a given engine application. In other words, the PROM unit is specifically programmed or "tailor made" for the system in which it is installed. The PROM assembly is a replaceable component which plugs into a socket on the ECM and requires a special tool for removal and installation.
On some 231 cu. in. V6 engines, the ECM controls the Electronic Spark Timing (EST) system, AIR control system and the EGR valve control. ON some 350 V8 engines, the ECM controls the Electronic Module Retard (EMR) system, which retards the engine timing 10 degrees during certain engine operations to reduce the exhaust emissions.
Electronic Spark Timing (EST) allows continuous spark timing adjustments to be made by the ECM. Engines with EST can easily be identified by the absence of vacuum and mechanical spark advance mechanisms on the distributor. Engines with EMR systems may be recognized by the presence of five connectors, instead of the HEI module's usual four.
To maintain good idle and driveability under all conditions, other input signals are used to modify the ECM output signal. Besides the sensors and switches already mentioned, these input signals include the Manifold Absolute Pressure (MAP) or vacuum sensors and the Barometric Pressure (BARO) sensor. The MAP or vacuum sensors sense changes in manifold vacuum, while the BARO sensor senses changes in barometric pressure. One important function of the BARO sensor is the maintenance of good engine performance at various altitudes. These sensors act as throttle position sensors on some engines. See the following paragraph for description.
A Rochester Dualjet carburetor is used with the C-4 system. It may be an E2SE, E2ME, E4MC or E4ME model, depending on engine application. An electronically operated mixture control solenoid is installed in the carburetor float bowl. The solenoid controls the air/fuel mixture metered to the idle and main metering systems. Air metering to the idle system is controlled by an idle air bleed valve. It follows the movement of the mixture solenoid to control the amount of air bled into the idle system enriching or leaning out the mixture as appropriate. Air/fuel mixture enrichment occurs when the fuel valve is open and the air bleed is closed. All cycling of this system, which occurs ten times per second, is controlled by the ECM. A throttle position switch informs the ECM of open or closed throttle operation. A number of different switches are used, varying with application. The V6 engines use two pressure sensors, Manifold Absolute Pressure (MAP) and Barometric Pressure (BARO) as well as a throttle-actuated wide open throttle switch mounted in a bracket on the side of the float bowl. V8 engines use a Throttle Position Sensor (TPS) mounted in the carburetor bowl cover under the accelerator pump arm. When the ECM receives a signal from the throttle switch, indicating a change in position, it immediately searches its memory for the last set of operating conditions that resulted in an ideal air/fuel ratio, and shifts to that set of conditions. The memory is continually updated during normal operation.
Many C-4 equipped engines with Air Injection Reaction systems (AIR) have an AIR system diverter solenoid controlled by the ECM. These systems are similar in function to the Air Management system used in the CCC System. Most C-4 Systems include a maintenance reminder flag connected to the odometer which becomes visible in the instrument cluster at regular intervals, signaling the need for oxygen sensor replacement.Computer Command Control (CCC) System
See Figures 2 and 3
The CCC has many components in common with the C-4 system (although they should probably not be interchanged between systems). These include the Electronic Control Module (ECM), which is capable of monitoring and adjusting more sensors and components than the ECM used on the C-4 System, an oxygen sensor, an electronically controlled variable-mixture carburetor, a three way catalytic converter, throttle position and coolant sensors, a Barometric Pressure (BARO) sensor, a Manifold Absolute Pressure (MAP) sensor, a "check engine" light on the instrument cluster, and an Electronic Spark Timing (EST) distributor.
Components used almost exclusively by the CCC System include the Air Injection Reaction (AIR) management system, charcoal canister purge solenoid, EGR valve controls a Vehicle Speed Sensor (VSS) a Transmission Converter Clutch solenoid (TCC), idle speed control, and Electronic Spark Timing (EST) system.
See the operation descriptions under C-4 System for those components (except the ECM) the CCC System shares with the C-4 System.
The CCC System ECM, in addition to monitoring sensors and sending a control signal to the carburetor, also control the following components or sub-systems; charcoal canister purge, AIR management system, idle speed control, automatic transmission converter lockup, distributor ignition timing, EGR valve control, EGR control, and the air conditioner compressor clutch operation. The CCC ECM is equipped with a PROM assembly similar to one used in the C-4 ECM.
The AIR management system is an emission control which provides additional oxygen either to the catalyst or the cylinder head ports (in some cases exhaust manifold). An AIR Management System, composed of an air switching valve and/or an air control valve, controls the air pump flow and is itself controlled by the ECM. A complete description of the AIR system is given elsewhere in this section. The major difference between the CCC AIR system and the systems used on other cars is that the flow of air from the air pump is controlled electrically by the ECM, rather than by vacuum signal.
The charcoal canister purge control is an electrically operated solenoid valve controlled by the ECM. When energized, the purge control solenoid blocks vacuum from reaching the canister purge valve. When the ECM de-energizes the purge control solenoid, vacuum is allowed to reach the canister and operate the purge valve. This releases the fuel vapors collected in the canister into the induction system.
The EGR valve control solenoid is activated by the ECM in similar fashion to the canister purge solenoid. When the engine is cold, the ECM energizes the solenoid, which blocks the vacuum signal to the EGR valve. When the engine is warm, the ECM de-energizes the solenoid and the vacuum signal is allowed to reach and activate the EGR valve.
The Transmission Converter Clutch (TCC) lock is controlled by the ECM through an electrical solenoid in the automatic transmission. When the vehicle speed sensor in the instrument panel signals the ECM that the vehicle has reached the correct speed, the ECM energizes the solenoid which allows the torque converter to mechanically couple the engine to the transmission. When the brake pedal is pushed or during deceleration, passing, etc., the ECM returns the transmission to fluid drive.
The idle speed control adjusts the idle speed to lead conditions, and will lower the idle speed under no-load or low-load conditions to conserve gasoline.
The Early Fuel Evaporative (EFE) system is used on some engines to provide rapid heat to the engine induction system to promote smooth start-up and operation. There are two types of system: vacuum servo and electrically heated. They use different means to achieve the same end, which is to pre-heat the incoming air/fuel mixture. They are controlled by the ECM.
Not all engines use all components. Component applications may differ.
Coolant Temperature Sensor
The coolant sensor is a thermistor (a resistor which changes values based on temperature) mounted in the coolant stream. The ECM supplies a five volt signal to the coolant sensor through a resistor in the ECM and measures the voltage. The voltage will be high when the engine is cold and low when the engine is hot. By measuring the voltage, the ECM knows the coolant temperature which affects most systems the ECM controls. A failure in this circuit should set a trouble code 14 or 15. The sensor itself can be tested by removing the sensor (as outlined in Engine & Engine Rebuilding ), then subjecting the sensor to changes in temperature and measuring the resistance at the terminals. This may be done with a water bath and a thermometer. Heat the water, watch the thermometer and measure the resistance across the terminals. Compare the readings to the chart. If the resistance values are out of range, replace the sensor.Manifold Air Pressure (MAP) Sensor
The MAP sensor measures changes in the intake manifold pressure which result from engine load and speed changes, and converts this to a voltage output. MAP is the opposite of what you would read with a vacuum gauge. When manifold pressure is high, vacuum is low. The ECM uses the MAP sensor to control fuel delivery and ignition timing. A failure in this circuit should set a trouble code 34. To test the MAP sensor, have the ignition ON with the engine not running. Check voltage from sensor terminal B to A. It should be within the value specified in the chart. Apply 10 in. Hg (68.95 kPa) of vacuum to the sensor with a vacuum pump. There should be a 1.2-2.3 voltage change. If the sensor did not meet either of these requirements, have the system diagnosed and repaired by a qualified technician.VAC Sensor
The differential pressure sensor is similar in appearance to the MAP and BARO sensors. However, it operates just the opposite of the MAP sensor in that it measures the difference between the manifold pressure and atmospheric pressure. The output of the sensor increases as the vacuum increases. A failure in this circuit should set a trouble code 34. To test this sensor, check vacuum at the sensor with a vacuum gauge. It should read at least 10 in. Hg (69 kPa) of vacuum. If it is not, repair before continuing. With the ignition ON and engine not running, check voltage from terminals B to A. It should be 0.50-0.64 volts. Connect a vacuum pump to the vacuum port on the sensor and apply 10 in Hg (69 kPa) of vacuum. Voltage should be 2.25-2.95 volts and respond quickly. If the sensor did not meet either of these requirements, have the system diagnosed and repaired by a qualified technician.Barometric (BARO) Pressure Sensor
The BARO sensor works like the MAP sensor, except that instead of measuring engine manifold pressure, it is open to the outside air, so it can measure barometric pressure. This allows the ECM to adjust for improved driveability at high altitudes. This sensor looks like the MAP sensor but it has a red insert in the harness connector cavity. A failure in the BARO circuit should set a trouble code 32. To test the BARO sensor, have the ignition ON with the engine NOT running. Check voltage from sensor terminal B to A. It should be within the value specified in the chart. Apply 10 in Hg (69 kPa) of vacuum to the sensor with a vacuum pump. There should be a 1.2-2.3 volt change. If the sensor did not meet either of these requirements, have the system diagnosed and repaired by a qualified technician.Throttle Position Sensor
See Figure 4
The Throttle Position Sensor (TPS) is mounted in the carburetor body and is used to supply throttle position information in the ECM. The ECM memory stores an average of operating conditions with the ideal air/fuel ratios for each of those conditions. When the ECM receives a signal that indicates throttle position change, it immediately shifts to the last remembered set of operating conditions that resulted in an ideal air/fuel ratio control. The memory is continually being updated during normal operations. The TPS is used to regulate the mixture control solenoid, idle speed, EST and TCC lockup. To test the TPS on carbureted engines, first clear any stored codes as outlined in Trouble Codes later in this section. With the engine running at specified idle speed, have an assistant put the car in Drive while applying both the service and parking brakes. Stand to the side of the car and fully depress the TPS plunger for 15 seconds. Ground the diagnostic terminal and check for a trouble code 21. If this test did not set the code, have the system diagnosed and repaired by a qualified technician.
Idle Speed Control (ISC)
The Idle Speed Control (ISC) on carbureted engines does just what its name implies; it controls the idle. The ISC is used to maintain low engine speeds while at the same time preventing stalling due to engine load changes. The system consists of a motor assembly mounted on the carburetor which moves the throttle lever so as to open or close the throttle blades.
The whole operation is controlled by the ECM. The ECM monitors engine load to determine the proper idle speed. To prevent stalling, it monitors the air conditioning compressor switch, the transmission, the park/neutral switch and the ISC throttle switch. The ECM processes all this information and then uses it to control the ISC motor which in turn will vary the idle speed as necessary. To test the ISC motor operation, ground the diagnostic connector with the ignition ON , engine NOT running. The ISC should pulse smoothly in and out. If it did not meet these requirements, have the system diagnosed and repaired by a qualified technician.Electronic Spark Timing (EST)
All 1980 models with the 231 V6 engine and all 1981 and later models use EST. The EST distributor, as described in an earlier section, contains no vacuum or centrifugal advance mechanism and uses a seven terminal HEI module. It has four wires going to a four terminal connector in addition to the connectors normally found on HEI distributors. A reference pulse, indicating engine rpm is sent to the ECM. The ECM determines the proper spark advance for the engine operating conditions and then sends an EST pulse back to the distributor.
Under most normal operating conditions, the ECM will control the spark advance. However, under certain operating conditions such as cranking or when setting base timing, the distributor is capable of operating without ECM control. This condition is called BYPASS and is determined by the BYPASS lead which runs from the ECM to the distributor. When the BYPASS lead is at the proper voltage (5 volts), the ECM will control the spark. If the lead is grounded or open circuited, the HEI module itself will control the spark. Disconnecting the 4-terminal EST connector will also cause the engine to operate in the BYPASS mode. To check EST performance, with the transmission in Park run the engine at fast idle and note timing change with a timing light as diagnostic lead is grounded.
If there is no change, stand to the side of the car and have an assistant in the car apply both the parking and service brakes. Perform the same test with the engine at idle in Drive as not all engines have EST operating when in Park. If there was no timing change in at least one of these tests, have the system diagnosed and repaired by a qualified technician.Electronic Spark Control (ESC)
The Electronic Spark Control (ESC) system is a closed loop system that controls engine detonation by adjusting the spark timing. There are two basic components in this system, the module and the detonation sensor.
The module processes the sensor signal and modifies the EST signal to the distributor to adjust the spark timing. The process is continuous so that the presence of detonation is monitored and controlled. The module is not capable of memory storage.
The sensor is a pieziolectric device (meaning that it generates its own voltage), mounted in the engine block that detects the presence, or absence, and intensity of detonation according to the vibration characteristics of the engine. The output is an electrical signal which is sent to the controller. To test the ESC system, run the engine at fast idle and note rpm. Use a steel rod (eg. socket wrench breaker bar) to tap the front area of the intake manifold.
Tap the manifold rapidly with medium to heavy taps. Observe engine speed drop of 200 or more rpm. The engine should return to original rpm within 20 seconds after the tapping stops. If the ESC system did not meet this requirement, have the system diagnosed and repaired by a qualified technician.Transmission Converter Clutch (TCC)
All 1981 and later models with an automatic transmission use a TCC. The ECM controls the converter by means of a solenoid mounted in the transmission. When the vehicle speed reaches a certain level, the ECM energizes the solenoid and allows the torque converter to mechanically couple the transmission to the engine. When the operating conditions indicate that the transmission should operate as a normal fluid coupled transmission, the ECM will de-energize the solenoid. Depressing the brake will also return the transmission to normal automatic operation.
The TCC may lock up early and give a feeling of engine lugging or vibration if you install over-size tires on your vehicle. This is because the clutch engages at a certain vehicle speed, not at a certain engine rpm. You can usually install tires one size larger than the original equipment tires, but if they are two sizes or more larger than original equipment, you may experience this form of engine roughness. Testing of the TCC should be left to a qualified technician.
REMOVAL & INSTALLATION
Coolant Temperature Sensor
- Disconnect the negative battery cable.
- Disengage the sensor electrical connection.
On most vehicles it will be necessary to drain the engine cooling system to a level just below the sensor, or the system will drain itself as the sensor is removed.
- Carefully back out the coolant sensor.
- Installation is the reverse of removal.
- Disconnect the negative battery cable.
- Disengage the electrical harness connection.
- Disconnect the vacuum hose.
- Remove the sensor from its mounting bracket.
- Installation is the reverse of removal.
See Figures 5, 6 and 7
Adjustment of the throttle position sensor is required after its replacement. This requires the use special tools. Review the procedures below before attempting replacement of the sensor.
- Disconnect the negative battery cable.
- Remove the air cleaner and vacuum hose.
- Disengage the idle speed control or idle speed solenoid electrical connections.
Remove the air horn:
- Attaching screws and remove the idle speed control, idle speed solenoid or idle load compensator.
- Upper choke lever from the end of choke shaft by removing the retaining screw. Rotate upper choke lever to remove the choke rod from slot in lever.
- Choke rod from the lower lever inside the float bowl casting. Remove rod by holding lower lever outward with a small screwdriver and twisting rod counterclockwise.
- (E4ME) Remove the retainer from the pump link, and remove the link from the lever. DO NOT remove the pump lever from the air horn.
- (E2ME) With tool J-25322 or a 3 / 32 in. (2.4mm) drift punch, drive roll pin (pump lever pivot pin) inward until end of pin is against air cleaner locating boss on air horn casting. Remove pump lever and lever from pump rod.
- Front vacuum break hose from tube on float bowl.
- Air horn-to-bowl screws; then remove the two countersunk attaching screws located next to the venturi. DO NOT drop the screws down the throttle bores.
- Air horn from float bowl by lifting it straight up.
- Remove the solenoid-metering rod plunger by lifting it straight up.
- Remove the air horn gasket by lifting it from the dowel locating pins on the float bowl. DISCARD GASKET .
Remove the staking holding the TPS in bowl as follows:
- Lay a flat tool or metal piece across the bowl casting to protect the gasket sealing surface.
- Use a small screwdriver to depress TPS sensor lightly and hold against spring tension.
- Carefully pry upward with a small chisel or equivalent to remove bowl staking, make sure prying force is exerted against the metal piece and not against the bowl casting.
- Push up from bottom on electrical connector and remove TPS and connector assembly from bowl.
- Align and install TPS and connector assembly with aligning groove in bowl casting. Push down on connector and sensor assembly so that connector and wires are located below the bowl surface. Be sure the green TPS actuator is in place in the air horn.
- Install air horn, holding down on pump plunger assembly against return spring tension, and aligning holes in gasket over TPS plunger, solenoid plunger return spring, metering rods, solenoid attaching screw and electrical connector. Position gasket over the two dowel locating pins on the float bowl.
- Install solenoid-metering rod plunger, holding down on air horn gasket and pump plunger assembly, and aligning slot in end of plunger with solenoid attaching screw.
- Carefully lower air horn assembly onto float bowl while positioning the TPS adjustment lever over the TPS, and guiding pump plunger stem through seal in air horn casting. To ease installation, insert a thin screwdriver between air horn gasket and float bowl to raise the TPS adjustment lever positioning it over the TPS.
- Install air horn attaching screws. Tighten all screws evenly and securely, following the air horn tightening sequence. Don't forget the countersunk screws in the venturi area.
- Install the front vacuum break and bracket assembly on the air horn, using two attaching screws and tighten securely.
- (E4ME) Re-install pump rod into hole in pump lever and insert retainer pin. (E2ME) Hook upper end of pump rod into hole in pump lever and place lever between raised bosses on air horn casting, making sure lever engages TPS actuator plunger and the pump plunger stem. Align hole in pump lever in holes in air horn casting bosses. Using a small drift or rod to diameter of the pump lever roll pin will aid alignment. Using sidecutting pliers on the end of the roll pin, pry the roll pin only enough to insert a thin blade screwdriver between the end of the pump lever roll pin and the air cleaner locating boss on the air horn casting. Use screwdriver to push pump lever roll pin back through the casting until the end of the pin is flush with the casting bosses in the air horn.
Use care installing the roll pin to prevent damage to the pump lever bearing surface and casting bosses.
- Install the choke rod into the lower choke lever inside the bowl cavity. Install choke rod in slot in upper choke lever, and position lever on end of choke shaft, making sure the flats on the end of the shaft align with the flats in the lever. Install attaching screw and tighten securely. When properly installed, the lever will point to the rear of the carburetor and the number on the lever will face outward.
- Install idle speed control motor.
- Reconnect all electrical connections
- Reconnect negative battery cable.
To adjust the TPS it is necessary to remove the plug covering the TPS adjusting screw first.
- Using a 5 / 16 in. drill, drill a 1 / 8 in. deep hole in the aluminum plug covering the TPS adjustment screw. Use care in drilling to prevent damage to the adjustment screw head.
- Start a No. 8 1 / 2 in. long self-tapping screw in the drilled hole turning screw in only enough to ensure good thread engagement in hole.
- Placing a wide-blade screwdriver between screw head and air horn casting, pry against screw heads and remove plug. DISCARD PLUG.
- Connect a digital voltmeter from TPS center terminal "'B" to bottom terminal "C."
- Adjustment is required if voltage reads different than 0.31 volts on the E2ME or 0.40 volts on the E4ME, by more than plus or minus0.05 volts. If it is within these specifications, no adjustment is needed.
- Using Tool J-28696 or equivalent, remove TPS adjustment screw.
- With ignition ON , engine NOT running, reinstall TPS adjustment screw and with Tool J-28696, BT-7967A quickly adjust screw to obtain 0.48 volts with the A/C OFF and throttle in curb idle position.
- After adjustment, install new plug in air horn, driving plug into place until flush with raised boss on casting.
- Disengage the electrical connector with the ignition OFF.
- Remove the ISC motor and bracket.
- Installation is the reverse of removal.
Refer to Engine Electrical for module removal and installation.Electronic Spark Control System
- To remove the ESC Knock Sensor, disconnect the negative battery cable and the sensor connector.
- Using the proper size socket loosen the sensor.
- Remove the sensor from the engine block
- Apply a sealer such as soft tape to the threads then install sensor.
- Engage the sensor connector and battery cable.
- To remove the ESC Module, disconnect the electrical connector and the attaching screws.
- Remove the ESC Module.
- Installation is the reverse of removal.
The following explains how to activate the Trouble Code signal light in the instrument cluster and gives an explanation of what each code means. This is not a full C-4 or CCC System troubleshooting and isolation procedure.
Before suspecting the C-4 or CCC System or any of its components as faulty, check the ignition system including distributor, timing, spark plugs and wires. Check the engine compression, air cleaner, and emission control components not controlled by the ECM. Also check the intake manifold, vacuum hoses and hose connectors for leaks and carburetor bolts for tightness.
The following symptoms could indicate a possible problem with the C-4 or CCC System.
- Stalls or rough idle - cold
- Stalls or rough idle - hot
- Poor gasoline mileage
- Sluggish or spongy performance
- Hard starting - cold
- Hard starting - hot
- Objectionable exhaust odors
- Cuts out
- Improper idle speed (CCC System)
As a bulb and system check, the "Check Engine" light will come on when the ignition switch is turned to the ON position but the engine is not started.
The "Check Engine" light will also produce the trouble code or codes by a series of flashes which translate as follows. When the diagnostic test lead (C-4) or terminal (CCC) under the dash is grounded, with the ignition in the ON position and the engine not running, the "Check Engine" light will flash once, pause, then flash twice in rapid succession. This is a code 12, which indicates that the diagnostic system is working. After a longer pause, the code 12 will repeat itself two more times. The cycle will then repeat itself until the engine is started or the ignition is turned OFF .
When the engine is started, the "Check Engine" light will remain on for a few seconds, then turn off. If the "Check Engine" light remains on, the self-diagnostic system has detected a problem. If the test lead (C-4) or test terminal (CCC) is then grounded, the trouble code will flash three times. If more than one problem is found, each trouble code will flash thee times. Trouble codes will flash in numerical order (lowest code number to highest). The trouble codes series will repeat as long as the text lead or terminal is grounded.
A trouble code indicates a problem with a given circuit. For example, trouble code 14 indicates a problem in the cooling sensor circuit. This includes the coolant sensor, its electrical harness, and the Electronic Control Module (ECM).
Since the self-diagnostic system cannot diagnose every possible fault in the system, the absence of a trouble code does not mean the system is trouble-free. To determine problems within the system which do not activate even thought the "Check Engine" light is not on, the trouble code must be evaluated. It must be determined if the fault is intermittent or if the engine must be at certain operating conditions (under load, etc.) before the "Check Engine" light will come on. Some trouble codes will not be recorded in the ECM until the engine has been operated at part throttle for about 5-19 minutes.
On the C-4 System, the ECM erases all trouble codes every time the ignition is turned OFF . In the case of intermittent faults, a long term memory is desirable. This can be produced by connecting the orange connector/lead from terminal "S" of the ECM directly to the battery (or to a "HOT" fuse panel terminal). This terminal must be disconnected after diagnosis is complete or it will drain the battery.
On the CCC System, a trouble code will be stored until terminal "R" of the ECM has been disconnected from the battery for 10 seconds.
An easy way to erase the computer memory on the CCC System is to disconnect the battery terminals from the battery. If this method is used, don't forget to reset clocks and electronic programmable radios. Another method is to remove the fuse marked ECM in the fuse panel. Not all models have such a fuse.
ACTIVATING THE TROUBLE CODE READOUT
On the C-4 System, activate the trouble code by grounding the trouble code test lead. Use the illustrations to locate the test lead under the instrument panel (usually a white and black wire or a wire with a green connector). Run a jumper wire from the lead to ground.
On the CCC System locate the test terminal under the instrument panel. Ground the test lead. ON many systems, the test lead is situated side by side with a ground terminal. In addition, on some models, the partition between the test terminal and the ground terminal has a cut out section so that a spade terminal can be used to connect the two terminals.
Ground the test lead or terminal according to the instructions given in "Basic Troubleshooting" above.
See Figures 8 and 9