Driving Paccar’s TX-18 Transmission: An Iron Fist in a Velvet Glove

If you’re looking to showcase a new severe-duty automated transmission for Class 8 trucks, Michigan in January is not a bad choice. With a foot of fresh snow on the ground, and the mercury struggling to climb over the 14-degree mark, I found myself crunching across the Eaton Proving Grounds in Marshall, Michigan. I was there to drive the latest Peterbilt heavy-duty tractors fitted with the new Paccar TX-18 transmission and the extreme-duty TX-18 Pro. Why this location? Paccar and Eaton partnered on the development of the new transmission.

There were three Peterbilt tractors to experience: a Model 567 loaded to 140,000 lbs., and a Model 579 and an old school, long-nosed Model 389, both loaded to 80,000 lbs. I headed straight for the 140,000-lb. Model 567 with the TX-18 Pro.

The Eaton Proving Grounds are divided into an on-highway section with a 1.5-mile oval track and an extensive off-road area. The latter includes a steep, 15% grade to test startability, hill-hold capability, and the ability of the transmissions to hold gears on a steep down-slope while heavily loaded.

Smooth Skip-Shifting

If you’re familiar with Paccar TX-8 and TX-12, there’s not a steep learning required 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 the new gearbox offers automatically engage based on road and terrain conditions. So, as with most modern automated transmissions, the TX-18 is essentially a hands-off transmission that can be counted on to deliver optimal performance without a lot of input from the driver.

My first impression clambering up into the cab was how well the Paccar HVAC was working in the bitter cold conditions.

Once I was settled in with my seat belt fastened, getting the big Model 567 rolling was a simple matter of snapping the stalk control down to “D” and getting on the throttle. Even with a foot of snow and ice on the ground and 140,000 lbs. to move, 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. Some credit has to go to the 510-hp MX-13 diesel engine. But there wasn’t a lot of difference between the highway performance I was experiencing and say, a truck loaded to “only” 80,000 lbs. with a PX-12. The TX-18 Pro did an outstanding job of managing all the horsepower effectively, both during acceleration and when I was off the throttle and slowing down.

I noticed a few skip-shifts as well, both while accelerating and slowing down. In those instances, the gear shifts were seamless and smooth and hardly noticeable, unless you’re watching for them on the gear selection display in 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 down in preparation for turning off of the track and heading out to the “off-road” portion of the drive.

Truck-Rally Torque

On the back side of the proving grounds, my Model 567 test tractor weaved through 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 course begins with an 8% grade. After you crest the hill, the backside of the road goes down at a sharp 15% drop. That made for 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 with left wheels popping off of the ground thanks to the massive power coming out of the drivetrains and thick clouds of black exhaust jetting out of the trucks’ stacks.

With the TX-18 Pro, for the first time, 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 despite the 140,000-lb. load, the 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 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 torque 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 that they can take over fully control of the TX-18 at any moment — including tough uphill starts or on steep downgrades — by simply pulling the 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” automated transmission with the TX-18 and TX-18 Pro transmissions. Despite the brute power it can deliver, the transmission still performs gracefully on the highway. But when extra power is needed, it delivers in spades.