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2018 Sierra Denali 8-Speed Transmission Shifting Rough - Is this normal?

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Chris P. · rep 0 · May 22, 3:48 AM · 1 replies · solved · Question

Vehicle: 2018 GMC Sierra 1500 Denali

Hey everyone,

I've got a 2018 GMC Sierra 1500 Denali with the 6.2L engine and the 8-speed automatic transmission. Lately, I've noticed some really harsh and clunky shifts, especially between 1st and 2nd gear, and sometimes when downshifting. It feels like a jolt, and it's getting pretty annoying.

I bought it used a few months ago, and it wasn't doing this initially. I've heard about some issues with these 8-speed transmissions. Is this a common problem? Should I be worried about long-term reliability?

What should I tell the dealership when I take it in? Any specific TSBs or known fixes I should reference? I want to make sure they don't just tell me it's 'normal operation.'

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 3:48 AM

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Hello Chris,

Rough shifting in the 8-speed automatic transmission (8L90/8L45) in 2016-2019 GM trucks and SUVs, including your 2018 GMC Sierra 1500 Denali, is a widely reported issue. It's often referred to as 'transmission shudder' or 'harsh shifts' and is not considered normal operation.

Here's what you should know and do:

  1. Common Causes:

    • Torque Converter Issues: A common culprit is the torque converter, which can degrade the transmission fluid prematurely.
    • Fluid Degradation: The original transmission fluid (Dexron HP) in some of these units was prone to breaking down, leading to friction and shudder.
    • Software Calibration: Sometimes, updated transmission control module (TCM) software can help mitigate the issue.
    • Valve Body Problems: Less common, but internal valve body wear can also contribute.

  2. Potential Solutions & TSBs:

    • Fluid Flush & Refill: GM released TSB 18-NA-355 (superseding earlier TSBs) which recommends flushing the transmission fluid and refilling with a new, improved fluid (Mobil 1 Synthetic LV ATF HP or ACDelco Dexron ULV). This often resolves the shudder for many owners.
    • Torque Converter Replacement: If the fluid flush doesn't work, or if the shudder is severe, a torque converter replacement might be necessary.
    • Software Update: Ensure your dealer checks for the latest TCM calibration updates.

  3. When to See a Shop:

    • You should take your Sierra to a certified GMC dealership or a reputable transmission specialist as soon as possible. Continuing to drive with severe shudder can potentially cause further damage to the transmission.
    • Mention the specific symptoms (harsh shifts, especially 1-2 and downshifts) and ask them to check for TSB 18-NA-355.
    • While not always present, a check engine light with codes like P0700 (Transmission Control System Malfunction) or P0716 (Input/Turbine Speed Sensor Circuit Range/Performance) could indicate a more severe issue, but often the shudder occurs without codes initially.
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