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2018 Sierra 1500 8-Speed Transmission Hard Shifting & Hesitation

0

Kevin B. · rep 0 · May 22, 3:39 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 harsh shifts, especially from 1st to 2nd gear, and sometimes from 2nd to 3rd. It feels like it's slamming into gear, and there's a noticeable hesitation before it finally engages.

It's got about 75,000 miles on it. I've heard these 8-speeds can be a bit finicky. The check engine light isn't on yet, but the shifting is definitely getting worse. Has anyone else dealt with this? Any ideas on what might be causing it or what I should check first?

Thanks for any help!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 3:39 AM

0

ClearTheCode admin answer:

It sounds like you're experiencing common issues with the GM 8L90 (8-speed) transmission, which has been known to have some quirks. Here's a breakdown of potential causes and steps:

  1. Fluid Degradation/Contamination: The original transmission fluid (Dexron HP) in some early 8-speed units was prone to breaking down, leading to shuddering and harsh shifts. This is often referred to as 'torque converter shudder'.
  2. Torque Converter Issues: A failing torque converter can cause shuddering, hesitation, and hard shifts as it struggles to transfer power efficiently.
  3. Valve Body/Solenoid Problems: The valve body controls fluid flow to engage gears. Worn solenoids or issues within the valve body can lead to erratic shifting.
  4. Transmission Control Module (TCM) Software: Sometimes, a simple software update for the TCM can resolve shifting anomalies.

Diagnostics & When to See a Shop:

  • Check for Codes: Even without a Check Engine Light, a professional scan tool can often pull 'pending' or 'history' codes from the Transmission Control Module (TCM). Look for codes like P0700 (Transmission Control System Malfunction) or specific gear ratio errors.
  • Fluid Inspection: Have a qualified technician inspect the transmission fluid. They'll check for color (should be reddish, not dark brown or black), smell (burnt smell indicates overheating), and metal particles.
  • Fluid Flush & Refill: If fluid degradation is suspected, a full transmission fluid flush using the updated Mobil 1 Synthetic LV ATF HP fluid (or equivalent) is often recommended, along with a new filter. This can sometimes resolve the issue, especially if caught early.
  • Torque Converter Replacement: If the fluid change doesn't help, or if significant shudder persists, the torque converter may need replacement. This is a more involved repair.
  • Professional Diagnosis: Given the complexity and potential cost, it's highly recommended to take your Sierra to a reputable transmission specialist or a GMC dealership. They have specialized diagnostic tools and experience with these transmissions to accurately pinpoint the root cause.
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