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GM launched the Hydra-Matic 8L90 in 2015 and the smaller 8L45 alongside it. They share a design language, an eight-speed architecture with four simple planetary sets, and unfortunately they share the same torque converter shudder reputation. What they do not share is size, torque capacity, or interchangeable hard parts.
People conflate them constantly, buy the wrong rebuild kit, and then find out at the parts counter. The short version: 8L45 is the light-duty box, 8L90 is the heavy-duty box. If your engine makes more than about 400 lb-ft, GM did not put an 8L45 behind it.
Side by Side
| Spec | 8L45 | 8L90 |
|---|---|---|
| RPO code | M5N | M5U |
| Class | Light duty | Heavy duty |
| Max input torque | ~407 lb-ft | ~553 lb-ft |
| Approx dry weight | ~190 lbs | ~230 lbs |
| 1st gear ratio | 4.62 | 4.56 |
| 8th gear ratio | 0.67 | 0.65 |
| Reverse | 2.95 | 3.82 |
| Overall ratio spread | ~6.9:1 | ~7.0:1 |
| Fluid | DEXRON HP / Mobil 1 Synthetic LV ATF HP | DEXRON HP / Mobil 1 Synthetic LV ATF HP |
| Approx total fill | ~9.0-9.7 qt | ~10.0-11.7 qt |
| Drain and fill volume | ~4.5-5.0 qt | ~5.0-6.0 qt |
| Hard parts interchange | No. Different case, different clutch pack counts, different converter. | |
The gear ratios differ only slightly, which is why the two feel similar to drive. The reverse ratio is the giveaway: the 8L90 has a much deeper reverse, which is part of why it backs a trailer up a ramp so much more willingly than an 8L45-equipped truck.
Which Vehicles Got Which
8L45 (RPO M5N)
- Chevrolet Camaro with the 3.6L LGX V6 and 2.0L turbo LTG
- Chevrolet Colorado and GMC Canyon 3.6L V6
- Silverado 1500 and Sierra 1500 with the 4.3L V6 and some 5.3L V8 builds
- Cadillac CT4 and ATS applications
8L90 (RPO M5U)
- Silverado 1500 and Sierra 1500 with the 6.2L L86 and many 5.3L L83 builds
- Chevrolet Tahoe, Suburban, GMC Yukon, Cadillac Escalade
- Camaro SS 6.2L LT1
- Corvette C7 (as the 8L90E)
- Cadillac CTS-V and CT6 V-Sport applications
Note the overlap: the 5.3L trucks got both, depending on year and trim. That is exactly why you cannot order parts based on engine size alone.
How to Identify Which One You Have
Three reliable methods, in order of speed.
- RPO sticker. Open the glovebox on trucks and SUVs, or look at the spare tire well on cars. Find the block of three-character codes. M5N is the 8L45. M5U is the 8L90. M5T, if you see it, is the later 8L80. This takes 30 seconds and is definitive.
- Transmission ID tag. Stamped or barcoded on the case, usually on the passenger side above the pan rail. It carries the model designation directly.
- Pan bolt count and size. Useful when the tag is unreadable and the RPO sticker is gone. The 8L90 case is visibly longer and the pan is noticeably wider than the 8L45. If you have both in front of you the difference is obvious; from underneath a single vehicle it is not, so use the RPO sticker.
Do not identify by VIN decoder alone on the 5.3L trucks. Mid-year production changes make VIN-only lookups unreliable for this specific pair.
Shared Problem: The Shudder
Both units built a reputation for a torque converter clutch shudder that owners describe as driving over rumble strips, typically between 25 and 80 mph under light throttle. It became the subject of a well-publicized class action covering GM 8-speeds. It is not a subjective complaint; it is a real, repeatable lock-up clutch behavior.
GM's own fix path went through several revisions before landing on a fluid change as the first-line repair. The original DEXRON HP fill was replaced by Mobil 1 Synthetic LV ATF HP (GM part number 19417577 per quart), and a large share of shudder complaints resolve with a complete fluid exchange to that spec.
The correct shudder repair order
- 1. Complete fluid exchange to Mobil 1 Synthetic LV ATF HP. Not a drain-and-fill. The converter holds several quarts of the old fluid, and a single drain leaves half the problem in the vehicle.
- 2. Check for and apply the current TCM calibration. GM revised converter clutch apply strategy multiple times, and an out-of-date TCM will re-create the shudder in fresh fluid.
- 3. If it survives both, the converter itself is the repair. At that point the valve body should be inspected for the debris the converter has been shedding.
Skipping step 2 is the most common shop mistake. New fluid plus an old calibration buys about six months.
Other Common Failures on Both
- Harsh 2-3 and 4-5 shifts. Usually valve body bore wear or a sticking pressure control solenoid. Fluid contaminated with converter clutch material accelerates this.
- Clunk on deceleration or at a stop. Frequently traced to the torque converter and the low-speed lock-up strategy rather than to driveline slack.
- TEHCM faults. The control module is integrated into the valve body assembly, so a module failure is a valve body assembly job, not a standalone module swap on most builds.
- Fluid contamination from the converter. This is the root cause behind a large share of valve body failures on both units. If you replace a valve body without replacing the converter that contaminated it, you will do the job twice.
- Line pressure faults. Codes in the P0700, P0741, P0776, P0796 and P2714 families are common on both.
The 8L45 fails earlier when it is worked hard, because it is the smaller box. A Colorado towing near its rated limit is asking more of an 8L45 than a Silverado 6.2 towing the same trailer asks of an 8L90.
Repair Cost Comparison
Independent shop labor at $120-175/hr. Dealer pricing runs well above the top of these ranges.
| Repair | 8L45 | 8L90 |
|---|---|---|
| Full fluid exchange (shop) | $300-480 | $350-550 |
| Pan, filter, fluid (DIY parts) | $160-260 | $190-300 |
| TCM recalibration | $120-220 | $120-220 |
| Valve body / TEHCM assembly | $900-1,500 | $1,000-1,700 |
| Torque converter replacement | $1,600-2,500 | $1,900-2,900 |
| Full rebuild | $3,000-4,200 | $3,600-5,000 |
| Reman unit installed | $4,200-5,800 | $4,800-6,800 |
Get the fluid spec right and half of this table never applies to you. Both transmissions are far more sensitive to fluid condition than the 6L80 that came before them.
Mobil 1 Synthetic LV ATF HP
Fluid exchange adapter kits
Which Would You Rather Have?
If you are cross-shopping two used trucks and one has an 8L90, take the 8L90. It has more torque capacity, a deeper reverse, more clutch surface area, and better parts availability in the aftermarket. The rebuild costs more, but it also takes longer to get there.
The exception is the light vehicle case. An 8L45 in a V6 Camaro or a Canyon that never tows is operating well inside its design envelope and will be perfectly reliable, assuming the fluid is right and the calibration is current. The 8L45 gets its bad reputation from 5.3L trucks that tow, which is the application where it is closest to its limit.
Neither one tolerates neglected fluid. If you take one thing from this page: service both of these on a real interval, and use the HP spec fluid.
Need Parts? Ships Same-Day
Filters, pan kits, solenoids, valve body components and overhaul kits for GM 8-speeds. Tell us your RPO code and we will match the right part. Same-day ship before 2pm Eastern.
Shop Gearbox Insider →FAQ
Can I swap an 8L90 in place of an 8L45?
Not as a bolt-in. Case length, mounting, driveshaft length, wiring and TCM calibration all change. It is a full engineering job, not a parts swap.
Do 8L45 and 8L90 share a rebuild kit?
No. Clutch counts, drum sizes and seal dimensions differ. Order by RPO code, not by "GM 8-speed."
How often should I change the fluid?
GM's severe-service schedule is the one to follow, and 45,000 to 60,000 miles is a realistic real-world interval on both. Trucks that tow should be closer to 45,000.
Is the shudder covered under warranty?
It depends on mileage, model year and the terms of the class settlement that applies to your vehicle. Start with a dealer service history check against the applicable bulletins before paying out of pocket.
Which one is behind the 5.3L?
Both, depending on year and trim. This is the exact pair that gets misidentified most. Check the glovebox RPO sticker for M5N versus M5U.