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Saturday, June 29, 2013

I Drive the Detroit DT12 Automated Manual 12 Speed Transmission - Road Test
Two more gears, "Mexican Overdrive", and a totally different experience!

Several months ago, I drove a tractor equipped with an automatic transmission for the first time in my life. See the following link to that post: I Drive the Smart Shift Automatic Transmission

I was not at all happy with it, especially driving it on slippery, snow covered roads.

A few weeks ago one of our drivers picked up a new test tractor in Denver for our location to try out. He brought it back to Salt Lake City and we have been running it in our fleet. The tractor is a Freightliner Cascadia conventional sleeper with a 13 liter Cummings and the new Detroit DT12 Automated transmission in it. "The Detroit DT12 is a 12-speed non-synchronized manual transmission with pneumatic X-Y shift and clutch actuators." We were to test it pulling Rocky Mountain doubles but before that ever happened, the powers that be found out that the tractor is not warranted to pull over 90,000 lbs gross combination vehicle weight. Add to that, the 13 liter engines have 300 to 400 lbs less torque than our current 15 liters. That immediately made the tractor impractical for us as we pull a lot of combination trailer sets in the Rocky Mountain region with gross combination weights of up to 128,000 lbs!
Detroit DT12
We still have the tractor for now and I had the chance to drive it from Salt Lake to Richfield, Utah a few days ago on a round trip of about 300 miles. I pulled a single 48 foot trailer and was grossing about 75,000 lbs to Richfield and came back empty.

It had a paddle shift on the right side of the steering wheel that was somewhat similar to the Smart Shift. One switch to choose neutral, drive or reverse and another to choose automatic or manual. For a manual downshift, you push the lever towards the dashboard away from you and for up-shifts pull the paddle towards you. One difference is that the Jake brake is also controlled by the paddle. Having the paddle in the raised position towards the top of the steering wheel has the Jake off and pushing the paddle sequentially towards the floor gives you three choices of Jake brake retarding (all the way down is full on).
Smart Shift Automated

The Bobtail  After pre-tripping the tractor at the shop I had to bobtail it a couple of miles to pick up the loaded trailer. I decided to just put it in drive and see what would happen. Exiting the driveway I was surprised at how smoothly the transmission up-shifted. Compared to the Smart Shift, it was a very big difference. The Smart Shift was much more of a sudden, unpredictable and rough shift, and when I drove it on snow, the wheels would constantly break traction (even while trying hard to stay light on the throttle) while bob-tailing with the rough shifts.

Another big difference with the DT12 was that it would skip shifts if not needed to pull a load. The Smart Shift, on automatic, would always start in second gear and shift through every gear all the way to 10th, no matter what. Bob-tailing was ridiculous with the constant slack and surge from running through every gear! The DT12 would actually start out in 4th gear bob-tailing and skip to 6th on the next shift, I don't know how it could figure I didn't have a trailer!

The Load  After hooking my loaded trailer, I took off out of the yard a few blocks before coming onto the freeway entrance going south. Under load and level terrain, the tractor would start in second gear (same as I would if doing it myself) and up-shift through every gear if you kept your foot hard on the throttle. If you had some momentum and backed off the throttle some, it would sense that it could skip a gear and do it. The only bad side was that it seemed a bit sluggish and slow shifting, but the shifts were predictable and I could feel when the they were coming, unlike the Smart Shift!

The Hills  There is supposed to be a performance automatic mode option available but the transmission I had did not have it. While pulling a load with the cruise control set, no matter what the grade of a hill you are approaching, the tranny will not downshift until it lugs down to about 1100 RPM. With the smaller 13 liter engine and a big hill coming up, I found that an earlier downshift at about 1300 or 1400 RPM provided much better performance. The only way to get that earlier downshift would be to floor the accelerator with my foot (or have it in manual mode). If you didn't want to lose your momentum, you had to constantly monitor upcoming grades and take action with your foot.

The "Mexican Overdrive"  One interesting feature of this transmission is "eCoast".  With the cruise control set, it disengages the transmission (shifting it to neutral)  from the drive-train while the truck is coasting, allowing the engine to idle at 600 RPM and reducing mechanical drag. This is something that I was taught never to do as a driver with a manual transmission, as the truck could easily get away from you on a downgrade, consequently not being able to get it back in gear as you start overheating the brakes. If the truck is coasting below 4 MPH over your cruise control set speed, the eCoast will engage. If your speed increases too much, it will disengage, downshift and apply the Jake as needed. Behold the wonders of technology!

The Empty  After swapping trailers in Richfield, I returned to Salt Lake with an empty. With the light trailer, the tranny would often skip a couple gears if it didn't need them. The shifts were surprisingly smooth and I could feel when they were coming. When decelerating from highway speed, the transmission will downshift using the Jake brake if you have it on.  The DT12 will skip-shift more often than not, even when decelerating, which really smoothed things out. As Jim Park remarked in his test, "I found it useful to shut off the engine brake below fifth gear, because it got a little "torque-y" in the cab at low speed".

The Verdict  I was impressed with DT12. I could get used to driving with this transmission, but if it was up to me, I would still go with a manual 10 or 13 speed. Being more of an "old school" driver, I prefer the satisfaction of using my skills to shift smoothly and work my truck efficiently. Seems to me that a good old mechanical tranny and clutch would be a lot cheaper, more reliable and subject to fewer breakdowns and problems. Although, unlike the Smart Shift, which was a near constant irritation to drive, I could live with the DT12. I averaged 8.5 MPG on my round trip (only loaded one way) which was very good mileage. I am sure the MPG was a result of the smaller engine and not only the transmission. This particular engine-tranny combo will not work for us but it would seem to be a good choice for fleets that drive out east.

Link to my related post:
I Drive the Smart Shift Automatic Automated Transmission



  1. I enjoyed your article, but one thing is confusing me I thought this transmission was only available with a Detroit Diesel. In May 2013 it was only available with the 15 L and in September 2013 if became available with the 13 L Detroit Diesel. You are saying it had a 13 L Cummins.

    1. Thanks for the comment! I was told it was a 13 Liter Cummins and that's what I believe was in it, though I drove it only once and really didn't inspect the motor. They were trying to sell us on the smaller motor and auto tranny, but the gvwr with doubles that we pull was much higher than the recommended mac gvwr of 90,000 lbs.