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2017 CT6 8-Speed Transmission Harsh Shifting - TSBs?

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Dan C. · rep 0 · May 22, 4:01 AM · 1 replies · solved · Question

Vehicle: 2017 Cadillac CT6 Luxury

My 2017 Cadillac CT6 Luxury, with the 3.6L V6 and 8-speed automatic, has developed some really harsh shifts, especially when going from 1st to 2nd and 2nd to 3rd gear. It feels like a jolt, particularly when the engine is cold, but it's noticeable even when warm.

I've heard about issues with GM's 8-speed transmissions. Is there a known TSB (Technical Service Bulletin) for this specific problem on the 2017 CT6? I'm concerned about the long-term reliability of the transmission and want to know if this is something Cadillac acknowledges and has a fix for.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 4:01 AM

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ClearTheCode admin answer: Harsh or erratic shifting in GM's 8-speed automatic transmissions (8L45/8L90) is a well-documented concern across various models, including your 2017 Cadillac CT6. This issue often relates to fluid degradation, torque converter shudder, or software calibration.

  1. Transmission Fluid: One of the primary causes identified is the original transmission fluid (Dexron HP) breaking down and causing friction material to clog the valve body. GM has released a revised fluid (Mobil 1 Synthetic LV ATF HP) designed to address this. A transmission fluid flush and refill with the new fluid is often the first recommended step.
  2. Software Updates/Relearn: There are often software updates available for the Transmission Control Module (TCM) that can improve shift quality. After a fluid change or software update, a transmission relearn procedure might be necessary to allow the TCM to adapt to the new conditions.
  3. Technical Service Bulletins (TSBs): Yes, there have been TSBs related to 8-speed transmission shift quality. For example, TSB 18-NA-160 (and its revisions) specifically addresses shudder and harsh shifts in these transmissions, often recommending the fluid exchange. Your dealership can check for the most current and relevant TSBs for your specific VIN.
  4. Torque Converter: In more severe or persistent cases, the issue could stem from the torque converter itself, which might require replacement. This is typically a last resort after fluid and software solutions have been exhausted.

When to See a Shop: You should schedule an appointment with a Cadillac dealership or a transmission specialist as soon as possible. They can:

  • Scan for diagnostic trouble codes (DTCs) such as P0700 (Transmission Control System Malfunction) or P0716 (Input Speed Sensor Circuit Range/Performance), though these may not always appear for harsh shifts.
  • Check for applicable TSBs and recall information.
  • Perform the recommended transmission fluid exchange with the updated fluid.
  • Apply any available software updates and perform a relearn procedure.

Addressing this issue promptly can prevent further wear on transmission components and maintain the reliability and longevity of your vehicle's drivetrain.

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