The Chevrolet Silverado ZR2 and the mystery of the “4WD Off”

With the arrival of the 2022 Chevrolet Silverado ZR2 earlier this year, our selection of gas-powered mega-pickup trucks has grown to three. While the Chevy gets magic Multimatic DSSV coil dampers and a 6.2-liter V-8, it doesn’t offer the decadent horsepower or outrageous pre-racer bodywork of the Ford F-150 Raptor and Ram 1500 TRX. But the ZR2’s narrowness can be seen as an advantage when your off-road environment is more suited to rock bouncers than Best in the Desert events.

The truck meets Ledge.

Ezra Dyer|Car and driver

Being 5.4 inches narrower than the Raptor and 6.8 inches narrower than the TRX, the ZR2 is suited to the realities of East Coast trails, where speeds are low and trees are plentiful. A little too abundant, I would say, after my visit to the Uwharrie National Forest, where I accidentally discovered a transfer case mode called “4WD Off” that has been discussed a lot in GM truck forums. The bad news is that my encounter with 4WD Off resulted in minor bodily harm. The good news: if you own a GM truck with a full-time transfer case, AWD mode, I can now tell you what not to do and why.

Uwharrie trails tend to be narrow and steep. I once had to bail out early on a road trip in a Toyota Tundra long bed because it didn’t have the tipping angle to reach the tall, steep drainage berms that crisscross the trails. This is definitely not somewhere you would want to pick up a TRX. Even the ZR2, I thought, could push it. You know you’re dealing with tough terrain when the trails have guardrails.

chevy silverado zr2 flexible rear suspension

Nice flexibility.

Ezra Dyer|Car and driver

One problem with Uwharrie is that no matter how many times I go there, as soon as I turn onto the access road, I immediately get lost. So when I pointed the ZR2 to a trail named Daniel, I ended up on the wrong side. Daniel climbs and crosses a mini-mountain, and on the one hand it is a difficult but relatively easy climb. The other side . . . is where I ended up. Search “Uwharrie Roll Over” on YouTube and plenty of results will be where I got to with the ZR2. You can even find a Chevy. Short wheelbase Jeeps are particularly difficult. It’s called The Ledge, and if your truck can’t catch up, it really has nothing to do. The only time I rode The Ledge was in an American Expedition Vehicles Jeep Wrangler Unlimited Rubicon with a lift and 35-inch tires.

chevy silverado zr2 riding on a rock

Ezra Dyer|Car and driver

The ZR2 has 33-inch tires, tame by today’s standards. But, thanks to its thinner front bumper, it has excellent approach angles. In fact, its approach and breakover angles are better than the TRX and Raptor on its standard tires. The ZR2 is also armored with skid plates and it gets locking differentials front and rear. But not in the middle, as I will tell you all about.

chevy silverado zr2 on a trail

Ezra Dyer|Car and driver

The ZR2 is basically the perfect size for The Ledge. Its wheelbase is long enough to provide fore-and-aft stability as the nose rises skyward (a major issue here for two-door Wranglers), and it’s wide enough to ride the rut that runs along the right side of the approach . With the lockers and serious suspension articulation at the rear axle, the ZR2 idled without too much drama, even when at least one tire was usually in the air. While the ZR2 has a one-pedal drive mode for trails, I figured in this circumstance I would opt for the two-foot method, straddling the brake and throttle to keep the transmission wound up.

chevy silverado zr2 on a trail

Car and driver

Once you get past The Ledge, you can probably hike the rest of the way to the top without too much drama (unless you deliberately pick some insane lines), but I didn’t have time to hike it all the course, so I turned around. Which, again, made me grateful for the ZR2’s relatively tidy dimensions, since it’s not always possible to turn around on Daniel.

chevy silverado zr2 with a rear wheel in the air

Coming down from The Ledge is not necessarily easier than going up.

Ezra Dyer|Car and driver

Coming back down to The Ledge, I stopped to reevaluate my line and decided to back up and take a shallower line over the rocks, to be careful with the back tee angle. Differentials locked, transfer case in low range 4WD, I straddled the brake and throttle again to make sure the truck didn’t roll forward and drag the bumper. Just when I expect the ZR2 to start slowly rewinding the last 10 or so feet of the descent, the rear end shifts sharply to the left as the rear tires spin unsuccessfully. The fronts don’t seem to do anything, like it’s not in four-wheel drive. This suspicion was confirmed when I looked at the instrument cluster and saw “4WD Off” staring at me. The truck kicked out of four-wheel drive itself, straight out of low range. No stopping in neutral, no intervention on my part.

This is a picture

Ezra Dyer|Car and driver

Which was a problem, because that rear skid placed the driver’s door right next to a tree root, with no way to move the truck without scratching it. And so, after 4×4 driving the truck back, I got off the obstacle and returned home without further incident. But I wanted to know what happened there.

Thus began a long back-and-forth with GM in which Chevy sent the truck back to its Milford proving ground to figure out what had happened. Which, in this case, means “to understand what I have done”. Because GM would know why “4WD Off” happens, right? He put it there. But the engineers wanted to know how I had discovered it inadvertently. Of course, the truck recorded the whole incident on its black box. When I finally had a Zoom call with some engineers, I told them how I straddled the brake and throttle when the truck left four-wheel drive. “Yeah, that’s what we saw,” said one. “You were over 50 per cent throttle with your foot on the brake, that’s why he did that.” OK, other than secret data logging, why is that a problem?

This is a picture

Serious suspension.

Ezra Dyer|Car and driver

Well, it turns out that GM trucks with full-time transfer cases never really lock up, front to back, because they use a clutch for front-to-rear power distribution. So when you’re in high-end or low-range four-wheel drive, the front and rear axles aren’t really physically locked together. They have that clutch in there, doing its best, but with too much load it can kick. Read the forums, and it usually seems like 4WD Off pops up when the trucks are hard at work – in the middle of a mud bog, plowing snow, or on The Ledge in Uwharrie. In other words, the least ideal times. Something owners could possibly avoid if they knew the limitations of the system and hardware inside. In trucks with a part-time transfer case, the front and rear axles are physically locked, so maybe if you plan on plowing snow, get one.

It’s a compromise, I guess. Full-time all-wheel drive (all-wheel drive) is very nice in a pickup truck, especially when driving in the rain with nothing in the bed. It’s a bargain on the street. If it’s a liability in narrowly defined off-road situations, it’s a trade-off that probably makes sense. But if you have something like a ZR2 or a GMC Sierra AT4, pay attention to the transfer case during your off-road adventures. At this point, those GM engineers had one final warning: Terrain mode automatically holds the brake until you take over with the throttle. I like to use my two feet to control the speed of an off-road truck, but in this particular situation on the track, next time I’d let the truck have its say.

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