Transmission Shootout: Allison vs Eaton

By: Matt Wood

Matt Wood uses a Freightliner Century test machine to compare the Allison Automatic Transmission to the Eaton Automatic Manual Transmission (AMT).


Times are changing and, in the not too distant future, people will talk of trucks with gear sticks in the same way that they currently talk of the days when trucks had joey boxes (auxiliary transmissions).

However, like it or not, automated manual transmissions (AMTs) are going to be the way of the future.

Already it’s rare thing to see a European truck with a gear stick, and since the Eaton UltraShift AMT came along North American AMTs have started to look more attractive to operators.

Comfort may be one of the driving factors behind the rise of the auto, but there is another major factor that’s bringing the auto to the fore — the looming labour crisis that the Australian transport industry is facing.

Blame the tarnished image of trucking, blame the rise of IT as a career option but, whatever the reason there’s virtually an entire generation of drivers missing in Australian trucking. Take look around most fleets and there aren’t many 30- or early-40-something drivers around.

Gen X took one look at transport and kept on walking.

The emerging breed of drivers is going to have to learn very quickly; the days of working in the depot and learning the ropes are well and truly over. Companies are going to need to get bums on seats quickly and using auto transmissions takes one skill out of the mix when getting new drivers behind the wheel and onto the road.

I was recently able to drive two different examples of auto transmission in two identically specced Freightliner Century Class CST 112 prime movers at the same gross weight of 40 tonnes.

One was the tried and true Eaton UltraShift Plus AMT while the other was an Allison 4500 torque converter automatic.

The no-clutch pedal 18-speed UltraShift is essentially a manual Road Ranger with a servo-shift unit and computer doing all of the shifting. The Allison is a heavy duty 6-speed auto that uses the torque converter to get things rolling in first gear before mechanically locking up from second gear upwards.

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The Eaton UltraShift equipped CST 112 was fitted with a 34-inch (86.4cm) midroof integrated sleeper and residing under the bonnet was the 13-litre Detroit DD13.

This engine is rated at 450hp (331kW) and develops 1,650ft-lb (2,237Nm) of torque.

In this spec the CST seems ideal for a single trailer metro and country hauler. The 4.33 final drive ratio hints at a good middle ground between metro and highway.

The Allison 4500 RDS-equipped vehicle was a day-cab CST 112 on a slightly shorter wheelbase with the same DD13 powerplant and same power and torque ratings.

This CST used a lower 4.56 final drive ration. The transmission and drive spec suggests dusty building sites and quarries rather than the open road.



I jumped into the bobtail UltraShift CST first and idled back under the counterweighted trailer.

This is where the UltraShift gets the most criticism; low-speed tasks, such as hooking up trailers, can be difficult with the two-pedal AMT as the clutch engagement makes it a delicate balancing act, especially when you add a bit of friction onto the turntable from the trailer skid plate.

It can be very difficult not to slam back into the trailer pin when the clutch bites even with the AMT in the lowest R1 gear. Yes, that’s exactly what I did, but with the turntable jaws clamped around the pin and with the trailer hooked up I headed out into the traffic to play.

With a load on its back, the Eaton AMT skip shifts and changes nicely.

The transmission and engine communicate well, though the dialogue between the Detroit and Eaton ECM’s has traditionally been a strong point in these installations.

The only issue the UltraShift sometimes has is that if drivers plants their foot when taking off from a standstill, it will, like most AMTs, use as many gears as possible to get going.

When driven too hard, the UltraShift will take what seems like forever to clear an intersection or get up to speed.

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This is usually accompanied by a lot of engine roaring and jake changing.

The AMT responds best to considered throttle use, going easy on the right foot gives the tranny a chance to sense the load in the driveline and skip-shift accordingly. Taking this approach gets the rig rolling a lot sooner.

The smart-shift transmission selector paddle that Freightliner uses on the Eaton AMT is one of the best and most intuitive controllers to use by far.

Where others use push buttons or a T-barstyle selector, the smart-shift paddle means you can manually change gears without taking your hands of the wheel. The only criticism I’d make would be that sliding the selector button around on the end of the paddle can be a tad fiddly.

The Eaton ‘box is a good compromise for a mix of urban and country applications.


This was the first time I’d driven an Allison auto-equipped prime mover and I was very curious to see how the 6-speed tranny would behave behind the 13-litre Detroit.

Hooking up the trailer was a cinch compared with the Eaton; I was just able to idle backwards under the trailer wind the skid plate down onto the turntable and reverse onto the pin with a gentle clunk and all with no banging or crashing.

At first, the Allison seemed to transform the CST into a noisy hard-revving beast. But, if you are going to compare a 6-speed to an 18-speed, it stands to reason that the engine in front of the 6-speed is going to have to work harder and rev longer to jump between gear ratios.

The Allison makes the auto CST really just a plant the foot, point and shoot affair.

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On hitting the go-pedal, the tacho needle will wind up to 1,900-2,000rpm before heading to second gear and this continues until hitting fourth gear and the rpm start to wind back below the DD13’s peak horsepower zone of 1,800rpm and back to 1,500-1,650rpm.

What is very noticeable is how the Allison puts the power to the ground. The auto transfers as much torque as possible to the ground from an idling start. Power-wise it’s like starting off in a deep-reduction gear but being able to take off a lot quicker than you would in a manual shift.

The Allison CST also cleared intersections a lot quicker, albeit with a lot more noise, than the Eaton equipped UltraShift.

Again that tractability from a standstill came into play.

On the highway, the auto CST held its ground quite well; retardation, however, was an issue.

The DD13’s three-stage Jacobs engine brake works quite well but, behind the Allison, it’s not so great.

The reason being that at highway speeds, when the Jake is most likely to be used, the combination of Jake and Allison tries to stop the truck, rather then just bleed off speed.

Flicking on the Jake switch at 100km/h sees the transmission drop out of top gear and continue to try to down-shift. Unless switched off, it really affects the driveability of the truck.

The tranny really shouldn’t be trying to down-change at speeds over 70-80km/h.

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To conclude that one of these transmissions is better than the other in a heavy-duty prime mover doesn’t really make much sense.

The transmission choice in these two trucks comes down to the application the vehicle will be working in.

However, running these two transmissions back-to-back does highlight the strengths of these two different approaches to automated and automatic gear shifting.

For a bit of highway, a bit of country and a bit of metro, the Eaton ‘box has the goods.

With 18 gears to choose from, the tranny generally finds the one it needs in a hurry.

One of the selling points of the Eaton self- shifter has been that under the computer the hardware is all tried and true manual Road Ranger. This makes parts and service just that bit easier.

I still prefer the gear stick version especially for low-speed tasks but the UltraShift does take the stress out of stirring cogs on a daily basis.

The tough vocational hero Allison is much more application-specific than the Eaton and reduces the versatility of the CST. The sheer tractability of the auto makes it perfect for agitator and construction roles and the low-speed abilities of the auto could make it a handy tipper or truck and dog transmission.

Reversing a loaded dog trailer into a tight spot is going to be a lot easier with the Allison.

Another potential role I could see this truck and transmission filling is container side-loader work.

Side-loaders quite often have to drag a lot of weight around town and poke trailers into very tight spots to load or unload a container.

As a general around-town prime mover gearbox, though, it’s a bit of overkill.

The UltraShift has the versatility and economy, the Allison is for getting dirty.

Just think though, if musical instruments had been automated like truck transmissions a couple of decades ago, I may have actually been famous by now.

Though I also may have needed to be prettier, and of course talented.

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Make/Model: Freightliner Century Class CST 112

Engine: 12.8-litre DD13 inline six with EGR and active regeneration.

Power/Torque: 450hp (334kW) @ 1,800rpm / 1,650ft-lb (2,238Nm) @ 1,300-1,400rpm

Transmissions (as tested): 18-speed Eaton UltraShift Plus AMT; Allison 6-speed 4500 RDS full automatic

Final drive: UltraShift equippedCST-4.33. Allison equipped CST-4.56

Cab options: Day Cab, 34-inch (864mm) mid-roof sleeper, 48-inch (1,219mm) sleeper

GCM: Up to 58,000kg

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