» » »

Jeep Q&A

Ask Your Question

1990 Jeep Wrangler Question: engine cranks but wont start

 

Question

longislandpaul, 4.2L 6 Cylinder, Moultonborough, NH, August 26, 2009, 17:22
 Rookie

tested ignition wire for spark when cranking, no spark. is the problem the ignition coil because wires test ok, or is there an engine control module at fault?

1 Answer
Flag This
  • Answer #1

    backyard tech August 26, 2009, 20:36
     Master

    Vehicle Application: 1990 Jeep Wrangler 4.2L, Vin T, CARB, Eng Cfg L6

    Customer Concern: The engine cranks over but will not start. No spark present while cranking.
    Tests/Procedures: 1. Bypass the engine computer, which on this vehicle is called a MCU (Microprocessor Control UnitVehicle Application: 1989 Jeep Wrangler Islander 4.2L, Vin T, CARB, Eng Cfg L6

    Customer Concern: Cranks no start, no spark. Have replaced the ignition module and the distributor pick-up coil.
    Tests/Procedures: 1. Check for B+ on the Red wire at the module and at the ignition coil.

    2. Check for a good ground on the Black wire at the ignition module. This wire is grounded through the distributor pick-up coil.

    3. Verify that the Dark Green tachometer wire at the coil has B+ with the ignition module disconnected and the key on. Keep the coil connected.

    4. To test the module, cut the Light Green wire at the ignition module.

    5. Connect a test light to B+ and tap it to the cut wire on the ignition module side while checking for spark from the coil. There should be a spark from the coil whenever the test light is removed from the wire. The Dark Green wire should get grounded when voltage is applied to the cut wire.

    6. If the module test will produce spark, check the AC voltage from the distributor pick-up coil. It should be at least 300 mVAC cranking.
    Potential Causes: Ignition Coil
    Ignition Module
    Pick-Up Coil

    ) by doing the following: Cut the Orange wire at the ignition module (which is located under the coolant overflow bottle). Locate the Orange and Purple wires at the distributor pick-up coil and cut both of those wires. Using new wire, run the Orange wire from the pick-up coil to the Orange wire at the ignition module. Using a second piece of new wire, run the Purple wire at the pick-up coil to the Purple wire at the ignition module. Note: the Purple wire at the ignition module is in the connector but was not being used. After verifying all connections are secure, attempt to start the engine. If the engine now starts, the MCU likely has failed. Verify the MCU power and ground supplies. This part is very difficult to find as it is not being made anymore. Leaving the wiring in place will cause the feedback carburetor not to control fuel properly as the MCU does not have an RPM input. This may result in an emission test failure in some parts of the country.
    Potential Causes: Micro Control Unit (MCU)
    Wiring

    Tech Tips: The MCU may be hard to locate as it is not being made any longer.

    Reply
    Flag This