6L90 vs 47RE: Differences, Specs, and Which One to Choose

These two transmissions get cross-shopped constantly, and it is not because they are similar. The 6L90 is General Motors' heavy-duty six-speed automatic; the 47RE is Chrysler's four-speed overdrive automatic that lived behind the 5.9L Cummins. People end up comparing them for one of three reasons: they are choosing between a used GM HD truck and a used Ram, they are planning a swap into a project chassis, or they are deciding whether a tired 47RE is worth rebuilding or worth stepping away from entirely.

Here is the short answer. If your truck already has one of them, the choice is made for you and the real question is which parts fail and what to replace while you are in there. If you genuinely have a choice, the 6L90 is the stronger, more modern, better-geared unit, and the 47RE is the simpler, more forgiving one to work on in a home shop. Below is the detail behind that, with every spec attributed.

Side-by-side specifications

Specifications above are from the DieselHub 6L90 spec sheet and the DieselHub 47RH/47RE spec sheet. DieselHub also lists a maximum GCWR of 19,000 lbs for the 47RE, which was a class-leading one-ton rating in its day.

Gearing is the biggest practical difference

Look past the torque numbers and the ratio spread is what you actually feel. Per GM Authority's Hydra-Matic 6L90 (MYD) reference, the 6L90 carries an overall ratio spread of 6.04:1, giving it both a steep 4.027 first gear for getting a loaded trailer moving and a 0.667 sixth for low-rpm highway cruising. The 47RE has a 2.45 first and a single 0.69 overdrive, so it leans on the torque converter harder off the line and has one long step between third and fourth.

That single overdrive step is why 47RE trucks are so sensitive to torque converter lockup behavior, and why 6L90 trucks handle grade changes better while towing. The six-speed simply has more places to put the engine.

How each unit is controlled

The 6L90 uses a 32-bit transmission control module packaged inside the transmission itself, where temperatures stay more constant than a body-mounted module, per GM Authority, and that module compensates for normal clutch plate wear over the life of the unit. In the field this module is called the TEHCM, and Sonnax notes that an incorrect part number or an incorrectly programmed TEHCM can produce a no-movement or no-upshift condition all by itself, so the part number and program file must match the VIN.

The 47RE is the electronically-governed version of the 47RH. Per DieselHub, governor pressure on the RH is hydraulically controlled while the RE controls it electronically, but the RE still retains a hydraulic valve body. That hybrid arrangement is exactly why 47RE shift-timing complaints so often trace back to governor pressure hardware rather than to clutches.

Known weak points: 6L90

Sonnax's tech article on GM 6L80/6L90 no-Reverse conditions is the best single summary. It notes the 6L90 is the heavy-duty derivative of the 6L80, with roughly 25 percent of internal components upgraded and a case lengthened by 1-3/8 inches to house stronger input and output gearsets. Common no-Reverse causes it lists include an out-of-place #5 checkball, an incorrect valve body separator plate or gaskets, an improperly indexed snap ring, cracked welds in the 3-5-R drum, a stuck 3-5-R clutch regulator valve, or a solenoid fault in the TEHCM.

One trap is worth memorizing: a 6L80 valve body will physically bolt to a 6L90 case, but Sonnax warns the center support holes in the upper casting have a different offset between the models. Install the wrong one and you get Forward but no Reverse, because the case ports do not line up with the upper casting.

Sonnax's companion article on 6L series no-movement conditions adds the pressure-side failures: a TEHCM line pressure solenoid stuck in exhaust mode leaves line pressure pinned around 60–68 psi with no rise at all, a Type 2 separator plate on a Type 1 valve body causes no Forward (the correct Type 1 plate, #24245720, uses seven checkballs), and clutch select valves or springs installed backwards will cause no movement with otherwise normal line pressure.

Known weak points: 47RE

The Sonnax 47RE unit catalog reads like a failure map. The recurring themes are the rear servo piston sticking or breaking in its bore, which Sonnax lists as a cure target for burnt bands and no Reverse (piston 22912C fits 1997-later units, 22912B fits 1994–1996); band strut length, where an extended-length strut 0.100 inch longer than OE addresses no Reverse and soft Reverse; the overdrive sprag pilot bushing, tied to OD roller clutch failure; the 4-spool switch valve, tied to lockup shudder, overheated converter and low cooler flow; and line pressure plug and sleeve wear, tied to Reverse slip and poor Forward and Reverse engagement.

Sonnax also catalogs heavy-duty input shaft kits for units fitted with a 35-spline turbine hub, which tells you where the strength ceiling sits once a 5.9L is turned up past stock.

The fix: which part, and why

For a 6L90 with harsh shifts, slipping in Reverse, or a burnt 3-5-R, the clutch pack is the repair. Per GM Authority, the 6L90 uses one additional clutch plate in each clutch compared to the 6L80, and DieselHub notes the same, so a 6L90-specific friction set matters. Do not source a 6L80 pack and hope it fills the drum correctly.

For a 47RE with slipping, delayed engagement, or a burnt band, the band and the servo hardware behind it are where you start, because Sonnax ties burnt bands directly to rear servo piston sticking. A fresh 47RE front transmission band is a low-cost item that should never be reused on a rebuild, and the rest of the transmission bands lineup covers the other RE-family units. Any 47RE that comes apart also gets a new filter; the 47RE transmission filter for 98-99 trucks is the correct service part for those years.

Replace these while you are in there

• 6L90: full friction and steel clutch sets, all case and pump seals, the filter and pan gasket, the pump stator and case bushings, and the valve body separator plate gaskets. Confirm the valve body casting matches a 6L90 before reassembly — Sonnax lists the D machined-down boss as the MYD/6L90 identifier, while cautioning that some valve bodies have no boss machined at all, so it is not foolproof on its own.
• 47RE: front and rear bands, band struts, rear servo piston, overdrive sprag pilot bushing, filter and pan gasket, and the governor pressure hardware. If the truck has any tuning or towing history, evaluate the input shaft and the 4-spool switch valve at the same time.
• Both: the torque converter. The 6L90 uses a 300 mm converter per DieselHub. Reusing a converter after a clutch failure sends the old debris straight back into a fresh build.

Cost and effort framing

The 47RE is the easier unit to work on. It is a hydraulic-valve-body four-speed weighing roughly 200 lbs dry per DieselHub, with adjustable bands and a valve body a competent DIYer can service on a bench. Figure a full removal and rebuild as a solid weekend-plus job for an experienced home mechanic, and roughly 8 to 12 labor hours of removal and installation in a shop depending on 2WD versus 4WD and transfer case.

The 6L90 is heavier at roughly 245 lbs assembled, requires DEXRON VI, and its internal control module makes it a scan-tool job. Expect similar removal and installation time but a meaningfully higher diagnostic and setup burden, plus module programming if the TEHCM is replaced. For most owners the 6L90 is a shop job and the 47RE is a driveway job.

So which one should you choose?

Choose the 6L90 if you want more torque capacity (531 lb-ft versus roughly 450), six ratios instead of four, a modern lockup and control strategy, and a unit still in production for HD trucks and vans. It is the better tow transmission on gearing alone.

Choose the 47RE if you are running a 1994–2003 Ram, you value serviceability and parts availability, and you are willing to address the known servo, band, and governor pressure items proactively. It responds very well to targeted upgrades and it does not require a scan tool to diagnose most of its complaints.

The comparison people actually need more often is within a family rather than across brands. If you are on the GM side, read 6L80 vs 6L90: which GM six-speed is in your truck and 4L80E vs 6L90. If you are on the Ram side, 47RE vs 48RE is the comparison that will actually change what you buy.

FAQ

Can you swap a 6L90 in place of a 47RE?
There is no factory relationship between the two. They are different manufacturers, different bellhousing patterns, different control systems, and different fluids. Any such swap is a full custom adapter, driveshaft, crossmember, wiring, and standalone-controller project, not a bolt-in.

Which is stronger?
On published input torque ratings the 6L90 at 531 lb-ft sits above the 47RE at roughly 450 lb-ft, per DieselHub. Per GM Authority, the 6L90 also has a strengthened input gearset with two additional pinion gears (six in total) and wider output gears compared to the 6L80.

Why does my 6L90 have Forward but no Reverse after a valve body swap?
Sonnax specifically calls this out: a 6L80 valve body bolts to a 6L90 case, but the center support hole offset in the upper casting differs, so the case ports do not align. Verify the casting is a 6L90 unit before you button it up.

Do I need ATF+4 in a 47RE?
Yes. DieselHub lists ATF+4 as the specified fluid, with 14.5 to 16.5 quarts for a dry fill and roughly 4 quarts for a routine service refill.

Sources

• DieselHub — GM 6L90 Six-Speed Automatic Transmission Specs & Ratios
• DieselHub — Chrysler/Dodge 47RH, 47RE Automatic Transmission Specs & Ratios
• Sonnax — Addressing GM 6L80, 6L90 No Reverse Condition
• Sonnax — GM 6L45/50, 6L80/90 No Movement Conditions: Analysis & Repair Tips
• Sonnax — Chrysler 47RE Transmission Parts & Tech Resources
• GM Authority — GM 6-Speed 6L90 (MYD) Transmission Info & Specs