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Easy to Understand - Jeep 4WD Systems

Posted in How To: Transmission Drivetrain on December 18, 2007 Comment (0)
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Easy to Understand - Jeep 4WD Systems
Photographers: Jeep Corp.

Quadra-Hydra-Trac II, Selec-a-Rock, Commando-Drive II ... Many of us have issues trying to keep all the Jeep four-wheel drive systems straight. What system came in which Jeep has been the subject of quite a few campfire arguments.

Well, we weren't even going to broach the subject until we got a nifty little brochure from Jeep Corporation that explains each one of the six current four-wheel drive systems, their features, and what vehicles they came in. So we took it upon ourselves to study the 120-page brochure and provide everyone with an easy-to-understand synopsis of the systems.

The base Jeep 4WD system has remained virtually unchanged since 1988. Sure, there were high pinions here and there, with vacuum disconnects, C-clips, and non C-clips, but we are talking about the bigger picture.

In the Wrangler, the Command-Trac system is based on a Dana 30 solid front axle, a choice of automatic or manual transmission, an NV231 (formerly NP231) transfer case, and a Dana 35 rear axle (with a Dana 44 optional over the years). Also optional for the rear axle is a Trac-Loc, which is a limited slip differential.

This is a part-time four-wheel drive system that is intended to be used as needed. With its new (for the time) shift-on-the-fly ability, this system was something to look at.

The Command-Trac is also available in the Liberty, but it's a different system.

The front axle is still called a Dana 30, but it is an independent suspension version of the 30, with an aluminum centersection. This isn't swappable with the solid axle Dana 30 and is prone to explosion when beat on with larger-than-stock tires. The NV231 remains essentially the same, and out back the Dana 35 is tossed out in favor of a Corporate 8.25 axle. There is no Dana 44 option for the Liberty, but Trac-Loc is still an option.

This is the other available system for the Wrangler that made a big splash with the introduction of the Rubicon Wrangler and, later, the Rubicon Unlimited. It features a 4:1 reduction in the transfer case and Tru-Lok locking differentials. The transfer case's 4:1 low range enables more control in technical off-road situations. The low-pressure, air-locked Tru-Lok differentials allow the driver to lock the front and rear axles from a switch on the dashboard, a first for Jeep vehicles.

Rock-Trac includes a Dana 44 front solid axle with a Tru-Lok locking differential and 4.10 gears, choice of an automatic or manual transmission, a heavy-duty transfer case (comparable to some pickup trucks with the only difference being a 4:1 low range), and a rear Dana 44 solid axle with 4.10 gears and Tru-Lok locking differential.

While both front and rear differentials are called Tru-Lok, only the rear functions as a limited slip when not in the locked position.

It's available in the Liberty (and will be in the '07 Wrangler), but the only thing that was included in our 120-page guide about this system was that it was the typical Command-Trac system.

That means it has two-wheel drive, four-wheel drive, and low-range four-wheel drive options. The low range is 2.72:1.

After some researching on the Jeep Web site, we were able to trick it into specifying a vehicle with this four-wheel drive system. The main thing that this system gives you over the NV231 is a heavier-duty transfer case. It's the same case that comes in the Rubicon Wrangler without the 4:1 low range. This includes a heavier-duty transfer case, which will outlive the Libby's front axle by a ratio of 5:1 (that's five exploded aluminum Dana 30s for every NV241 you could kill).

The transfer case is important because it includes a larger chain that will resist stretching longer than the NV231 chain. Also, it's a heavier case that is less likely to blow apart in extreme shock load kind of conditions.

The trick is how to get this system. There are limited ordering options to get the better power splitter. You've got three options for the '06 model year that can yield a good transfer case: the base sport model with Package B, the 3.7L V-6, and a six-speed manual or the Renegade Model with the 3.7L V-6, six speed manual, and either Package D or Package X. You'll find the NV241 under the Command-Trac HD part-time 4WD system. If you are left with shopping the '05 model year, there are four options; the three listed above, plus the Sport with the Package A, the 3.7L V-8, and the six-speed manual trans.

Available in the Liberty (but you can find it in a junkyard near you under many Cherokees and Grand Cherokees), this system allows the largest selection of ranges out of all the Jeep systems. With two-wheel drive, part-time four-wheel drive, full-time four-wheel drive, and low-range four-wheel drive, this system is ideal for regions that see snow, slush, and sleet as a normal part of the winter.

In two-wheel drive, it sends power to the rear wheels and can be used wherever you want. In part-time four-wheel drive, there are all kinds of cautions about only using the vehicle in low-traction conditions because the center differential mechanically locks up. And in low-range four-wheel drive, the transfer case multiplies torque by 2.72:1 and allows for more control over technical stuff. In general, it behaves just like the Wrangler's Command-Trac.

It comes with the independently suspended Dana 30 front axle and the Corporate 8.25 rear that all Libertys come equipped with.

The really nice thing here is that between the part-time four-wheel drive and the low-range four-wheel drive, there is another option: full-time four-wheel drive. This splits the power 48 percent to the front axle, and 52 percent to the rear axle and can be left in this setting for any road condition - from dry pavement to black ice. That means if you are heading out and there is sleet coming down, you can put it in the full-time range and forget about it. It'll greatly improve your control in the messy stuff without having to constantly worry about damaging it if you hit a dry spot of pavement. The full-time selection is also a blast for fire-road blasting. Talk about sticking to corners!

If you want a Jeep-branded Subaru (or insert choice of all-wheel drive wagon here), look no further than the Quadra-Trac I with the NV140 "transfer case." Available in the Commander and Grand Cherokee, Jeep touts no levers or buttons needed "To enjoy the benefits of continuous four-wheel drive." We simply give a good laugh at both the sell-out Trail Rated badge and the Jeep emblem on a vehicle that is basically no better than an all-wheel drive wagon.

With no fuel-saving, two-wheel drive range, all while offering no real four-wheel drive (with a 50/50 front to rear power split) and no four-wheel low-range whatsoever, this system is a joke. Run from it. Shun it. Make fun of those who buy it. Charge people who get stuck and need a tug if they have this pansy "four-wheel drive" system. OK, you get the point ... moving along.

Like the Quadra Trac I system, the Quadra-Trac II is available in the Grand Cherokee and Commander, but that's where the similarities end.

This system comes with the NV245 active transfer case, which allows full-time four-wheel drive and low-range four-wheel drive. It also does some fancy electrical dance that we don't really get. However, we'll explain it like our 120-page encyclopedia explained it to us.

Under normal driving conditions, this space-age transfer case sends 48 percent of the power to the front wheels, and 52 percent to the rear wheels. However, using some whiz-bang computer processing (electronically controlled clutch pack in the transfer case and brake traction control system), it is able to determine which axle has the better traction and can seamlessly transfer up to 100 percent of the power to either end.

In low range, it behaves just as the rest of the non-Rubicon Jeeps - a 2.72 reduction sends the power out to either end. This is a mechanical coupling, and it always sends 50 percent to both axles.

Also included is the independent front 7.875 (7 7/8 inch ring gear) Corporate Axle with an 8.25 Corporate axle in the rear. We are really anxious to see the miniature front ring gear take that 100 percent of the engine's power.

It is easy to confuse the Quadra-Drive II with the Quadra-Trac II. They are both available in the Commander and Grand Cherokee; both systems come with the independent front 7.875 axle and solid 8.25 Corporate Axle. Both systems are full-time four-wheel drive based around the super whiz-bang NV245 transfer case.

The big difference is the Quadra-Drive II features front and rear Electronic Limited Slip Differentials (ELSD). The electronic differential immediately detects tire slip and smoothly transfers engine torque to the tires with the most traction. In some cases, Jeep claims, the vehicle will anticipate low-traction conditions and preemptively eliminate tire slippage.

Now if you were paying attention, that means that the transfer case is constantly varying output between the front and rear axles while the differentials in the front and rear axles are constantly varying torque transfer left and right. It does this all the time, however many thousands times a minute, actively channeling power to where it's needed. We'd like to take one out in a massive rock course with at least two wheels off the ground or mud pit and play with it until the dashboard reads "ERROR ERROR ERROR," 'cause that's a lot of electrical wizardry goin' on.

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