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2018 Sierra 1500 8-Speed Transmission Shifting Hard and Clunking

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Ryan M. · rep 0 · May 22, 3:37 AM · 1 replies · solved · Question

Vehicle: 2018 GMC Sierra 1500 SLT

Hey everyone, I'm having some serious issues with the transmission in my 2018 GMC Sierra 1500 SLT, 5.3L V8. Lately, it's been shifting really harshly, especially between 1st and 2nd gear, and sometimes from 2nd to 3rd. It feels like a hard jolt or a clunk, and it's really noticeable when I'm accelerating from a stop or in stop-and-go traffic.

I've heard about some common problems with these 8-speed transmissions. Is this something others have experienced? The truck only has about 70,000 miles on it, and I've kept up with all the regular maintenance.

Sometimes, it even feels like it's slipping a bit before it finally engages the next gear. There's no check engine light on yet, but I'm worried this could lead to bigger problems down the road. Any advice on what I should check or if there's a known fix for this?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 3:37 AM

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ClearTheCode admin answer:

Hard shifting and clunking in the 8-speed transmission (8L90/8L45) in 2016-2019 GMC Sierra 1500 models is a widely reported issue. This is often attributed to a few common causes:

  1. Transmission Fluid Degradation: The original transmission fluid used in these units can break down prematurely, leading to shuddering and harsh shifts. GM has issued Technical Service Bulletins (TSBs) recommending a triple flush procedure with a specific Mobil 1 Synthetic LV ATF HP fluid to address this.
  2. Torque Converter Issues: A faulty torque converter can cause shuddering, especially at lower speeds, and contribute to harsh shifts. This is a more involved repair.
  3. Valve Body/Solenoid Problems: The valve body controls fluid flow within the transmission, and issues with its solenoids can lead to incorrect pressure and shifting problems.

What to do:

  1. Check for TSBs: Visit a GMC dealership or a reputable independent shop that specializes in GM vehicles. They can check for any active TSBs related to your specific VIN and transmission type. Many owners have found relief with the recommended fluid flush.
  2. Fluid Flush: If a TSB recommends it, have the transmission fluid professionally flushed and replaced with the specified fluid. This is not a simple drain-and-fill; it requires specialized equipment.
  3. Diagnostic Scan: Even without a Check Engine Light, a professional scan tool can often pull 'pending' or 'history' codes (like P0700 for transmission control system malfunction) or provide live data that can help diagnose the issue.

When to see a shop:

You should see a qualified mechanic or GMC dealership as soon as possible. Continuing to drive with harsh shifts can accelerate wear on other transmission components, potentially leading to a more expensive repair. Early intervention, especially with the fluid flush, can often prevent further damage.

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