What’s better than an mDrive automated manual transmission if you’re running Mack dump or mixer trucks? An mDrive HD, for (extra) heavy duty, the company says.

Mack engineers spent several years developing and testing a toughened version of the mDrive before releasing the HD for on/off-road duty. Mack executives announced the product in February at the World of Concrete show in Las Vegas.

There and at a late-spring briefing in Allentown, Pa., they explained that the HD has tougher gears and synchronizers than the on-highway mDrive that came out in 2010 (which more than half of all customers for road-going Macks now choose). The HD also has a more effective oil cooler and uses higher-viscosity synthetic lube oil. The 12-speed mDrive HD is rated for up to 2,060 lb-ft and is available only with Mack MP7, MP8 and MP10 diesels.

Weight and price are usually important factors in heavy trucks, so here the news is good: An mDrive HD weighs 237 pounds less and costs “significantly less” than a similarly rated Allison fully automatic torque-converter transmission, heretofore the only self-shifting alternative to a multi-speed manual in Mack vocational models, said Curtis Dorwart, Mack’s vocational segment marketing manager. A single-countershaft mDrive weighs about the same as a comparable Eaton manual transmission with twin countershafts and less than a triple-countershaft Mack manual, he said.

An mDrive HD will handle most vocational applications, but Mack execs say there are some where Mack will advise customers to stay with an 18-speed manual or stay with an Allison automatic. The mDrive HD will be standard in Granite vocational trucks and tractors and available in the Titan extra heavy duty model.

Taking the Mack mDrive HD for a Spin

Examples of those vehicles were all shined up and lined up waiting for us after the briefing, and I took two of them onto the nearby paved test oval and another out off premises for faster running. Having been spoiled by on-highway mDrives and their cousins, Volvo’s I-Shift AMTs, I expected these demo units to shift smoothly and choose the correct gears for any situation, and for the clutches to engage properly on the level as well as starting out on upgrades. And they did — mostly.

The exception was the Titan tractor. It was hitched to a heavy four-axle lowboy carrying a Volvo wheel loader, and the rig’s gross combination weight was 105,000 pounds, about 15,000 under an mDrive-equipped Titan’s rated GCW of 120,000. It had been sitting there for several hours so the “iron” was close to cold. Dorwart and I climbed in, I cranked over the engine and after about a minute I punched Drive on the selector, released the brakes and nudged the accelerator.

The MP 10 diesel surged and dropped off repeatedly as it and the transmission struggled to get the rig moving on a slight upgrade. I eased up, punched Neutral and let the rig roll back down the grade and stopped. After waiting a bit before trying again, the engine smoothed out and pulled strongly. It turns out that the cold engine had been electronically derated. Lesson learned: Let the engine warm up before attempting that kind of difficult startup.

Earlier, Dorwart rode along as I took a Granite dump truck around the paved test track and up and down hills, then a mixer on the same course. After trying out the Titan, we got back in the mixer and headed out onto public streets and high-speed freeways.

Technicians had put several tons of stone in the mixer’s barrel to simulate a load of concrete, and this helped settle down the vehicle as we banged over broken and bowed concrete that’s in abundance in northeast Pennsylvania. We still experienced a lot of bouncing, though, because there’s not much else a stiffly sprung truck can do in such circumstances.

Mack mDrive Transmission Details

The mDrive always seemed to pick the proper gear no matter how fast or slow I drove, and whether we were on a straightaway or turning corners. Three operating modes — Easy Shift, Enhanced Construction and Heavy Haul — are available, and this one had the construction setting, so shift points were a little higher on the tachometer than I was used to. Technicians at Mack service departments can program a transmission’s electronic controls to suit the customer, Dorwart said.

The dash-mounted selector included a “Perf” (for performance) button, which delays upshifts and adds power and torque as engine revs rise, making a truck noticeably livelier during acceleration. To me it was especially effective as the transmission reached higher gears and road speed climbed. Punching the Perf button again returns the tranny to an economy mode. I’ll bet drivers will like this feature, but managers bent on saving fuel might want to disconnect it, or order trucks without it.

As with the highway version, the heavier-duty mDrive will improve fuel economy, greatly ease the workload on a driver, and widen the pool of potential employees who needn’t learn to operate a non-synchronized manual transmission. With their mechanical gears, AMTs also feel like they’re delivering more power to the wheels. That the mDrive HD is stronger is something that it’ll have to prove out over time, miles and operating hours.