Chrysler Full-Size Trucks 1967-1988 Repair Guide

Diagnosis and Testing

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1972-83 MODELS



See Figure 1

The magnetic pickup and control unit replace the functions of the breaker points. Unlike the breaker points, the magnetic pickup and control unit normally show no signs of wear. Therefore, periodic checks of dwell are unnecessary; besides, the dwell cannot be altered.

There is, however, an adjusting slot on the distributor plate that is used to change the air gap between the reluctor teeth and the pole piece of the coil. Unlike breaker points, reducing the air gap will not retard the timing. Since dwell is determined by the control unit and is independent of the pickup unit, changing the air gap will not affect timing or dwell. The gap between the pickup and the reluctor should be properly set, however.

One of the main advantages of the electronic ignition system is improved starting. Removal of the breaker points eliminates the possibility of arcing across the points. However, a pickup gap that is too wide can cause starting problems. A no-start condition can exist if the gap is too wide.



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Fig. Fig. 1: The electronic control unit can give you a dangerous shock if touched while the ignition is ON

If you encounter a hard starting condition, don't immediately blame the pickup gap and change the adjustment. The entire system should be left alone, except as a last resort. Make sure that the fuel system and the rest of the ignition system are performing satisfactorily. Although setting the pickup gap correctly is a must when installing a new reluctor or pickup unit, the gap does not change in service (due to wear) and should not require periodic checking or adjustment. The main reason that the minimum air gap specification exists is to make sure that the reluctor does not contact the pole piece as the vacuum plate moves.

When checking the pickup gap, use a non-magnetic feeler gauge. This is because a feeler blade that is attracted to the magnetism of the pole piece will give a false feel or drag. If non-magnetic feeler blades are not available, use brass shim stock of the proper thickness.


WARNING
When working on a truck with electronic ignition, be careful not to touch the round transistor located in the control unit heat sink when the ignition is ON. It can give out a very large shock.

1984 V8 ENGINES WITH DUAL PICKUP DISTRIBUTORS AND VACUUM ADVANCE



See Figures 2 through 14

To properly test the Electronic Ignition System, special testers should be used. In the event they are not available, the system may be tested using a voltmeter with a 20,000 ohm/volt rating and an ohmmeter which uses a 9 volt battery for its operation. Both meters should be properly calibrated.

  1. Visually inspect all secondary cables at the coil, distributor and spark plugs for cracks and wear. Check for tight connections.
  2.  
  3. Check the primary wire at the coil and ballast resistor for a clean, tight connection.
  4.  



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Fig. Fig. 2: Coil terminals


WARNING
When removing or installing the wiring harness connector to the control unit, the ignition switch must be in the OFF position.

  1. Using a voltmeter, measure the voltage at the battery, in order to assure that enough current is available to operate the starting and ignition systems.
  2.  
  3. Remove the coil secondary wire from the distributor cap.
  4.  
  5. With the key in the ON position, use a jumper wire to momentarily touch the negative terminal of the coil to ground while holding the coil secondary wire approximately 1 / 4 (6mm) from a good engine ground. A spark should be observed.
  6.  



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Fig. Fig. 3: Checking for spark during cranking

  1. If no spark is observed, turn the ignition key to the OFF position, then disconnect the four-wire harness going to the Electronic Control Unit (ECU).
  2.  
  3. With the ignition key in the ON position, again use the jumper wire and ground the negative terminal of the coil while holding the coil secondary wire approximately 1 / 4 in. (6mm) from a good engine ground. If a spark is observed, replace the ECU.
  4.  



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Fig. Fig. 4: Use a special jumper to ground the coil negative terminal on 1972-83 systems

  1. If no spark is observed, measure the voltage at the coil positive terminal. The voltage should be within one volt of battery voltage.
  2.  
  3. If battery voltage is not present, check the wiring between the battery positive terminal and coil. Replace the starter relay if the wiring is correct.
  4.  



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Fig. Fig. 5: Testing the coil positive terminal for battery voltage

  1. If the current is not continuous between the battery and the coil positive terminal, replace the ignition resistor and repeat the test.
  2.  
  3. Check the battery voltage at the coil negative terminal. It should be within one volt of battery voltage.
  4.  



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Fig. Fig. 6: Testing the coil negative terminal for battery voltage

  1. If battery voltage is present at the negative coil terminal, but no spark is obtained when shorting the terminal with a jumper wire, replace the ignition coil.
  2.  
  3. If spark is obtained, but the engine will not start, turn the ignition switch to the OFF position and pull the ECU harness connector off, then turn the ignition switch to the ON position and check for battery voltage at cavity No. 2 of the ECU harness connector. The voltage should be within one volt of battery voltage.
  4.  



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Fig. Fig. 7: Testing for voltage at cavity No. 2

  1. If no battery voltage is present, turn the ignition switch to the OFF position and check for continuity between cavity No. 2 and the coil negative terminal. If no continuity is obtained, find the wiring fault, repair it and retest.
  2.  



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Fig. Fig. 8: Checking for continuity between cavity No. 2 and the coil negative terminal

  1. Check for continuity between cavity No. 1 of the ECU connector and the ignition switch. If none exists, find the fault, repair it and retest.
  2.  



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Fig. Fig. 9: Checking continuity between cavity No. 1 and the ignition switch

  1. If voltage is obtained at cavity No. 2 of the ECU connector, turn the ignition switch to the OFF position and, with an ohmmeter, check the resistance between cavities No. 4 and No. 5 of the ECU connector. The reading should range between 117 and 900 ohms.
  2.  



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Fig. Fig. 10: Checking resistance between cavities No. 4 and 5

  1. If the resistance is not between 117 and 900 ohms, disconnect the distributor pickup leads. Measure the resistance at the pickup leads. The resistance should be between 117 and 900 ohms. If the resistance is not within the accepted range, the pickup coils are bad and must be replaced.
  2.  



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Fig. Fig. 11: Testing resistance of both pickup coils



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Fig. Fig. 12: Testing for a short at each pickup coil terminal



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Fig. Fig. 13: Testing the ECU pin 5 for ground

  1. If the resistance at the pickup leads is within specifications, this indicates that wiring between cavities No. 4 and No. 5 is open or shorted, or the dual pickup start/run relay is defective. Repair and retest as required.
  2.  
  3. Check pin No. 5 of the ECU for ground. If no ground is obtained, check the ECU for poor or dirty connections and tight mounting screws.
  4.  
  5. Reinstall all connections and check for spark. If no spark occurs, replace the ECU.
  6.  



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Fig. Fig. 14: Pickup coil air gap

Dual Pickup Start/Run Relay
  1. Remove the two-way connector from pins No. 4 and No. 5 of the dual pickup start/run relay.
  2.  
  3. Using an ohmmeter, touch pins No. 4 and No. 5. The meter should read 20-30 ohms. If not, replace the relay.
  4.  

Centrifugal Advance
  1. Connect a timing light according to the manufacturer's instructions.
  2.  
  3. Operate the engine at idle and remove the vacuum hose from the vacuum controller.
  4.  
  5. Slowly accelerate the engine to check for advance.
  6.  
  7. Excessive advance indicates a damaged governor spring (a broken spring will result in abrupt advance).
  8.  
  9. Insufficient advance is usually caused by a broken governor weight or a malfunction in the cam operation. Correct as necessary.
  10.  

Vacuum Advance

See Figure 15

  1. Connect a timing light and adjust the engine speed to 2170 rpm.
  2.  
  3. Check for advance by disconnecting, then reconnecting the vacuum hose at the distributor, and watching the advance or retard at the crankshaft indicator.
  4.  
  5. For a more accurate determination of whether the vacuum advance mechanism is operating properly, remove the vacuum hose from the distributor and connect a hand vacuum pump.
  6.  



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Fig. Fig. 15: Distributor with a vacuum advance

  1. Run the engine at idle and slowly apply vacuum pressure to check for advance.
  2.  
  3. If excessive advance is noted, look for a deteriorated vacuum controller spring.
  4.  
  5. If insufficient advance or no advance is noted, this could be caused by linkage problems or a ruptured vacuum diaphragm. Correct as necessary.
  6.  

The electronic ignition system can be tested with either special ignition testers or a voltmeter with a 20,000 ohm/volt rating and an ohmmeter using a 9 volt battery as a power source. Since special ignition system testers have manufacturer's instructions accompanying them, be sure to refer to the procedural steps necessary to operate them. The following is based on usage of an ohm/volt combination meter.

1987-88 ENGINES WITH SINGLE PICKUP DISTRIBUTORS AND NO VACUUM ADVANCE; 1984-88 ENGINES WITH DUAL PICKUP DISTRIBUTORS AND NO VACUUM ADVANCE



Secondary Circuit
  1. Remove the coil wire from the distributor cap. Wrap it with cloth and hold it cautiously about 1 / 4 in. (6mm) away from an engine ground. Use a remote starter switch or have an assistant crank the engine while checking for spark.
  2.  
  3. If a good spark is present, slowly move the coil wire away from the engine and check for arcing at the coil while cranking.
  4.  
  5. If good spark is present and it is not arcing at the coil, check the rest of the parts of the ignition system.
  6.  

Ignition System Starting
  1. Visually inspect all secondary cables at the coil, distributor and spark plugs for cracks. Check for tight connections.
  2.  
  3. Check the primary wire at the coil and ballast resistor for clean, tight connections.
  4.  


WARNING
When removing or installing the wiring harness connector to the control unit, the ignition switch must be in the OFF position.

  1. Use a voltmeter to measure the voltage at the battery and confirm that enough current is available to operate the starting and ignition systems.
  2.  
  3. Remove the coil secondary wire from the distributor cap.
  4.  
  5. With the key in the ON position, use a jumper wire and momentarily touch the negative terminal of the coil to ground while holding the coil secondary wire approximately 1 / 4 in. (6mm) from a good engine ground. A spark should be observed.
  6.  
  7. Verify that spark is getting to the spark plugs. If the spark plugs are being fired, the ignition system is not responsible for the engine not starting.
  8.  
  9. If no spark is observed at the ignition coil wire, turn the ignition switch to the OFF position and detach the 10-way connector from the bottom of the Spark Control Computer. Turn the ignition switch to the ON position, and hold the ignition coil wire approximately 1 / 4 in. (6mm) away from a good engine ground.
  10.  
  11. With battery current flowing to the coil negative terminal, intermittently short the terminal to ground. If spark now occurs, replace the Spark Control Computer.
  12.  
  13. If the voltage is incorrect, check the continuity of the wiring between the battery and the coil positive terminal. Repair the wiring as required and retest.
  14.  
  15. With the ignition key in the ON position, check the voltage at the coil negative terminal. If it is not within one volt of the battery's voltage, replace the ignition coil.
  16.  
  17. If battery voltage (within one volt) is present, but no spark is obtained when shorting the negative terminal, replace the ignition coil.
  18.  
  19. If spark is obtained, but the engine will still not start, turn the ignition switch to the RUN position and, using the positive lead of the voltmeter, measure the voltage from cavity No. 1 to the disconnected ground lead of the computer. The voltage should be within 1 volt of the battery voltage noted earlier.
  20.  
  21. If battery voltage is not present, check the wire for an open circuit and repair. Retest as required.
  22.  
  23. Place a thin insulator between the curb idle adjusting screw and the carburetor switch, or make sure the curb idle adjusting screw is not touching the carburetor switch.
  24.  
  25. Connect the negative voltmeter lead to a good engine ground. Turn the ignition switch to the RUN position and measure the voltage at the carburetor switch terminal. The voltage should be approximately 5 volts.
  26.  
  27. If the voltage is not 5 volts, turn the ignition switch to the OFF position and detach the 10-way connector from the bottom of the Spark Control Computer. Turn the ignition switch back to the RUN position and measure the voltage at terminal 2 of the connector.
  28.  
  29. Voltage should be within 1 volt of the battery's voltage. If the correct voltage is not present, check the wiring between terminal 2 of the connector and the ignition switch for an open or shorted circuit or a poor connections.
  30.  
  31. Turn the ignition switch to the OFF position and detach the connector from the bottom of the spark computer, if not already done. With an ohmmeter, check the continuity between terminal 7 of the connector and the carburetor switch terminal. Continuity should exist between these two points. If not, check for an open circuit poor connection.
  32.  
  33. Check for continuity between terminal 10 of the connector and engine ground. If continuity exists, replace the Spark Control Computer assembly. If continuity does not exist, check the wiring for open circuits or poor connections. Repeat Step 18.
  34.  
  35. If the engine still fails to start, turn the ignition switch to the OFF position and, with an ohmmeter, measure the resistance of the start pickup coil between terminal 5 and terminal 9 of the 10-way connector. The resistance should be 117-900 ohms.
  36.  
  37. If the resistance is not within the specified range, disconnect the start pickup coil leads from the distributor. Measure the resistance at the lead going into the distributor. If the reading is now 117-900 ohms, an open circuit or faulty connection exists between the distributor connector and terminals 5 and 9 of the 10-way connector. If the resistance is not within specifications, the pickup coil is bad. Replace it and set the air gap to specifications.
  38.  
  39. Connect one lead of the ohmmeter to the engine ground and, with the other lead, check for continuity at each terminal of the leads going to the distributor. There should be no continuity.
  40.  
  41. If there is continuity, replace the pickup coils. Adjust the air gap to specifications.
  42.  
  43. Attempt to start the engine. If it fails to start, repeat the tests. If the engine still fails to start, replace the Spark Control Computer.
  44.  

Should the engine still fail to start with the replaced Spark Control Computer, Chrysler Corporation suggests reinstalling the original Spark Control Computer and repeating the tests. However, proper testing of the circuits and pickup should result in the engine starting, unless unrelated problems exist.

Basic Timing

See Figure 16

Correct basic timing is essential for optimum engine performance. Before any testing and service is begun in response to poor performance, the basic timing must be checked and adjusted as required. Refer to the underhood specifications label for timing adjustment specifications.



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Fig. Fig. 16: Inductive timing light

Carburetor Switch

Grounding the carburetor switch eliminates all spark advance on most systems.

  1. With the ignition key in the OFF position, detach the 10-way connector from the Spark Control Computer.
  2.  
  3. With the throttle completely closed, check the continuity between pin 7 of the 10-way connector and a good engine ground.
  4.  
  5. If no continuity exists, check the wires and the carburetor switch. Recheck the basic timing.
  6.  
  7. With the throttle open, check the continuity between pin 7 of the 10-way connector harness connector and a good engine ground. There should be no continuity.
  8.  

Engine Temperature Sensor
  1. Turn the ignition switch to the OFF position and disconnect the wire from the temperature switch.
  2.  
  3. Connect one lead of an ohmmeter to a good ground on the engine, or in the case of the charge temperature switch, to its ground terminal.
  4.  
  5. Connect the other lead of the ohmmeter to the center terminal of the coolant temperature switch.
  6.  
  7. Check for continuity using the following ohmmeter readings:
    1. Cold engine: continuity should be present with a resistance less than 100 ohms. If not, replace the switch. The charge temperature switch must be cooler than 60°F (15°C) in order to achieve this reading.
    2.  
    3. Hot engine at normal operating temperature: the terminal reading should show no continuity. If it does, replace the coolant temperature switch or the charge temperature switch.
    4.  

  8.  

Coolant Sensor

  1. Connect the leads of an ohmmeter to the terminals of the sensor.
  2.  
  3. With the engine cold and the ambient temperature less than 90°F (32°), the resistance should be between 170-1100 ohms.
  4.  
  5. With the engine at normal operating temperature, the resistance should be greater than 1300 ohms.
  6.  

The sensor will continually change its resistance with a change in engine operating temperature.

  1. If the resistance is not within the specified ranges, replace the sensor.
  2.  

Detonation Sensor
  1. Connect a timing light to the engine, according to the manufacturer's instructions.
  2.  
  3. Start the engine and run it with the fast idle cam on its second highest step (at least 1200 rpm).
  4.  
  5. Connect an auxiliary vacuum supply to the vacuum transducer and apply 16 in. Hg (54 kPa).
  6.  
  7. Tap lightly on the intake manifold near the sensor with a small metal object.
  8.  
  9. Using the timing light, look for a decrease in the spark advance. The amount of decrease in the timing is directly proportional to the strength and frequency of the tapping. The maximum decrease in timing should be 11° for 1984 models, or 20° for 1985 and later models.
  10.  
  11. Turn the ignition switch to the OFF position. With the engine stopped, disconnect the timing light.
  12.  

Electronic Exhaust Gas Recirculation (EGR) System

See Figure 17

The Electronic EGR control is located within the electronic circuitry of the Spark Control Computer, and its testing procedure is outlined.

  1. All the engine temperature sensors must be operating properly before the tests can be performed.
  2.  
  3. With the engine temperature cold and the ignition switch turned to the OFF position, connect one voltmeter lead to the gray wire on the EGR solenoid and the other lead to a good engine ground.
  4.  
  5. Start the engine. The voltage should be less than one volt. It will remain at this level until the engine has reached its normal operating temperature range and the electronic EGR schedule has timed out. The solenoid will then de-energize and the voltmeter will read charging system voltage.
  6.  
  7. If the charging system voltage is not obtained, replace the solenoid and repeat the test.
  8.  
  9. If the voltmeter indicates charging system voltage before the EGR schedule is complete, replace the computer or the externally mounted timer.
  10.  



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Fig. Fig. 17: EGR system-1982

The 318 cu. in. non-California engines with 2-bbl. carburetors have no thermal delay below 60°F (15°C) ambient temperature. They will follow the EGR time delay schedule only.

  1. If an engine is started with the temperature hot, the EGR solenoid will be energized for the length of the time delay schedule only. It will then de-energize.
  2.  

Electronic Throttle Control System

Incorporated within the Spark Control Computer is the electronic throttle system. A carburetor mounted solenoid is energized when the air conditioner, electric back light or electric timers are activated. The two timers which are incorporated in the ignition electronics operate two seconds after the throttle is closed, or after an engine start condition.

  1. Connect a tachometer to the engine.
  2.  
  3. Start the engine and run it until normal operating temperature is reached.
  4.  
  5. Depress the accelerator and release it. A higher than curb idle speed should be seen on the tachometer for the length of the EGR schedule.
  6.  
  7. On air conditioned vehicles, turning on the air conditioning or the back light and depressing the accelerator for a moment should give a higher than curb idle speed. Turning the air conditioning and back light off should produce the normal idle speed.
  8.  

With the air conditioning system on, the air conditioning clutch will cycle on and off. This should not be mistaken as a part of the electronic control system.

  1. If the speed increases do not occur, disconnect the three-way connector at the carburetor.
  2.  
  3. Check the solenoid with an ohmmeter by measuring the resistance from the terminal that contains the black wire to ground. The resistance should be 15-5 ohms. If it is not within specifications, replace the solenoid.
  4.  
  5. Start the engine and, before the delay has timed out, measure the voltage of the three-way connector's black wire. The voltmeter should read charging system voltage. If it does not, replace the computer.
  6.  
  7. Turning the air conditioning or the back light on should also produce charging system voltage after the time delay has timed out. If not, check the wiring back to the instrument panel for an open circuit.
  8.  

DUAL PICKUP START/RUN RELAY
  1. Remove the two-way connector from pins No. 4 and No. 5 of the dual pickup start/run relay.
  2.  
  3. Using an ohmmeter, touch pins No. 4 and No. 5. The meter should read 20-30 ohms. If not, replace the relay.
  4.  

Centrifugal Advance
  1. Connect a timing light according to the manufacturer's instructions.
  2.  
  3. Operate the engine at idle, then remove the vacuum hose from the vacuum controller.
  4.  
  5. Slowly accelerate the engine to check for advance.
  6.  
  7. Excessive advance indicates a damaged governor spring (a broken spring will result in abrupt advance).
  8.  
  9. Insufficient advance is usually caused by a broken governor weight or a malfunction in cam operation. Correct as necessary.
  10.  

 
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