Jeep Wrangler/YJ 1987-1995 Repair Guide

Solid State Ignition (SSI) System

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The Solid State Ignition (SSI) is standard equipment on all 4.2L engines. The system consists of a sensor and toothed trigger wheel located inside the distributor, a permanently sealed electronic control unit which determines dwell, along with ignition wires, spark plugs, and a coil.

The trigger wheel rotates on the distributor shaft. As one of its teeth nears the sensor magnet, the magnetic field shifts toward the tooth. When the tooth and sensor are aligned, the field is shifted to its maximum, signaling the electronic control unit to switch off the coil primary current. This starts an electronic timer inside the control unit, which allows the primary current to remain off only long enough to fire the spark plug. The timer adjusts the amount of time primary current is off according to conditions, thus automatically adjusting dwell. There is also a special circuit within the control unit to detect and ignore spurious signals. Spark timing is adjusted by both mechanical (centrifugal) and vacuum advance.

A wire of 1.35 ohms resistance is spliced into the ignition feed to reduce voltage to the coil during running conditions. The resistance wire is bypassed when the engine is being started so that full battery voltage may be supplied to the coil. Bypass is accomplished by the I-terminal on the solenoid.

DIAGNOSIS & TESTING



Secondary Circuit Test
  1. Disconnect the coil wire from the center of the distributor cap.
  2.  

Twist the rubber boot slightly in either direction, then grasp the boot and pull straight up. Do not pull on the wire, and do not use pliers.

  1. Hold the wire 1 / 2 in. (13mm) from a ground with a pair of insulated pliers and a heavy glove. As the engine is cranked, watch for a spark.
  2.  
  3. If a spark appears, reconnect the coil wire. Remove the wire from one spark plug, and test for a spark as above.
  4.  


WARNING
Do not remove the spark plug wires from cylinders 1 or 5, or sensor damage could occur!

  1. If a spark occurs, the problem is in the fuel system or with the ignition timing. If no spark occurs, check for a defective rotor, cap, or spark plug wires.
  2.  
  3. If no spark occurs from the coil wire in Step 2, test the coil wire resistance with an ohmmeter. It should be 7,700-9,300 ohms at +75ºF (24ºC) or 12,000 ohms maximum at +93ºF (34ºC).
  4.  

Coil Primary Circuit Test

  1. Turn the ignition ON . Connect a voltmeter to the coil positive (+) terminal and a ground:
    1. If the voltage is 5.5-6.5 volts, go to Step 2.
    2.  
    3. If above 7 volts, go to Step 4.
    4.  
    5. If below 5.5 volts, disconnect the condenser lead and measure. If the reading with the condenser lead disconnected is now 5.5-6.5 volts, the condenser is faulty and should be replaced. If the reading is not 5.5-6.5 volts, go to Step 6.
    6.  

  2.  
  3. With the tester connected as in Step 1, read the voltage with the engine cranking. If battery voltage is indicated, the circuit is okay. If not, go to Step 3.
  4.  
  5. Check for a short or open in the starter solenoid I-terminal wire. Check the solenoid for proper operation.
  6.  
  7. Disconnect the wire from the starter solenoid I-terminal, with the ignition ON and the voltmeter connected as in Step 1:
    1. If the voltage drops to 5.5-6.5 volts, the solenoid is faulty and should be replaced.
    2.  
    3. If the voltage does not drop to 5.5-6.5 volts, connect a jumper between the coil negative (-) terminal and a ground. If the voltage now drops to 5.5-6.5 volts, go to Step 5. If not, repair the resistance wire.
    4.  

  8.  
  9. Check for continuity between the coil negative (-) terminal and D4 , and between D1 to ground. If the continuity is okay, replace the control unit. If not, check for an open wire and go back to Step 2.
  10.  
  11. Turn ignition OFF . Connect an ohmmeter between the + coil terminal and dash connector AV . If above 1.40 ohms, repair the resistance wire.
  12.  
  13. With the ignition OFF , connect the ohmmeter between connector AV and ignition switch terminal 11 . If less than 0.1 ohms, replace the ignition switch or repair the wire, whichever is the cause. If above 0.1 ohms, check connections, and check for defective wiring.
  14.  

Coil Test

  1. Check the coil for cracks, carbon tracks, etc., and replace as necessary.
  2.  
  3. Connect an ohmmeter across the coil + and - terminals, with the coil connector removed. If 1.13-1.23 ohms at 75ºF (24ºC), the coil is okay. If not, replace it.
  4.  

Control Unit and Sensor Test

  1. With the ignition ON , remove the coil high tension wire from the distributor cap and hold it 1 / 2 in. (13mm) from a ground with insulated pliers. Disengage the 4-wire connector at the control unit. If a spark occurs (normal), go to Step 2. If not, go to Step 5.
  2.  
  3. Connect an ohmmeter to D2 and D3 . If the resistance is 400-800 ohms (normal), go to Step 6. If not, go to Step 3.
  4.  
  5. Disengage, then reconnect the 3-wire connector at distributor. If the reading is now 400-800 ohms, go to Step 6. If not, disengage the 3-wire connector and go to Step 4.
  6.  
  7. Connect the ohmmeter across B2 and B3 . If 300-800 ohms, repair the harness between the 3-wire and 4-wire connectors. If not, replace the sensor.
  8.  
  9. Connect the ohmmeter between D1 and the battery negative terminal. If the reading is 0 (0.002 or less), go to Step 2. If above 0.002 ohms, there is a bad ground in the cable or at the distributor. Repair the ground and retest.
  10.  
  11. Connect a analog voltmeter across D2 and D3 . Crank the engine. If the needle fluctuates, the system is okay. If not, either the trigger wheel is defective, or the distributor is not turning. Repair or replace as required.
  12.  

Ignition Feed-to-Control Unit Test

Do not perform this test without first performing the Coil Primary Circuit Test.

  1. With the ignition ON , unplug the 2-wire connector at the module. Connect a voltmeter between F2 and ground. If the reading is battery voltage, replace the control unit and go to Step 3. If not, go to Step 2.
  2.  
  3. Repair the cause of the voltage reduction: either the ignition switch or a corroded dash connector. Check for a spark at the coil wire. If okay, stop. If not, replace the control unit and check for proper operation.
  4.  
  5. Engage the 2-wire connector at the control unit, then unplug the 4-wire connector at the control unit. Connect an ammeter between C1 and ground. If it reads 0.9-1.1 amps, the system is okay. If not, replace the module.
  6.  

 
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