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Thread: 4.0 Jeep Crank Pulse Timing

  1. #1
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    4.0 Jeep Crank Pulse Timing

    Looking for information, diagrams, etc displaying relationship of crank pulses and piston / cylinder position. System provides four pulses every 120 degrees for total of 12. Odds are beginning and end of each pulse train coincides with TDC of paired cylinders, but how closely?

    I know, MB/Chryco hated to release anything technical to the masses.

    Here's the only thing I've found so far...


    Anyone know of a source for similar info?

  2. #2
    Fuel Injected! JeepsAndGuns's Avatar
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    According to the 1993 wrangler/cherokee FSM I have, the 4.0 has 3 sets of 4 notches which coincide with a pair of cylinders.
    It says the trailing edge of the 4th notch is 4 degrees below TDC of the corresponding cylinder.
    79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy

    93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver

    99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.

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    It says the trailing edge of the 4th notch is 4 degrees below TDC of the corresponding cylinder.
    That, sir, is gold. There are six notches 60 degrees apart. If the trailing edge of the last notch in a group is 4 deg retarded, then the center of the first notch of each group is between 54 and 60 deg BTDC. So if a guy fills in the "extra" notch and makes one more notch 10 degrees from one of the existing notches, then the result would look remarkably like this one:

  4. #4
    Fuel Injected! JeepsAndGuns's Avatar
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    Planning on converting a 4.0 to a GM ecm with DIS?
    79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy

    93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver

    99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.

  5. #5
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    Yes. I picked up a 95 Wrangler for short money. 2.5... bleh. I thought about forced induction but I stumbled on a 2000 4.0 and AX15 cheap. The 4.0 already has an aftermarket head so no worries there. And I've got this complete '87 258 sitting here so I'm thinking of going the budget stroker route. Either way, I don't have a controller so it makes sense to install the GM box.

  6. #6
    Fuel Injected! JeepsAndGuns's Avatar
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    Welcome to the YJ madness.
    I still have a 75% modded stock YJ engine harness setting here on the floor to go in mine. I am setting it up to run off a 1227727. I just have so much going on lately I have not had time to finish it up.
    The stock 4.0 system is really not a bad system. There is just no aftermarket support or tuning for the ECM. (the OBD1 ecm's that is)
    79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy

    93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver

    99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.

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