Author Topic: Tranny shifts hard. i have a 1981 chevy c20 454 with auto trans  (Read 1567 times)

Offline sammysamm

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i have a 1981 chevy c20 454 with auto trans and it shifts hard from 1st to second..   

its nothing major but feels like a lil jerk forward when i am going slowly.


i noticed when i step on it or im on the highway for example merging. it does NOT make the jerk and shifts smoothly.

any ideas what that could be?

pretty sure its a th400

thanks

Offline zmanabba

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Re: Tranny shifts hard. i have a 1981 chevy c20 454 with auto trans
« Reply #1 on: June 18, 2018, 10:17:53 PM »
My guess is some time in its 36 years the transmission has been rebuilt. A transmission shop will install a shift kit during the rebuild. One of the hardest things on a transmission is heat. A source of heat is a soft shift. The OEM's design a bit of slippage for passenger comfort but it does shorten the life of the transmission. To lengthen the life of the transmission the shift kit ups the line pressure and quickens the shifts. You are probably feeling the 1-2 shift more due to the increased torque multiplication of the lower gears. At highway speeds any clearance in the drivetrain will be taken up as opposed to the slow speed when the drive train is being loaded/unloaded. 

Offline ehjorten

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Re: Tranny shifts hard. i have a 1981 chevy c20 454 with auto trans
« Reply #2 on: June 20, 2018, 04:04:20 PM »
How hard is it shifting?  Is the engine revving up more than usual at part throttle?  If it is shifting pretty hard at cruise and the engine is revving up higher than normal at part throttle, perhaps you have a leak in the vacuum line running to the vacuum modulator on the tranmission or it is disconnected altogether?
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6

Offline sammysamm

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Re: Tranny shifts hard. i have a 1981 chevy c20 454 with auto trans
« Reply #3 on: February 21, 2019, 09:54:14 PM »
This problem. Was solved .mechanic just adjusted shifter cable.. or whatever. This truck shifts smooth as a baby.