LUXURY off-roaders sometimes leave us a bit bewildered. Not quite an oxymoron, but you’d have to be a moron to take an $80,000 four-wheel drive bush bashing, wouldn’t you? Any serious off-roader would stick with something cheap and durable with a washable interior. However, the folks at Mercedes-Benz certainly know how to throw a curve ball. With the arrival of the superb V6 ML320 CDI turbo-diesel the always impeccably suited Merc spin-doctor Toni Andreevski decided some brisk early morning mountain air and a rugged off-road session would convince the dismissive assembled motoring journos that the M-class was, indeed, a defender and not pretender when it comes to the rough stuff. What they hadn’t bargained on was rain of biblical proportions that turned a modest off-road drive through the Great Dividing Range into the mother of all endurance feats. And the verdict? If you plan to take your M-class off-road, go for the Off-Road Pro pack. Like the rest of the car, it’ll get you to hell and back without raising a sweat.

We like:
Build quality, comfy and supportive seats, space versatility, rear-seat leg and headroom, Off-Road Pro

We don't like:
Thick B-pillar obscures rear vision, slightly contrived rear-end styling, soft-touch tailgate but electric close function costs $1690

AS THE heavens opened, turning the already muddy track into a river and the river into a torrent, we wished we were at home beside a nice warm fire with a glass of port.

If this was fun someone ‘upstairs’ neglected to tell us. It was going to be a bad karma day with lots of muddy clothes, plenty of winching and lots of cursing.

The fallen tree blocking our path at the very start of our adventure should have been a portent of things to come.

The only saving grace was that our trusty guides had a chainsaw and we were cocooned in the lavish leather interior of the latest M-class turbo-diesel as the heavy rain and mist rolled in.

The arrival of the just-released ML320 CDI turbo-diesel brings to three the number of M-class variants now available in Australia. The other two are a petrol V6 or a petrol V8.

The turbo-diesel is a sophisticated third-generation 3.0-litre common-rail direct-injection V6 with the latest in piezo injectors.

If you’re familiar with Jeep Grand Cherokee you’ll also be aware that it shares its diesel with the M-class. They are, after all, part of the same German empire.

In the Merc, the sweet-sounding engine develops 165kW at 3800rpm and 510Nm from 1600rpm, endowing the car with a top speed of 215km/h and spirited zero to 100km/h time of 8.6 seconds.

The V6 CDI replaces the previous in-line five-cylinder engine of the ML270CDi but because of its lightweight compact construction Mercedes says it is about the same weight as the five-cylinder engine.

It’s a very quiet operator that feels more like a powerful petrol V6 than a diesel, yet pulls like a V8.

Enough of the soft-sell. How does it perform in a hostile environment?

Very well, in fact.

As we discovered, the beauty of a diesel is that it allows the driver to feather the accelerator – even in high-range - and use the massive amounts of torque at idle to delicately urge the car over every bolder and through every rut.

Unfortunately, our ML320’s outstanding engine attributes were marred by a recalcitrant electronic module in the steering wheel that confused the seven-speed gearbox – and driver, it must be said – about just exactly what gear we were in.

Left in low-range the car hunted up and down and the gear indicator on the dashboard kept cycling through the gears, making the already hazardous off-road situation even more hazardous.

In the end, even though we still had some distance to travel, a decision was taken to leave the diesel in high-range and just rev to the redline if necessary to keep the big beast moving through the rough stuff.

Fortunately with some ginger driving and utilising the engine’s low-down torque – a massive 510Nm from 1600rpm – the car adroitly traversed tracks we’d previously gone through in low-range. Were we impressed? Mightily.

At $82,900 the ML320 CDI sits in the middle of the M-class pack.

The V6 petrol ML350 is $79,900 while the range tops out at the moment with the $116,900 ML500 V8.

Mercedes expects the arrival of the 3.0-litre turbo-diesel V6 to account for up to 40 per cent of all ML sales but Aussie buyers are becoming more savvy to diesel and the ML320 CDI could do more traffic than even Mercedes anticipates.

Dynamics, not a strong point with the old car, are vastly improved.

On-road its offers almost BMW X5-dynamic responses while the steering and road-manners are commendably fluid with little bodyroll.

At 2.1 tonnes, the wagon is still a hefty vehicle but the turbo-diesel tempers the weight by offering almost hatchback fuel economy, passenger car ride, plus room for five and their luggage – ultimately up to 2050 litres with the rear seats folded.

According to Mercedes figures, the turbo-diesel ML will sip fuel below 10L/100km – it quotes a highway cycle of 7.5L/100km and city cycle of a still commendable 12.7L/100km.

We managed a respectable 11.1L/100km on a mix of 450km of highway and city driving.

Like its petrol siblings the CDI utilise Mercedes’ new seven-speed automatic and a comprehensive list of standard equipment wrapped in a stylish new body that’s bigger and roomier but weighs less than the previous 2.7-litre CDI.

The diesel enjoys the same equipment levels as the petrol V6 and runs the same suspension set-up, double wishbones up front and sophisticated four-link rear with anti-squat and anti-dive characteristics.

Like the V6 petrol engine, the CDI has full-time four-wheel drive, 17-inch alloys with a full-size spare, electronic stability control, downhill speed regulation, traction control, parking sensors, rain sensing windscreen wipers, dual front, side and curtain airbags, climate control, six-disc CD stereo, cruise control, tyre-pressure monitoring system, split fold rear seats and multi-function steering wheel.

The optional Off-Road Pro costs $10,150 and utilises the height-adjustable Airmatic air suspension with a two-speed transfer case and differential locks, which can be manually, or automatically locked, depending on your preferences.

As we discovered, it’s worth its weight in gold, allowing the car to crawl over rocky outcrops and trawl through deeply rutted wheel tracks without bottoming, or worst still, getting suspended on a rock.

Three levels of adjustment are available - 166mm, 211mm and 261mm with a maximum ground clearance of 291mm. The car’s standard ground clearance is 181mm.

If there is a disadvantage to the system it is that when on its highest setting, the suspension becomes rock-hard and passengers in the back will feel every bump.

Ultimately, when we did get stuck, it came down to the tyres, which were more suited to tarmac travel than an off-road endurance.

But we lived to tell the tale and ultimately that’s really what off-roading is all about. Telling tall tales.