Driving Mercedes' Quiet Diesels, Smooth-Shifting AGS

By Tom Berg, Truck Editor | September 28, 2010

MBE4000 heavy-duty (shown) and MBE900 midrange diesels now have a simplified form of EGR and updated turbos.
AGS uses electro-hydraulic mechanisms, automatic clutch and electronic controls to do most or all of the shifting work.
SmartShift selector is a paddle with small switches for Forward, Neutral and Reverse, and Automatic and Manual.

Mercedes-Benz engines for 2004 have been equipped with a simple form of exhaust-gas recirculation and refined turbochargers to meet more stringent emissions regulations, but the newly configured medium- and heavy-duty diesels remain powerful and quiet.

Meanwhile, a new-to-America Automated Gear Shift transmission will soon be available in midrange Freightliners and Sterlings. The M-B AGS is a "two-pedal" system with an automatically operated clutch, so all a driver has to do is release the brakes, select "D" and go.

Freightliner and M-B representatives recently showed off the new products in and near Las Vegas, where city streets and nearby Interstate 15 provided real-life driving experiences. I drove two semis and three straight trucks with MBE4000 and 900 diesels, and observed that the EGR seemed to have no effect on behavior compared to non-EGR, pre-'04 versions I drove in the past.

The engines will continue to cost less than most competitor engines installed in Freightliner, Sterling and Western Star trucks. That's because MBE's version of EGR uses simple reed valves instead of multiple flap-type valves, executives said. And the high-output turbo is waste-gated but has no expensive variable vanes. Then again, drivers won't get the pleasing rush that comes at low revs from Detroit and Cummins engines with their variable geometry turbos.

As before, the in-line six-cylinder MBE4000 displaces 12.8 liters and makes 350 to 450 hp and torque of 1,350 to 1,550 lbs.-ft. The 4000's standard turbo and compression brakes produce up to 500 retarding horsepower. Detroit Diesel, which handles marketing and service for MBE products, says the MBE4000 weighs 2,117 pounds—83 to 330 pounds less than competitive 11-, 12- and 13-liter diesels.

Midrange four- and six-cylinder MBE900s are now available with a front-driven PTO for snow plowing and other vocational work. Optional engine and exhaust brakes on the Sixes produce up to 250 retarding hp.

Also new for MBE900s are two displacements for each version: 6.4- and 7.2-liter Sixes rated from 170 to 330 hp and 420 to 1,000 lbs.-ft., and 4.3- and 4.8-liter Fours making 170 hp/420 lbs.-ft. and 190 hp/520 lbs.-ft., respectively.

The medium-duty Automated Gear Shift has electro-hydraulic shifting mechanisms applied to a Mercedes-Benz six-speed synchromesh gearbox. Electronic controls "talk" to those on the engine, and make gear changes according to driving and load conditions. The automatic clutch disengages and engages smoothly for each shift, whereas Eaton's UltraShift has no synchros and float-shifts without using its automatic clutch. To the driver, though, the two products feel the same.

I tried to fool the AGS into making mistakes. I jammed on the brakes, then stomped on the accelerator; AGS paused a bit to "think," then picked the right gear and we moved out. Dang. Then while cruising at 45 mph on a level boulevard, I mashed the gas and ha!—it went from 6th to 4th amid some high engine revs. Fifth gear would've been enough, I thought, but most drivers like performance and wouldn't complain.

They may or may not like Freightliner's paddle-type SmartShift, the only selector available. It's on a stalk under the steering wheel, and little switches include an M for Manual. Here, the tranny's up—and downshifts are done by pulling or bumping the paddle. After playing around a bit with the paddle, I switched to A for Automatic and let AGS do the work.

For now, AGS will be available only with MBE900 diesels in Freightliner Business Class M2 and Sterling Acterra medium-duty trucks.