GM 8L45 and 8L90 Shudder: The Class Action, the TSB, and the Real Fix

GM 8L45 and 8L90 Shudder: The Class Action, the TSB, and the Real Fix

If you own a 2015-2019 Silverado, Sierra, Escalade, Yukon, Camaro, Corvette, Colorado, Canyon, ATS, CTS or CT6 with the eight-speed automatic behind it, you already know the feeling: a low-speed vibration around 25 to 45 mph that feels like driving over rumble strips, a hard clunk on the 2-3 shift, or a long hesitation off the line followed by a lurch. Owners call it the shudder. Shops call it a torque converter clutch problem. Lawyers call it a class action. All three are describing the same GM 8L45 and 8L90 defect, and there is a real repair path once you understand what is failing.

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What owners are actually reporting

The complaints are remarkably consistent. Vehicles "shake, jerk or hard shift" during acceleration and deceleration, with one owner describing a shift so violent it felt like being hit by another car, per the case summary published by ClassAction.org. Others report hesitation followed by shuddering, jerking, clunking and vibration at highway speeds. A second filing, Ulrich, et al. v. General Motors (No. 2:24-cv-11007, E.D. Mich., filed April 17, 2024), describes vehicles that "suddenly lurch forward, shudder, and experience significant delays in acceleration," plus difficulty downshifting under braking.

Two things matter here for diagnosis. First, the shudder is load-sensitive: it shows up under light throttle at converter-lockup speeds, not at idle and usually not under wide-open throttle. Second, it is progressive. It starts as a faint buzz and gets worse over months. That progression is the clue to which repair you actually need.

Where the litigation stands

The transmissions at issue are the GM Hydra-Matic 8L45 and 8L90 in 2015-2019 model years, covering Chevrolet Silverado, Colorado, Corvette and Camaro; Cadillac Escalade and Escalade ESV, ATS, ATS-V, CTS, CTS-V and CT6; and GMC Sierra, Yukon, Yukon XL, Yukon Denali and Canyon. In August 2024 the 6th U.S. Circuit Court of Appeals declined to reverse class certification, and the certified class spans owners in 26 states (Duffy, et al. v. General Motors, Case No. 2:19-cv-11875-AC-MKM). GM petitioned for rehearing in September 2024.

The detail that matters most to a wrench-turner is buried in the Cohen Milstein filing summary: GM developed an internal transmission flush remedy referred to as "Mod1a" in 2018, and the complaint alleges the company applied it selectively rather than across roughly two million affected vehicles. Whatever a court eventually decides, the engineering takeaway is plain. The first-line factory remedy for this defect is a fluid procedure, not a teardown. That is where you should start too.

What is actually failing inside the transmission

There are two separate mechanisms, and they stack.

1. Fluid friction stability at the torque converter clutch. The 8L45 and 8L90 run the converter clutch in a controlled-slip mode almost constantly to keep engine speed down. That means the TCC friction material is slipping on purpose, all the time, and the ATF is what keeps that slip smooth. As the fluid's friction modifiers deplete, the slip turns from smooth to stick-slip, and stick-slip is what you feel as shudder. Transmission Digest reports that GM has issued multiple technical service bulletins on TCC shudder in the eight-speed 8L45 and 8L90, and that the problem touches over four million vehicles across 11 models and three GM brands in model years 2015 through 2020. GM's own remedy is a full fluid exchange using 20 quarts of a specific full synthetic ATF, not a drain-and-fill.

2. Converter feed limit valve bore wear in the valve body. This is the one most owners never hear about. Per Sonnax product information published by Transmission Digest, bore wear or a stuck converter feed limit valve in the 8L45/8L90 can cause converter overheat, incorrect TCC apply or release, inadequate lube and cooling, and converter over-pressurization. Read that list again: every one of those symptoms overlaps with "bad fluid." The fix is to ream the bore and install an oversized valve kit (Sonnax kit no. 154740-22K, with tool kit no. F-154740-TL22). Superior also markets a shudder kit for these units, part no. STL-8L90, which the company says installs from the bottom without removing the transmission in most cases.

Here is the practical rule: if the fluid exchange fixes it and it stays fixed, you had a fluid problem. If the fluid exchange fixes it for a few thousand miles and the shudder returns, you have a bore-wear problem and you are cooking the converter every time you drive. If the exchange does nothing at all, the TCC lining is already gone.

The fix, in the order you should attempt it

Work cheapest to most invasive. Skipping steps on this transmission wastes money.

  • Step 1 - Full fluid exchange, not a pan drop. A pan drop leaves the majority of the old fluid sitting in the converter and cooler lines, which is exactly where the depleted fluid does its damage. Use the correct GM-specified full synthetic ATF and exchange the full volume. Test drive at 25-45 mph under light throttle before declaring victory.
  • Step 2 - Friction modifier, if you want insurance. In a LubeGard-sponsored test published by Transmission Digest, friction and steel specimens lubricated with the GM-recommended ATF alone still exhibited shudder and slippage on a Falex MultiSpecimen machine; adding an ATF protectant at two ounces per quart eliminated it and held fluid friction stable through the test. Treat that as vendor test data, not an OEM position, but the mechanism is sound and the cost is trivial next to a converter.
  • Step 3 - Address the converter feed limit valve. If the shudder returns, the valve body comes off. Ream the bore and install the oversized valve kit. This is the step that makes the repair permanent instead of seasonal.
  • Step 4 - Converter and clutch packs. If the shudder has been present for a long time, the TCC lining is glazed or gone and no fluid will bring it back. At that point you are into a converter and, if you found debris in the pan, a clutch pack refresh.
8L90 Raybestos GPZ friction clutch pack for 2015-up GM eight-speed transmissions
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If you find the pan loaded with friction material, do not put fresh clutches into a dirty unit. The 8L90 Raybestos GPZ friction clutch pack is the right starting point for a 2015-up rebuild because GPZ lining holds up to the constant controlled slip these units live on. The rest of what you will need is in the 8L90 transmission parts catalog.

Replace while you are in there

If the transmission is down far enough to pull the valve body or the converter, these are the parts you do not want to leave alone:

  • Filter and pan gasket - non-negotiable any time the pan comes off.
  • Converter feed limit valve kit - the actual root-cause part, and the one you cannot reach later without doing the job twice.
  • Friction and steel clutch plates - if debris is in the pan, the clutches have already lost material.
  • Cooler line flush or cooler replacement - metal shavings that survive in the cooler will kill the new converter. If the old converter came apart, the cooler is contaminated.
  • Front and rear seals - cheap, and you are already there.

Cost and effort, honestly

A full fluid exchange is a one to two hour job for a shop with the right equipment, and it is realistically a driveway job only if you have a way to move 20 quarts through the cooler circuit. Pulling the valve body to address the feed limit valve is a four to six hour job at intermediate difficulty; the reaming step requires the correct tool kit and a steady hand, and getting it wrong ruins the valve body. A converter and clutch pack job means the transmission comes out, which is a full day plus for an experienced DIYer with a lift and a trans jack, and is not a first-time-rebuilder project. Related reading: GM 8L45/8L90 harsh shifts, flare and burnt clutches covers the accumulator side of these same units.

FAQ

Does the class action mean GM will fix my truck for free? No. Class certification is a procedural ruling about whether the case can proceed as a group, not an order that GM repair vehicles. Litigation status changes; check the docket or the class notice before assuming coverage, and do not delay a repair that is destroying your converter while you wait.

Will a plain drain-and-fill stop the shudder? Usually not for long. A pan drop replaces only part of the total volume, and the converter holds a large share of the fluid that has actually lost its friction modifiers. GM's own remedy calls for a full exchange for that reason.

How do I tell fluid shudder from a bad converter feed limit valve? By how long the repair lasts. Fluid-only shudder stays fixed after a proper exchange. Bore-wear shudder comes back, typically within a few thousand miles, because the valve is still failing to control converter feed pressure and the fluid is being overheated all over again.

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