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2018 Sierra 1500 Transmission Shudder and Hard Shifts

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Mark S. · rep 0 · May 25, 2:36 AM · 1 replies · solved · Question

Vehicle: 2018 GMC Sierra 1500 SLT

Hey everyone,

I've got a 2018 GMC Sierra 1500 SLT with the 5.3L V8 and the 8-speed automatic transmission. Lately, I've been experiencing some really frustrating issues. When accelerating, especially from a stop or at low speeds, the truck often shudders and feels like it's struggling to find the right gear. It's not smooth at all.

Sometimes, the shifts are really harsh, almost like someone bumped the truck from behind. It's most noticeable between 1st and 2nd, and then again between 3rd and 4th. There's no check engine light on yet, but it's definitely concerning. I've heard about these 8-speed transmissions having issues. Has anyone else dealt with this? What was your fix?

I'm worried about potential long-term damage if I don't address this soon. Any advice or shared experiences would be greatly appreciated before I take it to the dealer.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 2:36 AM

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ClearTheCode admin answer:

Transmission shudder and hard shifts in your 2018 GMC Sierra 1500, particularly with the 8-speed automatic, are commonly reported issues. Here's a breakdown of potential causes and steps:

  1. Transmission Fluid Degradation/Contamination:

    • Cause: The original transmission fluid (often DEXRON-HP) in some 8L90/8L45 transmissions was prone to breaking down prematurely, leading to friction material wear and shudder. Contamination can also occur.
    • Diagnostics: A common fix involves a transmission fluid flush and refill with a newer, improved fluid like Mobil 1 Synthetic LV ATF HP or AC Delco DEXRON-VI. This can often resolve the shudder by improving lubrication and reducing friction.

  2. Torque Converter Issues:

    • Cause: A failing torque converter can cause shuddering, especially under light acceleration or at specific speeds, as it struggles to lock up properly.
    • Diagnostics: This often requires more in-depth diagnosis and potentially replacement of the torque converter. If the fluid flush doesn't help, this is a strong candidate.

  3. Transmission Control Module (TCM) Software:

    • Cause: Sometimes, the issue can be related to outdated or faulty software within the TCM, which controls shift points and clutch engagement.
    • Diagnostics: A dealership can check for and apply any available software updates or perform a transmission relearn procedure.

  4. Valve Body or Solenoid Problems:

    • Cause: Internal components like the valve body or solenoids can wear out, leading to inconsistent fluid pressure and erratic shifting.
    • Diagnostics: These issues are more complex and typically require professional transmission repair.

When to See a Shop:

  • Immediately if you notice any new warning lights (like a Check Engine Light, which might display codes such as P0700 for a transmission control system malfunction), or if the shuddering or hard shifts worsen significantly.
  • If a transmission fluid flush and refill with the correct fluid doesn't resolve the issue within a few hundred miles.
  • If you hear any new grinding, clunking, or whining noises from the transmission area.

Addressing this promptly can prevent more extensive and costly transmission damage down the road.

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