48RE vs Allison 1000: Ratios, Fluid, and Which Is Stronger

Every diesel shop gets this question from a customer shopping used trucks: is the 48RE behind a 5.9 Cummins as tough as the Allison 1000 behind a Duramax? These two units were built to different ideas. The 48RE is the last version of Chrysler's TorqueFlite family, a hydraulic four-speed with two bands, a governor-pressure system and an add-on overdrive section. The Allison 1000 is a commercial-duty clutch-to-clutch unit from a company that builds transmissions for buses and medium trucks. It went into GM heavy-duty pickups starting with the 2001 model year.

This is how they compare from the bench: what Chrysler, GM and Allison actually publish, the ratio sets, fluid and service rules, how each one is controlled, where each one fails, and what to check before you order parts. The hard numbers come from the Chrysler service manual, Ram Body Builder data, GM, Allison, ATRA, ATSG and Sonnax, all listed under Sources.

Recommended 48RE and Allison 1000 parts

Order by unit, nothing crosses between them: a 2003-2007 master rebuild kit for the 48RE, and a valve body shift repair kit and deep pan filter kit for the Allison 1000. Secure checkout.

48RE Master Rebuild Kit 2003-2007

$265.99View part

Allison 1000 6-Speed Valve Body Shift Repair Kit

$109.99View part

Allison 1000/2000 Deep Pan Filter & Gasket Kit, Duramax

$49.99View part

The Short Version

What is missing from that table matters. Chrysler's 2004 Ram Body Builder data lists the HD 48RE as the automatic behind both the standard and high-output 5.9L Cummins, but it does not publish an input torque rating. GM's 2016 Silverado HD spec sheet does not publish one for the Allison either; it lists the engine paired with it, the 6.6L Duramax at 765 lb-ft at 1600 rpm. The big horsepower numbers you read for either transmission are aftermarket build claims, not factory data.

Gear Ratios: Four Wide Steps vs Six Close Ones

Chrysler's service manual lists the 46/47RE gearset at 2.45, 1.45, 1.00 and 0.69, with a 2.21 reverse, and the 48RE kept that ratio set. GM lists the six-speed Allison at 3.10, 1.81, 1.41, 1.00, 0.71 and 0.61 with a 4.49 reverse and a 3.73 axle behind the Duramax.

Run the numbers and the difference is clear. The Allison's first gear is about 27 percent deeper than the 48RE's, and its total spread (first divided by top gear) is about 5.1 to 1, against 3.55 to 1 for the 48RE. The 48RE leans on the Cummins' low-rpm torque and the converter to get a load moving, then takes a big 1-2 step. The Allison splits the same job into smaller steps and keeps the engine closer to its torque peak. In reverse the gap is even bigger: 4.49 against 2.21. That is why a Duramax can back a gooseneck up a grade on the gear alone, while a 48RE truck does more of that work in the converter, which means more heat in the fluid.

On a road test, do the ratio check with the right table. Compare input (turbine) speed to output speed in each gear. A 48RE has no turbine speed sensor in its stock setup, so you are looking at engine rpm against output speed with the converter locked. If one gear is off while the others hold, look at the clutch or band that is applied in that gear.

How Each One Is Built

48RE: bands, roller clutches and an overdrive box

ATSG's 46/47/48RE manual sets out the layout. First through third come from the front (direct) clutch, rear (forward) clutch, kickdown band, low/reverse band, low roller clutch and the planetary gearsets. Fourth comes from the overdrive unit at the rear, which has its own overdrive clutch, OD/direct clutch, overrun roller clutch and planetary. ATSG's component figure shows separate diesel and gasoline overdrive clutch packs, so the clutch stack-up depends on the application.

Sonnax's 48RE hydraulics write-up explains why a 48RE valve body is not just a 47RE valve body. On the 46/47RE, a circuit added around 1996 raises line pressure about 10-15 psi in third with the TCC on and in fourth. On the 48RE, two reduction points are exhausted at the boost valve in those ranges, so line pressure rises about 22-28 psi. That extra pressure clamps the converter clutch and overdrive clutch harder under a trailer. The 48RE also feeds the TCC solenoid from the manual valve in any drive range instead of from the 1-2 shift valve. Sonnax's warning is direct: a 46/47RE valve body bolted into a 48RE may seem to work, then the truck comes back with a burned converter clutch or overdrive clutch.

The input shaft is a well-known weak link on built 48REs. Sonnax sells a Smart-Tech big input shaft kit, 22121B-08K, with a 35-spline shaft that fits any 47RH/RE or 48RE converter with a 35-spline turbine hub. Sonnax rates it 35 percent stronger than the best heavy-duty 23- or 29-spline shafts. That tells you where the stock shaft lets go once the fuel is turned up.

Allison 1000: clutch-to-clutch, no bands

The Allison has no bands and no shift valves in the TorqueFlite sense. The TCM picks the gear by controlling clutch apply and release directly. ATRA's LCT1000 webinar sets out the changes by generation: 2001-2005 units are five-speeds, and a G solenoid was added in 2004 to lower line pressure in Park and Neutral to cut pump whine. Starting in 2006 the unit became a six-speed by releasing C3 and applying C4. The trim solenoids changed to pressure control solenoids and the neutral start switch moved inside as an internal mode switch. For 2010 the G solenoid was dropped and a main modulating solenoid took over line pressure control.

Controls and the Diagnostic Traps

48RE

The 48RE is still a hydraulic transmission. The PCM controls governor pressure through the governor pressure solenoid and sensor, plus the overdrive and converter clutch solenoids. The fluid temperature thermistor lives in the governor pressure sensor, and Chrysler's manual gives about 1000 ohms at room temperature. The PCM blocks OD and converter clutch apply below about 50 F. Above 260 F it forces a 4-3 downshift with third-gear lockup and lights the OD OFF lamp. A customer who says the truck "drops out of overdrive and the light comes on when I tow" may be describing that heat-protection mode, not a failure.

The 48RE's throttle input changed in 2005. ATSG notes that 2005-up diesel 48REs replaced the TV cable and its linkage with an electronic Transmission Throttle Valve Actuator. A push-in transmission range sensor replaced the screw-in Park/Neutral switch in 2002. On a 2003-2004 truck a misadjusted TV cable is still the first suspect for soft, late or harsh shifts. On a 2005-2007 truck, check the TTVA and its codes before you take anything apart.

Chrysler's pressure test is the baseline. In 1, 2 and D at 1000 rpm, the manual lists line pressure at 54-60 psi, rising to 90-96 psi as throttle pressure comes in. A pump or regulator problem shows up here before it shows up as a slip.

Allison 1000

The Allison's pressure switch manifold reports shift valve movement to the TCM, not manual valve position. ATRA's deck covers the traps that cost shops money. A restricted solenoid feed filter causes a no-movement complaint with no codes. The dash reads "Shift Inhibited" and the range still reads correctly on the scan tool. Ratio error codes P0735 or P0736 in top gear, with neutral shifts or no upshifts, often trace to transfer case wear sending an erratic speed signal: slider and fork, bearing and rear case, or chain and pump retainers. That is ATRA TSB 1256. Check the transfer case before you condemn the Allison. On 2001-2006 units, the engine speed sensor reads dimples on the converter shell. A damaged dimple can cause converter surge without setting a code, and the sensor was eliminated in 2007.

Fluid and Service: The Biggest Practical Difference

The 48RE is a small-sump transmission. Chrysler's 2004 Ram Body Builder capacity chart lists an 8 pint (3.79 L) service fill and 29-33 pints (13.7-15.6 L) at overhaul, depending on the cooler and lines. It takes ATF+4, Chrysler specification MS-9602. Fluid makers' spec sheets list that fluid for Chrysler units that originally called for ATF+3 or ATF+2 as well. The pan filter is all you get unless you add one, and on a towing truck the fluid works hard. Change it on a short schedule, check the cooler flow, and look at the pan for band and clutch material at every service.

GM's TechLink capacity chart lists the Allison LCT1000 (RPO MW7) at 14.5 L, 15.3 qt at overhaul with Castrol TranSynd. Allison's 1000/2000 operator's manual sets the intervals by fluid. With 100 percent TES 668 or TES 295 fluid and prognostics off, the fluid change is 150,000 miles or 48 months in general duty and 75,000 miles or 36 months in severe duty. Severe duty means stopping more than once per mile. With TES 389 fluid, the intervals drop to 50,000 miles general and 12,000 miles or 6 months severe. On a used Duramax with unknown history, assume a mixture and use the short schedule.

Do not cross the fluids. ATF+4 and TranSynd are different products for different friction systems, so never top off one with the other's jug.

Where Each One Fails

• 48RE under load: overdrive clutch and converter clutch burned by heat or low apply pressure, input shaft spline twist or breakage on built trucks, kickdown band wear and band adjustment neglect, and TV cable or TTVA problems that drop line pressure.
• 48RE after a rebuild: a 46/47RE valve body or parts installed in a 48RE (the Sonnax pressure warning above), a governor pressure solenoid or sensor not checked, and a TV cable not adjusted after the swap.
• Allison 1000: wrong valve body or solenoids for the generation, a plugged solenoid feed filter, transfer case wear that sets ratio codes, early engine speed sensor dropouts, and fluid run on the long schedule when it is really a mixture.

So Which Is Stronger?

Neither manufacturer gives you a published input torque rating to settle the argument. On design, the Allison has the advantage for heavy towing in stock form. It has two more gears, a much deeper reverse, clutch-to-clutch control with no bands to adjust, a spin-on filter and a fluid program built for commercial duty. The 48RE is simpler and cheaper to rebuild, every TorqueFlite builder knows it, and the aftermarket for it is huge. That is why so many Cummins trucks run built 48REs with billet shafts, upgraded converters and valve bodies.

In the shop, the more useful question is which parts this truck needs, because nothing crosses over between these two units. On a 48RE, confirm the model year (cable or TTVA), the valve body and the input shaft spline count before you order. On an Allison, identify the generation and valve body version first.

Frequently Asked Questions

Is the 48RE stronger than the Allison 1000?

Neither Chrysler nor GM publishes an input torque rating for these units. Chrysler lists the HD 48RE behind both the standard and high-output 5.9L Cummins, and GM pairs the Allison with a Duramax rated 765 lb-ft in 2016. In stock form the Allison has the edge for heavy towing with six gears, a 4.49 reverse and no bands, while the 48RE is simpler and cheaper to build up.

What are the 48RE and Allison 1000 gear ratios?

Chrysler lists the 46/47RE family ratio set used by the 48RE as 2.45, 1.45, 1.00 and 0.69 with a 2.21 reverse. GM's 2016 Silverado HD spec sheet lists the six-speed Allison 1000 at 3.10, 1.81, 1.41, 1.00, 0.71 and 0.61 with a 4.49 reverse.

What fluid and how much does each transmission take?

The 48RE takes ATF+4, with an 8 pint service fill and 29 to 33 pints at overhaul per Chrysler's 2004 Ram Body Builder chart. GM's TechLink chart lists the Allison LCT1000 at 15.3 quarts at overhaul with Castrol TranSynd. The two fluids are not interchangeable.

Why does my 48RE drop out of overdrive when towing?

Chrysler's service manual says the PCM forces a 4-3 downshift with third-gear converter lockup and lights the OD OFF lamp when fluid temperature passes about 260 F. If that is happening, check the fluid level and condition, cooler flow and load before condemning the overdrive clutch.

Can you put a 47RE valve body in a 48RE?

Sonnax warns against it. The 48RE valve body raises line pressure about 22 to 28 psi in third with the converter clutch on and in fourth, against 10 to 15 psi on the 46/47RE, and it feeds the TCC solenoid differently. A 46/47RE valve body may seem to work and then burn the converter clutch or overdrive clutch.