Paccar’s TX-18 AMT offers an iron fist in a velvet glove

It’s not Canada, but close. If you’re looking to showcase a severe-duty, automated manual transmission (AMT) for Class 8 trucks, winter in Michigan is not a bad choice. And so, with a foot of fresh snow on the ground, and the mercury struggling to climb above -10 Celsius on the thermometer, I found myself crunching across the Eaton Proving Grounds in Marshall for a day driving the latest Peterbilt heavy-duty tractors fitted with the new Paccar TX-18 transmission and extreme-duty TX-18 Pro.

The locale makes sense because Paccar and Eaton partnered in developing the AMT, but the setting also offers a controlled environment to put the equipment through its paces.

The Eaton Proving Grounds are divided into an on-highway section with a 1-1/2-mile oval track, as well as an extensive off-road area, including a steep 15% grade to test startability, hill-hold capability, and the ability to hold gears on a steep slope while heavily loaded.

Kenworth and Peterbilt both pride themselves on building tough trucks that can handle severe-duty roles on the highway and off roads. And the new TX-18 series transmissions are designed to fill out Paccar’s proprietary AMT lineup with tough, robust, and powerful options for fleets in those kinds of applications.

Smooth skip-shifts

Once the mandatory safety briefings were done, I made my way across the skid pad’s snow and ice to where three new Peterbilt tractors were warming up. These included a Model 567 loaded to 140,000 lb., as well as a Model 579 and an old-school, long-nosed Model 389 – each of which was loaded to 80,000 lb.

If you’re already familiar with Paccar TX-8 and TX-12 AMTs, there’s no steep learning curve to get up and moving with the new TX-18 Series gearboxes. The stalk-mounted controls are identical, and most of the new performance features automatically engage based on road and terrain conditions. So, as with most modern AMTs, the TX-18 is essentially a hands-off transmission that can still be counted on to deliver optimal performance without a lot of input from the driver.

Figuring I might as well start at the top and work my way down, my first drive of the morning was in the 140,000-lb. Model 567. My first impression clambering up into the cab was how well the Paccar HVAC heat was working in the cold conditions, keeping the cab interior nice and toasty warm.

Once settled in with my seatbelt fastened, getting the big Model 567 rolling was a simple matter of snapping the stalk control end down to “D”, and getting on the throttle. Even with a foot of snow and ice on the ground, and 140,000 lb. behind me, the TX-18 Pro was sure-footed and smooth as it accelerated, clipping through the short-step gears with virtually no powertrain rocking or pauses.

This held true as I turned onto the oval track and began to run up to highway cruising speeds. Again, the TX-18 Pro took full throttle inputs effortlessly in stride, shifting smoothly and briskly. I had a hell of a lot of weight behind me. So, some credit has to go to the 510-horsepower MX-13 diesel engine up front. But, in terms of highway performance, there wasn’t a lot of difference between the performance I was experiencing and say, a truck loaded to “only” 80,000 lb. with a standard-duty TX-12 AMT handling the power coming out of the MX engine.

The TX-18 Pro did an outstanding job of managing all the horsepower effectively during acceleration and when I was off the throttle. I noticed a few skip shifts as well, both while accelerating and slowing down. But, in those instances, the shifts were seamless and smooth, and hardly noticeable unless you’re watching the gear selection display on the center dash cluster.

As with the TX-12, the integrated engine brake on the TX-18 is actuated by shifting the right-hand control stalk downward, with increasingly aggressive braking being engaged the lower it goes. The brake performed admirably – easily slowing the heavily laden tractor-trailer as I prepared to turn off the track and head out to the “off-road” portion of the drive.

Pulling with torque

On the back side of the Eaton test track, my Model 567 test tractor weaved around the winding roads leading out to the grade course, ignored by a herd of whitetail deer who see this sort of thing all the time.

The front portion of the Eaton course begins with an 8% grade. After you crest the hill, the backside of the road goes down at a sharp 15% drop. So it’s an excellent opportunity to test the TX-18 Pro’s gradeability and hill-start capabilities. It was here that I really saw the full power and potential this new Paccar transmission has to offer fleets that need a little more low-end grunt to get their work done or heavy loads hauled.

We’re all familiar with tractor-trailer torque pulls – truck rallies where super-torquey diesel engines get heavy loads up and moving, popping wheels off the ground thanks to the massive power coming out of the drivetrains, and leaving thick clouds of black exhaust jetting out of the stacks.

With the TX-18 Pro I got a little taste of that kind of power and performance from a AMT. From a dead-stop about halfway up the grade, I released the throttle and held my right foot up in the air for a couple of heartbeats. As advertised, the 140,000-lb. Model 567 didn’t budge so much as an inch. In an impressive display of strength, the TX-18 Pro effortlessly held the truck still until I stomped my foot down and gave her full throttle.

The response from both the MX-13 diesel and the TX-18 Pro was immediate and impressive. The AMT began briskly running through the gears. And while the front left tire wasn’t about to start jumping off of the road, there was a definite hard rocking to the right as the entire truck frame tried to absorb the massive amounts of horsepower and 1,850 lb-ft of toque churning out of the drivetrain. Even with all that weight behind me, the TX-18 Pro powered up the 8% slope without breaking a sweat. Even better, the engine brake, obviously set to the most aggressive setting, held the truck in gear on the downgrade without any brake input at all. Experienced drivers will be happy to learn they can take over full control of the TX-18 at any moment – including tough uphill starts or on steep downgrades – by simply pulling the AMT stalk toward them for an instant upshift, or pushing forward to downshift.

Paccar and Eaton engineers have succeeded in designing a “best of both worlds” AMT with the TX-18 and TX-18 Pro transmissions. This is a transmission that – despite the brute power it can deliver – still performs gracefully on the highway. But when extra power is needed, it delivers in spades. It is a highly refined AMT that consistently delivers smooth engine horsepower, regardless of how severe the application might be.