6L90 vs 722.6: Differences and Which One to Choose

These two transmissions get cross-shopped for one reason: both ended up behind big American V8s, and both show up in the same swap threads and the same salvage yards. But they are not competitors in any engineering sense. The GM 6L90 is a heavy-duty six-speed built for 3/4-ton and 1-ton trucks. The Mercedes 722.6, sold under Chrysler as the NAG1, is a five-speed passenger-car and light-truck unit that Mercedes spread across half the industry. If you are choosing between them for a swap, a rebuild, or a used-truck purchase, the differences that matter are torque rating, gear count, control strategy, and what actually breaks.

The short answer

Pick the 6L90 if you are towing, hauling, or running a GM Gen IV drivetrain and want the deepest first gear with the tallest cruise gear. Pick the 722.6 if you are working on a Mercedes, a Jeep Grand Cherokee, a Dodge Charger or Challenger, or a Chrysler 300 that already has one, or if you want a compact five-speed with an enormous parts supply. Neither one converts into the other's application without significant work.

Hard specs, side by side

GM 6L90

Per DieselHub's 6L90 specification data, the gear ratios are 4.027:1 first, 2.364:1 second, 1.532:1 third, 1.152:1 fourth, 0.852:1 fifth, 0.667:1 sixth, and 3.064:1 reverse. Max input torque is listed at 531 lb-ft. The unit runs DEXRON VI at roughly 13 quarts, weighs about 245 lb including torque converter and fluid, and uses a die-cast aluminum case. Applications include Chevrolet Silverado and GMC Sierra 2500HD/3500HD from 2007 forward, plus Chevrolet Express and GMC Savana 2500/3500 vans from 2010 forward.

The critical internal note from DieselHub: the 6L90 is a heavy-duty version of the 6L80 with one additional plate in each clutch pack. That extra plate per pack is the entire reason the 6L90 exists, and it is why a 6L80 clutch set will not correctly fill a 6L90 drum.

Novak Conversions adds the packaging details that matter for a swap: the 6L90 is 1-3/8" longer than the 6L80, the case is a cast aluminum alloy with removable bellhousing and tailhousing for interchangeability with GM 90-degree engines, and the output shaft is a larger-diameter 29-spline for most truck applications with a 36-spline variant used on certain HD trucks and 2WD vans. Novak also flags the control constraint that ends most casual swaps: the transmission control module is integrated into the valve body rather than being a separate box, reprogramming is required even when swapping between physically identical modules with different serial numbers, and functional compatibility exists only with Generation IV GM engines and their matched engine control modules.

Mercedes 722.6 / NAG1

Per the Mercedes-Benz 5G-Tronic reference, the 722.6 family was produced from 1996 to 2020 as a converter-equipped five-speed automatic. It was built in several torque-rated variants: the W5A 330 for smaller-displacement passenger cars up to 330 N·m, the W5A 580 for V8 and V12 applications up to 580 N·m (roughly 428 lb-ft), and the W5A 900 for high-output turbocharged engines. Speedshift and AMG Speedshift variants added manual mode and active downshifting starting in 2001.

The application list is what makes this unit worth knowing even if you never touch a Mercedes. Beyond the Mercedes range it went into the Jeep Grand Cherokee and Wrangler, the Dodge Charger and Challenger, the Chrysler 300, and even Porsche 911 variants. Gear ratios run 3.932:1 first, 2.408:1 second, 1.486:1 third, 1.000:1 fourth, and 0.830:1 fifth as overdrive.

What the numbers actually tell you

First gear is close: 4.027 versus 3.932. The real spread is at the top. The 6L90's 0.667 sixth is a much taller cruise gear than the 722.6's 0.830 fifth, which is why the GM unit lets a 6.0L truck turn low highway rpm while still keeping a deep launch ratio. On torque, the strongest common 722.6 variant at roughly 428 lb-ft sits below the 6L90's 531 lb-ft input rating, and the 6L90 is the one of the two specified behind heavy-duty pickups and 3500-series vans. If your use case involves a gooseneck, that gap is the whole decision.

What fails, and how differently

6L90: clutch counts and valve body hydraulics

The 6L series fails in ways that are well documented and largely hydraulic. Sonnax's no-movement analysis for the 6L45/50 and 6L80/90 lays out the pattern. A TEHCM line pressure solenoid stuck in exhaust mode holds line pressure at roughly 60 to 68 psi with no rise on throttle whatsoever. A pump pressure regulator boost valve missing its retaining roll pin drops pressure to 30 psi or less at idle. A missing pump pressure relief ball and spring, which Sonnax notes occasionally shows up in rebuilt pumps, gives no movement and no line pressure at all. A stuck pump TCC control valve blocking apply oil is usually a no-movement condition and can also produce shudder during TCC apply or when coasting to a stop.

On the valve body side, Sonnax notes the 1-2-3-4 clutch regulator valve sticking causes no Forward and is "usually discovered after the repair/reinstall," and that the 3-5-R clutch regulator valve "is also known to stick and cause no Reverse and shift complaints." Line pressure typically reads normal with these conditions, which is exactly why they get misdiagnosed as clutch failures.

Sonnax also documents the assembly traps that bite rebuilders. A Type 2 separator plate on a Type 1 valve body causes a no-Forward condition, and early Type 1 valve bodies cannot be updated to the later eight-checkball plate; the correct plate, part #24245720, uses seven checkballs. That same Type 1 plate installed on a Type 2 valve body causes no 3rd and no Reverse. A 6L90 valve body installed on a 6L80 is a no-Reverse condition. Clutch select valves or springs installed backwards cause no movement; the valves go in first with springs outboard. A shrunken or displaced No. 1 checkball is no Forward, a displaced No. 5 checkball is no Reverse, and Sonnax notes this is sometimes intermittent. Every one of those reads normal line pressure.

722.6: converter circuit leakage and cold stall

The 722.6's signature complaint is completely different in character. Sonnax's teardown analysis in Mercedes 722.6: Cold Stall/TCC Shudder traces the classic cold-stall and shudder problem to wear in the torque converter apply circuit rather than to friction material choice. The article notes that early efforts focused on replacement friction material, that both smooth and grooved surfaces were tried, and that no material proved foolproof, including at the OEM level. The 722.6 converter uses a clutch pack to provide TCC clamping force and a three-path converter oil circuit: one path for charge oil to enter the converter, one for lube oil to exit, and a third to supply TCC apply pressure during lockup or exhaust it on release.

The measured example in that article is worth memorizing. Apply oil travels through the input shaft and exits between the smaller and larger smooth diameters at the end of the shaft. The larger input shaft mating diameter measured .660", the worn cover tower bore measured .683", and a good OEM tower bore measures .662". Sonnax reports that the difference between the .002" factory-intended clearance and the .021" clearance on the worn part produced a 7.5 gallon-per-minute leak, along with both cold stall and TCC shudder. The documented fix was removing the O-ring/lathe cut seal, blocking the lube bypass passage, and installing a radial lip seal.

Continuing downstream, Sonnax identifies the turbine (input) shaft sealing rings as the next leak point, and reports shops resolving cold stall on an ML430 and an E320 AWD by replacing the early-model dark gray scarf-cut turbine shaft rings with late-model tan interlock rings. TCC piston seals were the first item scrutinized historically, with the loose-fitting OEM seals replaced by snugger aftermarket seals fixing many early shudder cases.

The practical read: on a 6L90 you chase valve body plates, regulator valves, checkballs, and clutch counts. On a 722.6 you chase converter circuit clearance, shaft sealing rings, and TCC piston seals. Rebuilding either one with a parts-cannon approach and no pressure diagnosis wastes the same money in different places.

Which one to choose

Choose the 6L90 when

• You are towing or hauling under sustained load. The 531 lb-ft input rating and the extra clutch plate per pack are the reason it exists.
• You want the widest usable ratio spread. A 4.027 first paired with a 0.667 sixth beats anything the five-speed does at either end.
• You already have a Generation IV GM engine and its matched ECM. Per Novak, that is a functional requirement, not a preference.
• You want the deeper aftermarket for hard parts, upgraded pump components, and valve body correction kits.

Choose the 722.6 when

• You are servicing a vehicle that came with one. Putting a 6L90 into a Jeep, Charger, or Mercedes chassis is a fabrication and electronics project, not a bolt-in.
• Packaging matters. The 722.6 was designed around passenger-car tunnels and is the physically smaller unit.
• You want a five-speed with a 1996 to 2020 production run behind it, which means very deep used and remanufactured supply across Mercedes, Chrysler, Dodge, Jeep, and Jaguar platforms.
• Your power level fits under the W5A 580's roughly 428 lb-ft ceiling, or you can source a W5A 900 variant.

Rebuilding either one: what to replace while you are in there

Both units punish a partial job. If the transmission is on the bench, the following go back in new regardless of which side you are on.

6L90

• All frictions and steels at the correct 6L90 count. Remember the additional plate per pack versus the 6L80. Start with a 6L90 friction clutch kit and match the steels to it.
• Full gasket and seal set, filter, and pump seals.
• Torque converter, if there is any TCC shudder history. Sonnax ties shudder during apply and shudder while coasting to a stop back to the pump TCC control valve, so air-check that valve at the same time. Sonnax notes it can be air checked in-vehicle with the valve body removed.
• Positive identification of the separator plate as Type 1 or Type 2 before reassembly, given the documented mismatch failures.

A complete 6L90 master rebuild kit with overhaul gaskets covers frictions, steels, and the full seal package in one box, which avoids the classic mistake of mixing 6L80 clutch counts into a 6L90.

722.6

• All frictions, steels, and the complete seal package. A 722.6 overhaul rebuild kit is the baseline.
• Turbine shaft sealing rings, using the late-model interlock design rather than the early scarf-cut rings, per the Sonnax cold-stall findings.
• TCC piston seals, which Sonnax identifies as the first wear item technicians scrutinized on shudder complaints.
• Solenoids, if there is any shift-quality or code history. A 722.6 six-piece shift solenoid kit replaces the set instead of chasing one at a time.
• Converter cover tower bore inspection. Measure it. The Sonnax good-OEM reference is .662"; the worn .683" example produced a 7.5 GPM leak.

Part selection for both units lives in the rebuild kits collection and the clutch packs collection.

Cost and effort framing

Neither of these is a first-rebuild unit. Expect roughly 8 to 12 hours of removal and reinstallation labor on a 6L90 in a 2500HD or 3500HD depending on 2WD versus 4WD and transfer case, plus bench time on top of that. The 722.6 comes out faster in most passenger-car chassis but demands more precision on the converter circuit measurements, and Mercedes-specific service procedures apply. Difficulty on both is intermediate to advanced: the mechanical assembly is manageable, but the diagnosis is where the money is won or lost. On the 6L90 that means confirming line pressure rise before condemning clutches. On the 722.6 that means measuring the tower bore before buying friction material a third time.

The single largest hidden cost on the 6L90 is electronics. TEHCM programming has to match the VIN, and Sonnax notes that an incorrect part number or incorrectly programmed TEHCM can itself cause a no-movement or no-upshift condition. Novak's point that reprogramming is required even between physically identical modules is not optional advice. Budget for that step before you start.

FAQ

Is a 6L90 stronger than a 722.6?

By published input torque rating, yes. The 6L90 is rated at 531 lb-ft input, while the strongest common 722.6 variant, the W5A 580, is rated to 580 N·m, roughly 428 lb-ft. The 722.6 family does include a W5A 900 variant built for high-output turbocharged engines, but the mainstream V8 unit sits below the 6L90. The 6L90 also carries an extra clutch plate in each pack over the 6L80, which is the physical basis for its heavy-duty rating.

Can I swap a 6L90 in place of a 722.6, or the reverse?

Not as a bolt-in. Bellhousing patterns, output shaft splines, overall length, and crossmember locations are all different, and the control side is the harder problem. The 6L90 puts its control module in the valve body and, per Novak, has functional compatibility only with Generation IV GM engines and their matched ECMs. Moving either unit outside its native platform is a fabrication plus standalone-controller project, not a weekend swap.

My 722.6 stalls the engine on a cold start. Is the transmission finished?

Not necessarily, and it is often not a friction material problem at all. Sonnax traces cold stall to leakage in the TCC apply circuit: a worn converter cover tower bore, worn turbine shaft sealing rings, or worn TCC piston seals. Shops have resolved it by replacing early scarf-cut turbine shaft rings with late-model interlock rings, and by correcting the tower bore with a blocked lube bypass passage and a radial lip seal. Measure before you condemn the unit.

Why does my rebuilt 6L90 have no Reverse when line pressure looks normal?

Normal line pressure with no Reverse points at the valve body and checkballs rather than the pump. Sonnax lists several documented causes: a stuck 3-5-R clutch regulator valve, Type 1 separator plate #24245720 installed on a Type 2 valve body, a 6L90 valve body installed on a 6L80, and a displaced No. 5 checkball. Clutch select valves installed backwards can also take out Reverse, Forward, or both. All of those read normal on a pressure gauge, so the gauge alone will not find them.

Related reading

If you are comparing the lighter-duty GM six-speed instead, see 6L80 vs 722.6: differences and which one to choose.

Sources

• DieselHub – GM 6L90 6 Speed Automatic Transmission Specs & Ratios
• Mercedes-Benz 5G-Tronic transmission – 722.6 family variants, ratios and applications
• Sonnax – GM 6L45/50, 6L80/90 No Movement Conditions: Analysis & Repair Tips
• Sonnax – Mercedes 722.6: Cold Stall/TCC Shudder, Part 1
• Novak Conversions – The Novak Guide to the GM 6L80 & 6L90 Automatic Transmissions