Detroit DT12 is a Smooth Self-Shifter (2024)

Tom Berg, Truck Editor

Automated mechanical transmissions are a trend. In 2012, they were in 20 percent of all heavy trucks in North America, according to one supplier, and they are expected to reach 30 percent sometime this year. In Europe, fully 80 percent have AMTs. They are popular because they save money in fuel and maintenance, and greatly ease the work done by drivers.

So this recently introduced Detroit DT (for Detroit Transmission) 12 (for 12 ratios) AMT comes at a good time for Freightliner, Detroit’s sister company under Daimler Trucks North America, because it now has a proprietary product to sell alongside the Eaton UltraShift Plus transmissions it already had in its lineup. One more product heightens competition and should squeeze selling prices, which further works in the favor of any customers progressive enough to consider AMTs.

Test Set

Tractor: 2014 Freightliner Cascadia Evolution daycab, setback front axle, BBC 125 in., GVW rating 52,000 lb.

Engine: Detroit DD13, 12.8 liters/781 cu. in., 450 hp @ 1,800 rpm, 1,550 lb.-ft. @ 1,100 rpm, w/ Jacobs Engine (compression) Brake

Transmission: Detroit DT12-OB-1550 automated mechanical, 12-speed overdrive (11th gear 1 to 1, top gear 0.78 to 1), controls set for economy mode, w/ Detroit automated clutch

Front axle: 12,000-lb. Detroit DA-F-12.0-3 on 12,000-lb. dual taperleafs

Rear axles: 40,000-lb. Detroit DA-RT-40.0-4 HT w/ 3.23 ratio, on 40,000-lb. Freightliner Airliner air-ride

Wheelbase: 174 in.

Brakes: Meritor Q+ S-cam drum, 16.5x5-in. front, 16.5x8.62-in. rear, Meritor Wabco ABS w/ hill-start aid and automatic traction control

Tires, wheels: 275/80R22.5 Michelin XZA3+ front, XDA Energy rear, on Alcoa polished aluminum discs

Fifth wheel: SAF Holland FW35 ILS 12-in. air slide

Fuel tanks: Twin 80-gal. aluminum

DEF tank: 13-gal. polyplastic

Trailer: Fruehauf 48-ft. composite steel-aluminum

The DT12 is such a smooth-acting self-shifter that driving a truck becomes a pleasing task instead of a load of work, especially in stop-and-go traffic and on hilly highways. That’s my impression after several hours of operating this Freightliner daycab tractor in Indiana. Where are the hills in the Hoosier State? Try State Route 46 between Columbus and Bloomington. This is country that the ground-grinding glaciers of 10,000 to 15,000 years ago didn’t reach, evidently, because the road twists and turns as it climbs over numerous mounds of dirt and rock.

Route 46 was a good workout for the transmission and the engine ahead of it, a Detroit DD13. When announced about a year ago, the DT12 was first available only with the larger DD15; now the tranny comes in lower-rated versions that are good pairings to the less punchy 12.8-liter diesel—not that 450 horses and 1,550 lb.-ft. are weak numbers. And a precisely operating AMT efficiently moves the power and torque to the wheels.

The DT12 is based on a Mercedes-Benz AMT used in Europe and now produced in the United States. It’s paired with Detroit Diesel engines in Freightliner Cascadias, like this one, though truck-model availability might widen later. DT12s for now are meant for highway service and have no PTO mounting, but might later be approved for vocational use, according to Freightliner managers Mike McHorse and Doug Ackerman, who set up this drive for me. We began at Stoops Freightliner, a large dealership on the south side of Indianapolis. When we arrived, the tractor was hitched to an empty 53-foot van, so Brian Sutherland, a Detroit technical sales manager, and I dropped it and found a loaded flatbed parked among other trailers.

Right from the get-go, the automated clutch engaged like it was fashioned of silk, and the tranny made smooth, sensible gear changes. We headed out of the yard and onto nearby Interstate 465. Sutherland, who drove briefly at first, called out the gear choices made by the DT12’s electronic brain. It started out in a higher gear than most drivers would, usually 3rd, then quickly skip-shifted to 5th, and then to 7th or 8th, depending on grades and how much pressure he put on the accelerator. Upshifts were usually made at 1,400 or 1,500 rpm. At higher road speeds, it tended to use more ratios until settling into the overdrive 12th gear where we cruised at 1,300 and 1,400 rpm. (A direct-drive version is also available.) We circled through the suburbs so I’d get an idea of how the tranny operated. Then I took the wheel.

First. I acquainted myself with the column-mounted shift selector, a stubby stalk with these functions—a vertically rolling thumb switch changes the tranny from Neutral to Drive or Reverse; a horizontal sliding switch chooses Automatic or Manual mode; and nudging the stalk up or down induces an up- or down-shift if road speed isn’t too fast or slow. While in M, the tranny will hang in the latest gear chosen, but will usually shift out of it while in A. Sutherland told me how it works, but drivers with some experience would figure it out pretty quickly, and really, all they have to do is put it in D, release the brakes and drive.

I proceeded onto eastbound I-465, as before, but continued to I-65, then southeast through a construction zone and onto open road. The truck was so easy to operate and the Cascadia’s cab so quiet that I accelerated to higher speeds than I meant to. Sixty-five and more were on the speedo rather soon, and I had to consciously back off and set the cruise control at 60 or 62.

Within a few minutes, I had appraised the tranny and remarked to Sutherland, “It’s as good as a Volvo.” He made a face, and I said, “That’s a compliment,” because Volvo’s I-Shift and its brother, Mack’s mDrive, were the slickest automated transmissions available, at least in my experience. They, too, are 12-speeds.

Then I concentrated on observing the DT12’s operation. It down-shifted smoothly from top gear to 11th and 10th, then back up to handle varying speeds from traffic and the ups and downs over freeway interchanges.

Pushing the accelerator past a detent causes a kickdown of at least one ratio. Another feature is “eCoast.” When cruise control is engaged and no load is on the engine, the clutch disengages and the engine drops to idle; this reduces drag and saves fuel. When road speed falls to the set point, or if the driver touches the accelerator or brake pedal, the clutch re-engages and the engine quickly revs back up and resumes working. E-Coasting does not occur with CC off or if the Jake Brake is on, and certain limits can be set in the electronic controls..

The landscape was rather flat, but that changed when we exited onto SR 46 and headed west. As I said, this highway crosses some hills, and some of the up- and downgrades are surprisingly steep if usually short—a good test track for the DT-DD team. The engine ably powered us up the hills and, as directed, held back our speed on downgrades with its strong Jake Brake. With our weight near 78,000 or so pounds, we did slow to as low as 25 and 30 mph on upgrades and a string of cars formed behind us. There was nowhere to pull over and let them pass, but I sped up to 55, 60 and more on downgrades and level stretches, and then the motorists fell back.

The tranny never missed a shift and almost always picked the right gear for the situation. The one time it didn’t seem to was on a steep downhill section when it hung in 8th or so. I let it stay in that gear, and as we accelerated down the hill the tach needle went way past the 2,100-rpm redline—to 2,400, 2,500 and for an instant to 2,600—before I intervened by bumping the selector into a couple of upshifts to bring down revs, and stabbing the brake pedal. Sutherland said he had not seen that before, so he checked with Detroit engineers to see if it was normal. It was.

“Upshifting and/or braking to bring rpm down is in the hands of the driver, just as it is in a manual transmission,” he said later. Most drivers will probably like that. By the way, the retarder has three settings, and I was in 3, for maximum braking, and when the rig descended hills the exhaust made a pleasing Jake Brake rap.

The transmission otherwise acted without surprises. At Bloomington, we headed north on SR 37 toward Indy. By the time we had returned to Stoops Freightliner we had put about 120 miles on the odometer, and my early positive opinion of the DT12 was verified. It’s both smooth and smart.

Detroit DT12 is a Smooth Self-Shifter (2024)
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