Chrysler Caravan/Voyager/Town n Country 1984-1995

Spark Control Computer (SCC) System

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The Hall Effect electronic ignition is used in conjunction with the Chrysler Spark Control Computer (SCC) controlling the entire ignition on 1984-87 models. It consists of a sealed Spark Control Computer, specially calibrated carburetor and various engine sensors, such as the vacuum transducer, coolant switch, Hall Effect pick-up assembly, oxygen sensor and carburetor switch.

SCC SYSTEM COMPONENTS



Spark Control Computer (SCC)

During cranking, an electrical signal is sent from the distributor to the computer. This signal will cause the computer to fire the spark plugs at a fixed amount of advance. Once the engine starts, the timing will then be controlled by the computer based on the information received from the various sensors.

There are essentially 2 modes of operation of the spark control computer: the start mode and the run mode. The start mode is only used during engine cranking. During cranking, only the Hall Effect pick-up signals the computer. These signals are interpreted to provide a fixed number of degrees of spark advance.

After the engine starts and during normal engine operation, the computer functions in the run mode. In this mode, the Hall Effect pick-up serves as only one of the signals to the computer. It is a reference signal of maximum possible spark advance. The computer then determines, from information provided by the other engine sensors, how much of this advance is necessary and delays the coil saturation accordingly, firing the spark plug at the exact moment this advance (crankshaft position) is reached.

There is a third mode of operation which only becomes functional when the computer fails. This is the limp-in mode. This mode functions on signals from the pick-up only and results in very poor engine performance. However, it does allow the vehicle to be driven to a repair shop. If a failure occurs in the pick-up assembly or the start mode of the computer, the engine will neither start nor run.

Hall Effect Pick-Up

The Hall Effect pick-up is located in the distributor assembly and supplies the engine rpm and ignition timing data to the SCC to advance or retard the ignition spark as required by current operating conditions.

Coolant Sensor

See Figures 1, 2 and 3

The coolant temperature sensor is located on the thermostat housing and provides the SCC with engine temperature data. The SCC uses this data to control various engine functions such as spark advance, fuel mixture, emission controls operation and radiator fan.



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Fig. Fig. 1: Coolant temperature sensor location-2.2L engines



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Fig. Fig. 2: Coolant temperature sensor location-2.5L engines



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Fig. Fig. 3: Coolant temperature sensor location-3.0L engines

Vacuum Sensor

See Figure 4



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Fig. Fig. 4: Spark control vacuum transducer location-2.2L engines

The vacuum transducer is located on the Spark Control Computer (SCC) and informs the SCC as to the manifold vacuum during operation. The engine vacuum is one of the factors that will determine how the computer will advance/retard the ignition timing, and with the feedback carburetor, how the air/fuel ration will be changed.

Carburetor Switch

See Figure 5

The carburetor switch is located on the left side of the carburetor; it provides the SCC with throttle open or throttle closed signal.



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Fig. Fig. 5: Carburetor switch location-2.2L engines

Oxygen Sensor

See Figure 6

The oxygen sensor (used with feedback carburetors) is located in the exhaust manifold and signals the computer how much oxygen is present in the exhaust gases. Since this amount is proportional to rich and lean mixtures, the computer will adjust the air/fuel ration to a level which will maintain operating efficiency of the catalyst system and engine.



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Fig. Fig. 6: Oxygen sensor location-2.2L engines

SYSTEM DIAGNOSIS & TESTING



See Figures 7, 8 and 9



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Fig. Fig. 7: Jumping cavities 2 and 3 of the distributor harness



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Fig. Fig. 8: Testing cavities 2 and 9, then cavities 3 and 5 for continuity



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Fig. Fig. 9: Checking voltage between cavities 2 and 10

Apply the parking brake and block the wheels before performing any engine running tests, including idle or timing checks and adjustments.

Testing for Spark at Coil

Remove the coil secondary cable from the distributor cap. Using a suitable tool, hold the end of the cable about 1 / 4 in. (5mm) from a good engine ground. Crank the engine and look for a good, constant spark at the coil secondary wire. If spark is constant, have a helper continue to crank the engine while moving the coil secondary cable away from ground. Look for arcing at the coil tower. If arcing occurs, replace the coil. If no arcing occurs, the ignition system is producing the necessary high secondary voltage. Make certain this voltage is getting to the spark plugs by checking the distributor rotor, cap, spark plug wires and spark plugs. If all check in good condition, the ignition system is not the cause of the problem.

If spark is weak, not constant or not present, continue with Failure To Start test.

Ignition System Starting Test
  1. With a voltmeter, measure voltage at the battery and record it. Battery specific gravity must be 12.20. If if is not, charge the battery to specification.
  2.  
  3. Turn ignition switch ON .
  4.  
  5. Remove the coil wire from the distributor cap. Using a suitable tool, hold wire about a 1 / 4 in. (5mm) away from a good ground.
  6.  
  7. Intermittently jump coil negative to ground while looking for a good spark at coil wire.
  8.  
  9. If there is spark at the coil wire it must be constant and bright blue in color. If the spark is good, slowly move the coil wire away from ground while looking for arcing at the coil tower. If arcing occurs replace coil. If spark is weak or not constant, proceed to "Failure to Start Test".
  10.  

See Figure 10



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Fig. Fig. 10:

Failure to Start Test

See Figure 11



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Fig. Fig. 11: Special jumper wire construction for grounding the coil

Before proceeding with this test, make certain that "Testing for Spark at Coil'' has been performed. Failure to do so may lead to unnecessary diagnostic time and incorrect test results. If a good spark was obtained during the Ignition Starting Test, go to Step 8.

  1. Turn the ignition switch to the OFF position and disconnect the SCC 10-way connector. Turn the ignition switch ON and remove the coil wire from the distributor cap. Using a suitable tool, hold the end of the wire 1 / 4 in. (5mm) away from a good ground.
  2.  
  3. Intermittently short the coil negative wire to ground. If spark is obtained, replace the spark control computer.
  4.  
  5. If no spark is obtained, check for battery voltage at the coil positive terminal with ignition switch ON . It should be within 1 volt of battery voltage. If voltage is correct, go to Step 5.
  6.  
  7. If voltage is incorrect, check continuity of wiring between battery and coil positive terminal. Repair wiring and repeat Step 3.
  8.  
  9. Check for battery voltage at coil negative terminal, it should be within 1 volt of battery voltage. If it is correct go to Step 7.
  10.  
  11. If voltage is incorrect, replace coil.
  12.  
  13. If voltage is correct, but no spark is obtained when shorting negative terminal, replace coil.
  14.  
  15. If spark is obtained, but engine will not start, turn ignition switch to the RUN , position and with positive lead of a voltmeter, measure voltage from cavity 1 of SCC 10-way connector, to the ground of the disconnected lead from computer. Voltage should be within 1 volt of battery voltage. If voltage is correct, proceed to Step 10. If not, continue to Step 9.
  16.  
  17. If voltage in Step 8 is not correct, check wire for an open between the coil and the SCC 10-way connector. Repair wire and repeat Step 8.
  18.  
  19. Place a thin insulator (piece of paper or cardboard) between curb idle adjusting screw and carburetor switch.
  20.  
  21. Connect negative lead of a voltmeter to a good engine ground.
  22.  
  23. Turn ignition switch to RUN position and measure voltage at carburetor switch.
    1. If voltage is approximately 5 volts, proceed to Step 14.
    2.  
    3. If voltage is not at least 5 volts, turn ignition switch to OFF position. Turn ignition switch back to RUN position and measure voltage at terminal 2 of SCC 10-way connector. Voltage should be within 1 volt of battery voltage.
    4.  
    5. If voltage is correct, repeat Step 10.
    6.  
    7. If voltage is incorrect, check wiring between terminal 2 and ignition switch for open, shorts or poor connections.
    8.  

  24.  
  25. Turn ignition switch OFF , check for continuity between terminal 7 of SCC 10-way connector and carburetor switch terminal. There should be continuity between these 2 points.
    1. If there is no continuity, check wire for opens, shorts or poor connections.
    2.  
    3. If there is continuity, check for continuity between terminal 10 and engine ground.
    4.  
    5. If there is continuity between terminal 10 and engine ground, replace Spark Control Computer (SCC).
    6.  
    7. If there is no continuity, repeat Step 13; only proceed to Step 14 if engine still does not start.
    8.  

  26.  
  27. Turn ignition switch to OFF position and, with an ohm-meter, measure resistance between terminals 5 and 9 of SCC 10-way connector for run pick-up coil and between terminals 3 and 9 for start pick-up coil. The resistance should be between 150-900 ohms.
    1. If resistance is correct, proceed to Step 15.
    2.  
    3. If resistance is not correct, disconnect pick-up coil leads from distributor. Measure resistance at lead going into distributor.
    4.  
    5. If resistance is now between 150-900 ohms, this means there is an open, shorted, or poor connection between the distributor connector and terminals 5 and 9 or 3 and 9 of the SCC 10-way connector.
    6.  
    7. Repair wire and repeat Step 14. If resistance is still out of specification, pick-up coil is bad. Replace pick-up coil and repeat Step 14.
    8.  

  28.  
  29. Connect 1 lead of ohmmeter to an engine ground and, with other lead, check for continuity at each terminal lead going into distributor. There should be no continuity. Reconnect distributor lead and proceed to Step 16. If there is continuity, replace pick-up coil.
  30.  
  31. Remove distributor cap and check air gap of pick-up coil. If it is not within specification, adjust it. If it is within specification, proceed to Step 17.
  32.  
  33. Install distributor cap, reconnect all wiring and try to start engine. If engine still fails to start, replace Spark Control Computer.
  34.  

TESTING FOR POOR ENGINE PERFORMANCE



Before performing test, make sure "Testing for Spark at Coil'' has been carried out. Failure to do so may lead to unnecessary diagnostic time and incorrect test results.

Correct basic engine timing is essential for optimum vehicle performance and must be checked before any of the following testing procedures are performed. Refer to the individual vehicle section for ignition timing procedures and/or refer to the vehicle information label, located in the engine compartment.

Spark Control Computer System (SCC)
CARBURETOR SWITCH TEST

See Figure 12



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Fig. Fig. 12: Testing the carburetor switch

Grounding carburetor switch lead wire will give a fixed air/fuel ratio.

  1. With the ignition key OFF , unplug the 10-way dual connector from the computer.
  2.  
  3. With throttle completely closed, check continuity between cavity 7 of 10-way connector and a good ground. If there is no continuity, check wire and carburetor switch.
  4.  
  5. With throttle open, check continuity between cavity 7 of connector and a good ground. There should be no continuity.
  6.  

COOLANT TEMPERATURE SENSOR TEST

See Figure 13



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Fig. Fig. 13: Testing the coolant temperature switch

  1. With the key in the OFF position, unplug the wire connector from the coolant sensor.
  2.  
  3. Connect 1 lead of ohmmeter to terminal 1 (common) of coolant sensor.
  4.  
  5. Connect other lead of ohmmeter to terminal 3 (fan control circuit).
  6.  

Resistance between terminals 1 and 3 should be:



Below 150°F (66°C): 20-200 ohms
 
150-200°F (66-93°C): 100-1500 ohms
 
Above 200°F (93°C): 400-6000 ohms
 

  1. Remove the ohmmeter lead from terminal 3.
  2.  
  3. Connect the ohmmeter lead to terminal 2 (SCC control circuit). Resistance between terminals 1 and 2 should be:

    50-100°F (10-38°C): 3,300-36,000 ohms
     
    140-245°F (60-118°C): 176-3,900 ohms
     

  4.  

ELECTRONIC THROTTLE CONTROL SYSTEM

Incorporated within the spark control computer is the electronic throttle system. A solenoid, which regulates a vacuum dash-pot is energized when the air conditioner or electronic timers are activated. The 2 timers, incorporated within the ignition electronics, operate when the throttle is closed, plus a time delay of 2 seconds or after an engine start condition. To test the system:

  1. Connect a tachometer to engine.
  2.  
  3. Start engine and run it until it reaches normal operating temperature.
  4.  
  5. Depress accelerator and release it. A higher than curb idle speed should be seen for a specified time.
  6.  
  7. On vehicles equipped with A/C, a slight decrease in idle speed will be noted when A/C is turned on; turning off the A/C will produce normal idle speed.
  8.  

The A/C clutch will cycle on and off as the system is in operation. This should not be mistaken as part of the electronic control system.

  1. As the A/C compressor clutch cycles on and off, the solenoid kicker plunger should extend and retract.
  2.  
  3. If the plunger does not move with the A/C clutch cycling or after a start-up, check the kicker system for vacuum leaks.
  4.  
  5. If the speed increases do not occur, disconnect the 6-way connector at carburetor.
  6.  
  7. Check the solenoid with an ohmmeter by measuring resistance between the terminal that contains the black wire and ground. Resistance should be between 20-100 ohms. If not within specifications, replace the solenoid.
  8.  
  9. Start the vehicle and before time delay has timed out, measure the voltage across the vacuum solenoid terminals. The voltage should be within 2 volts of charging system voltage. If not within specifications, replace the computer.
  10.  
  11. Turning A/C on should also produce charging system voltage after time delay has timed out. If not, check wiring back to instrument panel for an open circuit.
  12.  

SPARK ADVANCE OF SPARK CONTROL COMPUTER

Incorporated in the digital microprocessor electronics are some unique spark advance schedules, which will occur during cold and warm engine operation. These commands have been added to reduce engine emissions and improve driveability. Because they will be changing at different engine operating temperatures during the engine warm-up, all spark advance testing should be done with the engine at normal operating temperature.

  1. Adjust the basic timing to specifications.
  2.  
  3. Have engine at normal operating temperature. The coolant temperature sensor must be connected and operating correctly.
  4.  
  5. Remove and plug the vacuum hose at the vacuum transducer.
  6.  
  7. Connect an auxiliary vacuum supply to the vacuum transducer; draw and hold vacuum to 16 in Hg. (54 kPa).
  8.  
  9. Start and raise engine speed to 2000 rpm. Wait 1 minute and check specifications.
  10.  

The use of a metal exhaust tube is recommended for this test. Using rubber hose may result in a fire due to high temperatures and a long test period.

  1. The advance specifications are in addition to basic advance. If correct advance is not obtained, replace spark plug control computer.
  2.  

 
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