8L90 Shudder Fix: GM TSB 18-NA-355, Additives, And Valve Body Repair

Quick Answer

• What It Is: 8L90 shudder is torque converter clutch (TCC) shudder at steady cruise between 25 and 80 mph. GM covers it in TSB 18-NA-355.
• How To Confirm It: Run GM's three-mode test. The vibration must be present in normal operation and gone with the TCC commanded open and commanded locked.
• How To Fix It: Three paths: a full exchange to blue label Mobil 1 LV ATF HP, a LubeGard additive, or valve body repair.

A 2015–2019 GM truck, Corvette, or Cadillac comes in with a vibration at steady cruise. The customer says it feels like rumble strips or rough expansion joints.

Your first suspect is 8L90 shudder from the torque converter clutch (TCC). GM has documented it, and it hits the 8L45 too.

The Technical Service Bulletin for GM 8-speed transmission shudder is TSB 18-NA-355. GM's correction is a complete fluid exchange to the blue label Mobil 1 Synthetic LV ATF HP. That is one of three fixes, and it is the wrong one when the vibration is not TCC shudder.

Diagnose first. Then pick the path.

What TSB 18-NA-355 Covers

Condition: a shake or shudder during light-throttle acceleration between 25 and 80 mph (40–128 km/h) at steady speed, when the transmission is not shifting.

Cause: torque converter clutch shudder.

Correction: a model-specific fluid exchange to the blue label HP fluid.

GM adds two cautions. First: "Sometimes shudder is not caused by the torque converter clutch (TCC). In some instances, shudder is fish bite, chuggle, surge or vibration."

Second, the procedure is done once per vehicle. A unit that comes back shuddering after a correct exchange needs real diagnosis, not a second exchange.

The bulletin applies to vehicles sold in the United States, Canada, and the Middle East. Mexico and export markets follow bulletin 19-NA-018.

Vehicle Compatibility

Vehicles built after GM's 2019 production breakpoints shipped with HP fluid. On those, follow normal diagnostics instead of the bulletin.

Diagnose First: The Three-Mode Test

GM published this test in the bulletin through May 2019, then removed the diagnostic section that July. Run it anyway. It is the step that keeps you from selling the wrong repair.

Drive on a smooth road with sump temperature between 122°F and 158°F (50–70°C). On full-size trucks and SUVs, hold the tow/haul button for 5 seconds to disable grade braking. Then run three modes with a scan tool:

• Mode A — normal operation. The vibration must be present here. If it is not, you are not chasing TCC shudder.
• Mode B — TCC commanded disabled (open).
• Mode C — TCC commanded enabled (locked).

If the vibration is present in A and gone in both B and C, it is TCC shudder. If it stays in B or C, the root cause is something else. No fluid or additive will fix it.

Read TCC Slip As A Pattern, Not A Number

Healthy: consistent TCC slip, typically 5 to 40 RPM of variation. Some passenger cars and light-duty trucks run 20–30 RPM of slip, and that is normal.

Shudder: an erratic saw-tooth trace where slip climbs as much as 100 RPM and drops back to near zero.

A steady 25 RPM of slip is a healthy transmission. Do not condemn it.

On a scope, TCC shudder holds a stationary frequency in 8th gear. If the frequency tracks vehicle or engine speed, it is not TCC shudder.

Three Ways To Fix 8L90 Shudder

Once the three-mode test confirms TCC shudder, the right fix depends on the vehicle, its history, and who is paying.

Path 1: The TSB Fluid Exchange

A drain and fill will not do this job. Service refill is roughly 3.5 quarts against a total capacity of about 11 quarts. A pan service leaves about two thirds of the old fluid in the converter, cooler, and lines.

GM's exchange runs through the cooler lines with the DT-45096 TransFlow. The quantity depends on the vehicle:

• Silverado, Sierra, Colorado, Canyon, Yukon Denali, and Escalade: 20 quarts loaded, then 2 more quarts through the cooler and lines — one in each direction.
• Camaro, ATS, and CTS: 20 quarts, plus 4 more on cars with a rear differential cooler.
• Corvette and CT6: a drain and fill performed three times, from 24 quarts.

Fluid: blue label Mobil 1 Synthetic LV ATF HP, GM p/n 19417577 (US) / 19418066 (Canada). Not DEXRON VI.

GM is explicit: intermixing other fluids or aftermarket additive packages during this procedure "will not provide satisfactory results." If you are doing the TSB, do the TSB as written.

It is a controlled exchange, not a high-pressure flush. GM warns that removing more than 3 quarts at a time can cause pump cavitation.

With the pan down, check for debris. Earlier versions of GM bulletin 16-NA-175 called for a new 8L90 filter when debris is found.

Set The Level Correctly

The 8L90 has no dipstick. Level is set at the standpipe plug in the pan:

• Transmission fluid temperature between 95°F and 113°F (35–45°C)
• Above 45°C the unit ends up under-filled; below 35°C, over-filled
• Engine running with the plug out, or you lose fluid
• Level set plug torqued to 9 N·m (80 lb-in)

On Silverado, Sierra, Yukon, and Escalade, bring the fluid to 194°F (90°C) first. That opens the thermal bypass valve and fills the cooler. Shut down, let it cool to 95–113°F, then check.

For what an over-fill does, see why overfilling your transmission can be fatal.

Improvement comes right away, but full effect can take up to 200 miles and at least two cold-to-hot drive cycles. Set that at the counter and you avoid a comeback that is not a comeback.

Path 2: Additive Treatment

Outside warranty, many owners will not pay for a 20-quart exchange. LubeGard built its GM 8-speed program for that customer.

LubeGard's published recommendation for the 8L45 and 8L90 is LubeGard Platinum ATF Protectant at 2 fl oz per quart. That is two 10 fl oz bottles per transmission.

Add it to the existing HP fluid without a flush, or after an exchange to keep shudder from returning. LubeGard reports eliminating shudder in friction-plate testing at that treat rate. Shops can stock the 12-pack.

For lock-up shudder, LubeGard's concentrated friction modifier is Instant Shudder Fixx, a 2 fl oz tube added through the fill port.

Be straight with the customer about the trade-off. On a vehicle still under GM coverage, an additive goes against the bulletin. On a high-mileage truck that is out of warranty, it is a reasonable first step. If the shudder survives it, move to Path 3.

Path 3: Valve Body Repair

If shudder is still there after a correct fluid exchange, stop treating the fluid. Look at valve body wear.

Worn pulse damper (accumulator) bores are a known 8L45 and 8L90 problem. Even light wear disturbs the solenoid signal to the clutch regulator valves, and TCC apply suffers with it.

TransGo lists its 8L90-PDP-OS oversize pulse dampers for "TCC shudder that is still present after a flush and refill with Mobil 1 LV ATF HP." The same part covers delayed reverse, 2-3 upshift flare, erratic shifts, and coast-down clunk.

They install with the 8L90-PDP-TKC carbide reamer tool kit, so the valve body does not need replacing.

For broader control problems, there is the TransGo 8L90 shift kit (SK 8L90). It targets TCC shudder apply and release issues, an overheating converter, low cooler flow, and codes P0741 and P0218.

If P0741 is stored, work through our P0741 torque converter clutch diagnosis guide before condemning the converter.

Frequently Asked Questions About 8L90 Shudder

What Causes 8L90 Transmission Shudder?

GM attributes it to torque converter clutch shudder and corrects it with Mobil 1 LV ATF HP. Worn pulse damper bores in the valve body can produce the same complaint. For shudder in general, see transmission shudder: vibration or gear problem.

Is Water In The Fluid The Cause?

Not according to GM. TSB 18-NA-355 does not mention moisture. Earlier versions of bulletin 16-NA-175 say to stop if the fluid is cloudy, milky, or contaminated with water or coolant. Do not do the exchange. Run a cooling system leak test instead.

Water means a leak to find. Read more on water in the transmission.

Can An Additive Fix 8L90 Shudder?

LubeGard publishes a treat rate for exactly this: Platinum at 2 fl oz per quart. It is a sound option for out-of-warranty units. GM does not approve additives inside the TSB procedure. On a warranty vehicle, follow the bulletin.

Should I Delete The 8L90 Thermal Bypass?

The TSB removes and reinstalls the thermal bypass block. It does not delete it.

Delete kits like the TransGo cooler flow bypass delete are an aftermarket choice for low cooler flow and heat, not a shudder fix.

Does The TSB Apply To 2019 And Later Trucks?

Only to vehicles built before GM's February or March 2019 breakpoints, depending on the model. On later builds, start with the three-mode test.

Get The Right Parts For The Job

Diagnose first. Then source the right parts.

We stock the 8L90 filter, LubeGard Platinum and Instant Shudder Fixx, the TransGo 8L90 shift kit, and TransGo pulse damper parts. For units past saving, we carry overhaul kits. See 8L90 Automatic Transmission Parts.

For more TCC diagnosis, see how to fix code P0740 and P2764 explained.

We have supplied transmission shops for 30 years, and we ship from Miami across the United States, Latin America, and the Caribbean.

Not sure which part fits your unit? Call us at +1 786 360 1516 or contact us on WhatsApp. Tell us the vehicle and what the three-mode test showed. We will tell you which parts you need — and which ones you do not.

Sources

• General Motors Service Bulletin 18-NA-355: April 2020 revision for the fluid exchange, and May 2019 revision for the diagnostic section
• General Motors Service Bulletin 16-NA-175, August 2017 version
• LubeGard: The GM 8-Speed TCC Shudder Solution
• TransGo instruction sheets: SK 8L90 and 8L90-PDP-OS