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Thread: Hard 1-2 shift

  1. #1
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    Default Hard 1-2 shift

    I have a newly rebuilt 4l60e ('94) that has a very hard 1-2 shift. I have talked to the builder, who took a second spring out of the 1-2 accumulator, didn't help.

    Back-story and what's been done to date. Shift problem from day-1. I talked to my Tuner and sent him short video's of the scan tool in real time. The PCM was not commanding higher line pressure during low load acceleration, but was commanding more pressure during a medium/high load take off. Shift at low load is just as hard as high load.

    - Shift points are where the Tuner and I discussed, 2-3 shift is perfect, lockup is perfect, 3-O.D./lockup is perfect.

    - TPS has been checked and adjusted to 5.6v on idle stop and 4.488v at WOT

    - MAP voltage: 5.039v engine off 1.409v engine idling (Lt GRN wire)

    - Idle vacuum is 18

    As I understand, and this could be a mile off reality, the 1-2 accumulator, separator plate hole #8, and the 2nd gear servo affect this shift. The builder used the "Corvette" servo which has more surface for the oil pressure to act against. The Tuner suggested I swap the Corvette servo for the Truck version to correct the hard shift. I also read that the Corvette servo also does a better job of locking up 3rd and/or 4th (O.D.) which would be a good thing.

    Way back in the day, I put a shift kit in a TH400 and part of that magic was to drill out that hole slightly. Made a difference back then. The builder said he drilled out the #8 hole to .070, which he does routinely.

    Now, here's what I do not understand. Sometimes the 1-2 shift is beautiful, light/firm with no slide-bump. This happens without fail while the trans oil is cold, first startup of the day. After about 5-6 minutes it starts the severe shifts into 2nd, no matter if I use the absolute minimum throttle to get up to speed or give the throttle a heavy shoe. But, maybe 10-15% of the time it will give me that beautiful smooth, controlled shift, and then go right back pounding the drive train. There is no pattern to when the softer shift is going to happen and I can't seem to repeat it.

    Short recap: the 1-2 shift now is what I would want on a full throttle high RPM shift, but not all the time and not poking around town from stop light to stop light.

    I would really appreciate help from anyone who has an educated guess as to what is causing this problem, or point me in a direction. The rebuild was total with top of the line parts, the builder does this service for most of the racers around here. He has not stepped up to help me much on this issue though. That's something between the two of us and going back to him is not an option until I have a better understanding of a solution.

    Thank you for your time.

    Dave
    [/FONT][/COLOR]
    Last edited by daveb; 05-18-2022 at 05:28 PM. Reason: Forum keeps kicking me off-line

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    Default

    Dave, I'm not a transmission guru, I'm not even a transmission flunky.
    But, This doesn't sound right;
    - TPS has been checked and adjusted to 5.6v on idle stop and 4.488v at WOT

    If this is not a misprint your TPS is bad. Did you mean .56 volts and 4.488 volts?

    If so, disregard this and carry on.
    Nolan
    '94 Z28 Bone stock,4l60e
    '97 SS SLP,T56,385 stroker,AFR210,XFI292,NO traction til top of 3rd gear.
    '57 Chevy 3100 p/u LT1/T56

  3. #3
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    You are absolutely spot on, Nolan. I don't know how I can loose my concentration in the 10 seconds it took to look in my records and type the [I]wrong[I] number in the post, just lucky, I guess. .56v is correct.

    Dave

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  5. #4
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    if the trans is built put a stock trans tune table in there. Frank would be yelling right now if it was one of his transmissions. The only thing i ever tuned around on a built cpt was stall wall, always ran stock line pressure tables because the line pressure was already physically modified inside the transmission along with the servo etc. frank explained that adding linepressure in the tune on top of having the trans built would kill it in short order and caused hard shifts.
    2021 Durango R/T Tow-n-Go
    2021 Charger Scat 392
    1994 Firebird Formula 381 stroker - Carrying the torch!

    The wrench/driver for LTConvert's 94 Z28- Ellwein 383/LE Trickflows/ D1SC / CPT Ultra Pro Race 4L60E
    10.78 @ 125mph. Shooting for 9.99 for Frank Cahall!

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    Mine is not a CPT rebuild and the tuner did use a Trans Am table for engine and trans with minor mods for tire size, weight, gear ratio, etc.

    Dave

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    Thank you for all your help.

    Dave

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    Did you ever get it figured out?
    '94 Z28 Bone stock,4l60e
    '97 SS SLP,T56,385 stroker,AFR210,XFI292,NO traction til top of 3rd gear.
    '57 Chevy 3100 p/u LT1/T56

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    Quote Originally Posted by daveb View Post
    Mine is not a CPT rebuild and the tuner did use a Trans Am table for engine and trans with minor mods for tire size, weight, gear ratio, etc.

    Dave
    doesnt matter who rebuilt it. if the line pressure is increased mechanically in the trans it CANNOT be increased in the tune. if it is, it will cause problems like you are describing
    2021 Durango R/T Tow-n-Go
    2021 Charger Scat 392
    1994 Firebird Formula 381 stroker - Carrying the torch!

    The wrench/driver for LTConvert's 94 Z28- Ellwein 383/LE Trickflows/ D1SC / CPT Ultra Pro Race 4L60E
    10.78 @ 125mph. Shooting for 9.99 for Frank Cahall!

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