[8L90] 5th gear clunk

NWI Denali · November 24, 2020

Ever since my truck was new, it has always had a clunk shifting into 5th gear at light throttle at neighborhood speeds right at 24-25 mph. It is more pronounced when cold and less intrusive when the truck and trans fluid warms up. I can drive around it somewhat if I am heavier on the gas (>2k rpms at the 4-5 shift point.)

I've had the valve body replaced for other reasons. It was done in 2018 before the new mobil 1 atf. This 5th gear clunk has always been there before the dealer did trans work. Wondering if the new fluid would help me or if a hard relearn would help? Anyone else have this clunk?

lucas287 · November 24, 2020

This doesn't surprise me. It's a "clunky" transmission in general. I can induce driveline "clunk" at any speed and any gear. It's exaggerated and influenced mostly by aggressive DFCO (deceleration fuel cutoff). Add leaf springs, 6 ft of driveshafts and you end up with even more clunkiness. You go from positive torque under throttle to negative torque back to positive and it feels really clunky. I've experimented by disabling DFCO (via HP Tuners) and it ends up being much smoother.

Gregory Krisa · November 24, 2020

This doesn't surprise me. It's a "clunky" transmission in general. I can induce driveline "clunk" at any speed and any gear. It's exaggerated and influenced mostly by aggressive DFCO (deceleration fuel cutoff). Add leaf springs, 6 ft of driveshafts and you end up with even more clunkiness. You go from positive torque under throttle to negative torque back to positive and it feels really clunky. I've experimented by disabling DFCO (via HP Tuners) and it ends up being much smoother.

Are you sure that's not a worn out u joint pretty typical symptoms.

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NWI Denali · November 25, 2020

My clunk is most certainly in the actual shift to 5th. Not driveline slop. It almost kinda sounds like someone who isn't smooth driving a manual trans.

Gregory Krisa · November 25, 2020

My clunk is most certainly in the actual shift to 5th. Not driveline slop. It almost kinda sounds like someone who isn't smooth driving a manual trans.

Right I was just commenting lucas I have no clue what your issue is. It does seem more mechanical I don't think it's a solenoid issue but could be.

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M1ck3y · November 25, 2020

Is this a 6 or 8 speed? If its been doing it since new one might think it has something to with the assembly or parts used for assembly. But a clutch relearn sounds like a good place to start. Usually you hear about 1-3 being a problem, the higher gears not so much.

lucas287 · November 25, 2020

4 hours ago, Gregory Krisa said:

Are you sure that's not a worn out u joint pretty typical symptoms.

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It very well could be! I'm still under CPO warranty and will try to get GM to replace em if not I have one of the most sought after driveshaft shops in San Antonio I can go to and have them replace and rebalance.

NWI Denali · November 25, 2020

16 hours ago, M1ck3y said:

Is this a 6 or 8 speed? If its been doing it since new one might think it has something to with the assembly or parts used for assembly. But a clutch relearn sounds like a good place to start. Usually you hear about 1-3 being a problem, the higher gears not so much.

8 speed. I do get the harsh 1-2 upshift and harsh 3-2 downshift sometimes as well.