8-Speed Automatic Transmission Assembly for 2018 Chevrolet & GMC • Sierra 1500, Silverado 1500
Product Notes Read before buying
This is an 8-speed automatic transmission (RPO code M5U) for 4WD models with the 6.2L V8 engine. Some 2018 vehicles were also available with a 10-speed automatic transmission. Please verify your vehicle is equipped with the 8-speed automatic before ordering.
Voice or type your vehicle details and Wrenchy will confirm fitment in seconds.
Vehicle Fitment Guide
This part fits the following vehicles:
| SubmodelSUB- MODEL |
Body Style | Years | Engine | Attributes |
|---|---|---|---|---|
| High Country | Crew Cab Pickup | 2018 | V8 6.2L | 4WD | Automatic |
| LTZ | Crew Cab Pickup | 2018 | V8 6.2L | 4WD | Automatic |
| LTZ | Extended Cab Pickup | 2018 | V8 6.2L | 4WD | Automatic |
| SubmodelSUB- MODEL |
Body Style | Years | Engine | Attributes |
|---|---|---|---|---|
| Denali | Crew Cab Pickup | 2018 | V8 6.2L | 4WD | Automatic |
| SLT | Crew Cab Pickup | 2018 | V8 6.2L | 4WD | Automatic |
| SLT | Extended Cab Pickup | 2018 | V8 6.2L | 4WD | Automatic |
Know This Transmission Before You Buy
The research behind our purchase interview — recalls, bulletins and real install economics for this transmission family:
A quick look at your own vehicle makes sure the right transmission shows up:
- Verify RPO code M5U on the glove box sticker or driver's side door jamb QR code.
- Confirm the engine is the 6.2L V8 (8th VIN digit 'J' for L86).
- Confirm the vehicle is 4WD/4x4 (check for front driveshaft and dashboard selector switch).
- Locate the transmission tag on the passenger side of the case to match the 4-character code.
What the install really takes — plan and budget with confidence:
Special tools this job needs:
- Heavy-duty transmission jack
- GM-compatible scan tool (GDS2 / Techline Connect / SPS2)
- Torque converter depth measurement tool or calipers
- Transmission cooler line flush canister/solvent
- Fluid pump or transfer tool
Budget for these consumables at install:
- Mobil 1 Synthetic LV ATF HP (GM Part No. 19417577, 'Blue Label' / DEXRON HP)
- AC Delco TF922 Transmission Filter Kit (GM Part No. 24295812 or 24274402)
- New oil pan gasket
- Front pump/torque converter seal (GM Part No. 24268875)
- Rear output shaft seal
- Transmission cooler line flush solvent
What your vehicle’s computers will need after the swap:
What’s involved: Solenoid Valve Characterization Reprogramming (MCVM Operations) and Transmission Service Fast Learn
The GM 8L90 uses variable force solenoids that are individually calibrated. You must program the replacement transmission's unique Transmission Unique Number (TUN) into the vehicle's TCM using GM's SPS2/Techline Connect system to retrieve the solenoid characterization flow data from the GM cloud. Skipping this step causes extremely harsh shifting, slipping, and immediate internal damage. A 'Transmission Service Fast Learn' must follow.
Pro tips for a smooth first start-up — hand them to whoever does the install:
- Torque Converter Seating: The torque converter must be fully seated (recessed by approx 1 inch / 25.4mm and clicking three times) before bolting the transmission to the engine. If not fully seated, tightening the bellhousing bolts will crush and destroy the pump sprocket/chain assembly.
- Fluid Level Check Procedure: There is no dipstick. Fluid level must be checked via the standpipe on the bottom of the pan while the engine is running, the vehicle is level, and the fluid temperature is strictly within the specified range (sources differ: 95°F-113°F vs 167°F-185°F).
- Thermal Bypass Valve (TBV) Upgrade: Highly recommended to replace the stock 194°F TBV with an updated GM unit or an aftermarket low-temp bypass (PPE/Surecool) to prevent future overheating.
- Mandatory Cooler Line Flush: Debris from the old failed transmission will contaminate and destroy the replacement unit if the cooler lines and radiator are not thoroughly flushed.
Easy to get right once you know them:
- Using standard Dexron VI fluid instead of the required Mobil 1 Synthetic LV ATF HP (Blue Label).
- Failing to program the Solenoid Characterization Data (TUN/PUN) into the TCM, leading to immediate harsh shifting and clutch damage.
- Improper torque converter seating resulting in a cracked front fluid pump upon bellhousing bolt tightening.
- Checking the transmission fluid level at the wrong temperature, leading to severe overfill or underfill.
- Reusing old contaminated fluid or failing to flush the transmission cooler lines.
Not issues with the units on this page — the manufacturer documented these for every transmission of this family, wherever you buy one. Handy for your installer:
🔎 Recommended at install: Complete fluid exchange (triple flush) using updated Mobil 1 Synthetic LV ATF HP fluid ('Blue Label', GM Part No. 19417577). Severe cases require torque converter replacement.
🔎 Recommended at install: Updating TCM calibration, performing 'Transmission Service Fast Learn' adaptive relearn, or replacing valve body/solenoids with updated OEM parts.
🔎 Recommended at install: Replacing TBV with updated GM unit or installing an aftermarket low-temperature bypass kit (e.g., PPE or Surecool) opening at 158°F.
Researched from factory recalls, TSBs and OEM service data (reference unit 400-10273-5255566). Unit-specific details — mileage, condition, warranty, freight — are covered in the checkout interview.
Policies, in plain English.
Shipping & Handling
- Ships by LTL freight on a pallet — a real freight truck, not a parcel van
- Your checkout freight quote covers delivery to your door (liftgate service included for residential addresses)
- The carrier calls ahead to schedule a delivery window — someone must be present to receive it
- Typically palletized and picked up within 2–3 business days; transit time depends on distance
120-Day Warranty
This part includes a 120-day warranty.
- If the part fails within the warranty window, we'll replace it or refund per the coverage terms
- Coverage is parts-only — installation labor is not included
- Warranty covers defects — not installation errors, misuse, or damage after installation
Your pre-purchase interview reviews these exact terms with you before anything is charged.
Returns (30 Days)
We accept returns of uninstalled parts within 30 days of delivery for this part.
- Defective parts: We will replace the part or issue a refund at our discretion
- Programming: "Needs to be programmed to my vehicle" is not a valid return reason. Many modules require dealer programming — we're happy to help connect you with resources if your dealer says it's not programmable
- Return shipping: This part ships by freight (LTL) — the part price is refundable per the terms above, but shipping costs are non-refundable and return freight follows the seller's return terms. Your pre-purchase interview walks through the exact numbers before you buy
- Parts must be returned in original packaging and unmodified condition
- Poorly packaged returns that arrive damaged will not be credited
Damaged in Shipping?
Freight damage is rare — and you're fully protected when you do two things at delivery:
- Inspect before you sign — you're allowed time to look the pallet over while the driver waits
- Write any damage on the delivery receipt before signing — that one line is what makes a freight claim succeed
- Severe damage? Refuse the delivery — it goes back at no cost to you and we make it right
- Photograph the pallet before unwrapping; report concealed damage within 24 hours of delivery
We handle the carrier claim — you just help document it. Your pre-purchase interview walks through this checklist too.
Order Cancellation
Cancel before shipment for a full refund. Once the pallet has been built, a preparation charge may apply — and once the freight carrier picks it up, shipping costs are non-refundable and standard return terms apply. Your pre-purchase interview shows the exact numbers before you pay.
Expert Guides
2018 Silverado & Sierra 1500 6.2L 8L90: Problems, Identification, and Used Replacement
Identify the M5U 8-speed automatic, understand the Chevy Shudder and thermal bypass failures, and learn when to swap in a used OEM transmission.
Read Full Guide →Related Trouble Codes (OBD-II)
26 matching guidesIf your scan tool is showing one of these trouble codes, this part is a common fix. Each guide explains the code, how to diagnose it, and exactly what to replace.
Miscellaneous Information
OEM Part Numbers:
GM: 19433301, 19434876, 19540197, 24274907, 24293323
Fits Years: 2018
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This unit ships with the selling yard's 120-day warranty. Longer plans can be requested during checkout.
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