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  1. #1
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    Carl Zachary Decker
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    1996 Chevy Corvette LT1
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    Default 96 LT1 crank no start issue

    Have a 96 LT1, motor was running while testing the tune and just died. Key on fuel pump primes, have good pressure, cranks but will not start. Have no spark at coil. Performed check of ICM harness and have all but 1-4 volts while cranking at terminal B. Performed Opti harness checks and all were good. Performed Fleet service manual checks for PCM, saying I have bad PCM. Snap On scanner communicates with PCM, shows TPS volts and %, Map, etc. But no RPMs when cranking. Question is do I have a bad Opti or bad PCM from tuning/shipping.

  2. #2
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    Default

    Have you checked to see if your rotor spun off?

    Sent from my SM-G955U using Tapatalk
    Same as everybody else...350 small block, double hump heads, 3/4 cam

  3. #3
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    Default

    Have not broke into Opti yet. Was hoping to narrow it down a bit more if possible.

  4. #4
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    Default

    Do you have spark at all the plugs?

    Do you have fuel pressure?

    Do you have a noid light to verify the injectors are pulsing?

    Have you verified the Opti signals are present, by checking for the pulse on the white wire to the ICM?

    http://shbox.com/1/4th_gen_tech2.html#opti_test

    Is the SECURITY light on while cranking?

    94 Formula A3+1: 381ci forged stroker - Callies Stealth, Oliver 5.85 billet rods, BME nitrous pistons / CNC LT4 heads / Comp Cams solid roller / TH400+GearVendors OD / 4.11's/Strange 12-bolt / 300-shot (dry) N2O / Spohn Suspension / 6-pt roll bar /AutoMeter instrumentation / MoTeC M48 Pro engine management system /a few other odds 'n ends.

  5. #5
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    Carl Zachary Decker
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    No spark at coil or plugs

    Fuel pressure KOEO is 44psi and holds steady

    No test on injector pulse yet

    Followed all steps on shbox.com write up and all checked good except for ICM white wire have no voltage when cranking

    Security light comes on key in but goes off and does not come on when cranking

    I appreciate all helping out.

  6. #6
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    Default

    And you got 5 volts on pins A and B in the Opti harness?

  7. #7
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    Yes with harness disconnected from Opti and key on got 5 volts on pins A and B.

  8. #8
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    Lack of the ignition pulse to the ICM can be the result of no low resolution pulse from the optical cam position sensor in the Opti. When the PCM doesn’t see the low res pulse it shuts down the fuel system. There’s a code for a missing low res pulse, but it can’t set unless the PCM also sees the high resolution pulse. The code for the low res pulse will set while the engine is cranking, if the high res pulse is present, and you crank the engine for a longer than normal length of time. That code does NOT turn on the SES light. Have you scanned it for codes?

    When the engine is cranking, does the tach needle move off the stop, up to 200-300 RPM? That is another indication the PCM is receiving the signal from the Opti.

    Another check is to reconnect the Opti, and connect the volt meter to the red/black wire on pin A (+) and the pink/black wire on pin D (-). Key on, slowly rotate the engine. For each revolution of the crank, the Opti low res signal should pull the 5 volts down to ~0 volts four times. You can do the same with the high res wire, measuring from the purple/white wire on pin B to the pink/black wire. But that wire should have 180 5 volt / 0 volt switches per crank rev. Plus, the PCM does not need the high res signal to run the engine. It just improves the timing of the spark and injectors to reduce spark scatter.

    http://shbox.com/1/1996_ec_03_ign_system.jpg

  9. #9
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    Carl Zachary Decker
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    Default

    shbox gave me another tip to take a spare Opti, connect it to harness, turn key on, with meter hooked to ICM pin B spin Opti and check for 1.4 volts. Got almost 4 volts along with spark from coil and injector pulse. Appreciate all the help. Now time to change Opti.

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