How To Make A Two-Wheel-Drive Survive, Eaton Style


In an op-ed article written a few months ago, the topic of two-wheel drive versus four-wheel drive was discussed at length. Sourcing opinions from industry and racing figures, the article came to the conclusion that just because 2WD has less capability than 4WD, does not mean off-road fun cannot be had. Rock-crawling may be off the menu, of course, but mud, sand, and snow can still be navigated with these types of trucks and SUVs.

IMG_4555The reason we bring this up again is because, in that article, we had the good fortune of speaking to Eaton regarding upgrades to open differential vehicles. As experts in improving traction to all types of platforms – drag racing, road racing, off-road, and more – the company offers great options. One of those options, the Detroit Truetrac limited slip differential (LSD), is a product that uses planetary gears to enhance grip when the going gets tough.

Applying the Truetrac to our 2000 Toyota 4Runner SR5, we foresee a lot less time spent recovering, and a lot more time spent exploring the hidden treasures of Southern California. This will be a worthwhile upgrade for years to come.

Tale Of A Toyota


A third-gen 4Runner has some off-road chops in its stock form. It is equipped with a 3.4-liter V6 making 183 horsepower and 217 lb-ft of torque, a 36-degree approach angle, a 29-degree departure angle, and about 10 inches of ground clearance. Ours received enhancements in the form of a cold air intake kit from K&N, as well as a set of killer 31-inch BF Goodrich All-Terrain KO tires.


This mud pool was no match for the 4Runner.

However, as experience has demonstrated time and time again, there have been occasions when the 4Runner (okay, the driver) bit off more than it could chew and wound up getting stuck in embarrassing locations. Sand in particular seemed to be its Achilles heel, as the IFS does nothing but steer toward doom. Meanwhile, the rearend acts like an MSNBC post-debate straw poll – always picking the losing side.

To mitigate this as much as possible, we are putting our trust in Eaton’s Detroit Truetrac (PN 913A610). Going by its design and history, we believe this LSD is just what the doctor ordered. We spoke with Eaton’s new Product Development Program Manager Jeff Saxton to get a better understanding of the Truetrac and its development, benefits, and limitations.

The Truth About Truetrac


For the first five model years of the third-generation 4Runner, Toyota made the unique decision to offer an electronic locking differential in the rear, something not commonly seen on midsize SUVs (or trucks for that matter, since Toyota also offered it in the Tacoma). These days, it makes for a highly sought after option in the off-road community, and it isn’t unheard of to find out about project vehicles given an E-locker from a scrapped 4Runner or Tacoma.

In the interest of swapping out new for old, however, sticking with a fresh Truetrac LSD is a logical option for several reasons. There to help us with these reasons, Saxton gave us the full scoop, including its background, intended purpose, and how it compares to other Eaton differentials.


The Truetrac dates back as far as the 1960s, when Eaton’s helical-gear style limited-slip differentials were first introduced in commercial applications.

The Truetrac, as Saxton explained, was “originally developed in the late 1960s for use in commercial truck applications. It was an obvious choice to introduce the design to the consumer segment in the 1970s for use in on and off-road applications due to its smooth, durable, ‘drop-in replacement’ performance.”

The goal behind the Truetrac’s development was to offer limited-slip performance to higher torque applications where long-term, smooth, and consistent performance was required. “Low maintenance, durability, and smooth performance combine to make it ideal for on-road performance, hardcore off-roading, and towing applications,” added Saxton.

Testing for the Truetrac is very important to Eaton, and is performed at the corporation’s 750-acre proving grounds in Marshall Michigan. “In this location, we can do a bunch of tests that ensure a quality product. We do 3-D computer-aided design and simulation, various assembly tests using an in-axle dynamometer and in-vehicle testing, and much more involving precise measurements and inspections,” said Saxton.

Truetrac Features

  • Helical-style pinion gears
  • Limited-slip operation
  • Smooth and automatic engagement – no driver action needed
  • Parts are maintenance free meaning more time on the road
  • Validated performance and durability
But the Truetrac is not the only limited-slip differential offered by Eaton. It’s the younger brother to the more famous Posi (also known as Positraction, a term from when it was still an exclusive offering to GM vehicles), but just because they share similar functions does not mean they operate the same.

Saxton explained their differences in detail: “Many of our customers tend to apply the Eaton Posi clutch-plate type limited-slip differential to moderate applications where a ‘traditional’ limited-slip function and ‘feel’ is preferred. This would often include the classic restoration market where consistency with the vehicle’s original design intent is desired.”

Saxton continued: “The Posi is both tunable through various pre-load systems, and is also rebuildable. It is an excellent choice for things such as road racing and drifting. In contrast, the higher torque handling capacity of the Truetrac makes it an excellent choice for demanding applications such as drag racing, towing, and off-roading.”

Those looking to get the most out of Eaton’s Truetrac on their 4×4 can do even better by getting one for the front, too. “In the case of a 4X4 vehicle, the combination of a front-axle Truetrac with a rear-axle, driver-selectable Eaton ELocker results in an extremely reliable and capable off-road vehicle that can still be used very comfortably as a daily driver,” said Saxton.



From Bummer To Stunner


Before raising the 4Runner up on our Bendpak Ranger lift, we did a once-over on the vehicle to make sure everything was in order, and then loosened the lug nut bolts on the two rear tires. The SUV was then raised over our heads and a jack stand was placed up front to balance things out. We waited for all of the gear oil to drain, and then the wheels, drum brakes (with axles still attached), and parking brake cable were removed. A clamp went over the central brake line to prevent leaking during the following stages.

A clamp was placed on the central brake line to stop any leaking during the installation.

A clamp was placed on the central brake line to stop any leaking during the installation.

The rear driveshaft flange was unbolted and moved up and away from the third member. After the 10 bolts holding the third member to the axle housing were removed, we proceeded to jar the third member out.

Now, we had to examine the backlash of the gears. The pinion and ring gear must be like yin and yang — perfectly in mesh with one another at all times — and the best way to get accurate readings was to use a dial indicator. This device measured the angle of the ring gear teeth against the angle of the pinion teeth, and told us where we had to get the Truetrac to line up once it was installed.

Left to right: The 4Runner's rear axle is a banjo type. Here, we see the third member comes out of the axle housing; the third member is laid bare; the gear lash is monitored and measured.

Left: A center punch was used to leave indentations so we would know which end bearing cap went. Right: The center bearing caps' bolts are removed.

We took a center punch and marked the end bearing caps to show left and right side markings, because to switch them up later during reassembly would be catastrophic. The end bearing retainers were removed, as were the end bearing caps, and the carrier popped right out of the third member.

A comparison of the old third member (left) against the new TrueTrac (right).

A comparison of the old third member (left) against the new Truetrac (right).

The carrier has bolt retainers that hold the ring gear in place, and had to be pried down using a hammer and chisel before the bolts could come out. The ring gear was then cleaned and made ready to pair with the Truetrac.

With the third member already out, a set of new bearings and seals was ready for installation. This was a good idea since we had no idea how old the bearings were. Not to mention, changing the bearings and seals is a recommended step by Eaton to ensure smooth operation.

The Truetrac seated perfectly with the ring gear, and we used Loctite on the bolts for added peace of mind. The new bearings were installed onto the left and right side of the Truetrac before placing the new LSD into the third member. The backlash was set on the ring gear to where it was with the old differential, and then the retaining clips were bolted back on.

Fresh bearings for either side of the Truetrac, as recommended by the Eaton installation guide, are installed.

Fresh bearings for either side of the Truetrac, as recommended by the Eaton guide, are installed.

The third member’s seating surface with the axle housing showed signs of an old gasket, so we stripped it off and applied a thick layer of silicone (instead of a new gasket) to the axle housing. After it had set, the third member was reattached and bolted down. From there, it was just a matter of wrapping everything up in reverse: bolting on the driveshaft, reinserting the axles, and refilling the 80W-90 gear oil, which took about three quarts.

Left to right: The gear lash is set; the third member goes back in, with a silicone 'gasket' ready; the driveshaft bolts are tightened; and the axles go back in.


We used about three quarts of 80W-90 gear oil to refill the axle.

The Difference In Differential


The Truetrac’s function was confirmed as the 4Runner was still up in the air by putting the gear selector in neutral, and releasing the parking brake. One tire spun forward, while the other one spun backward, just as it was supposed to.

Low maintenance, durability, and smooth performance combine to make the Truetrac ideal for on-road performance, hardcore off-roading, and towing applications. –Jeff Saxton, Eaton

Nonetheless, we had been waiting all day to go out and drive around the back hills near the shop. We aired up the tires a little bit and set off, taking it easy, but still having fun. Fire trails lining the Interstate 15 freeway made for some interesting terrain to check out, and any time to inject some joy into our day was time well spent.

When it came time to take the 4Runner out for a spin, we had to temper our expectations as we recalled an important point: although the Truetrac was installed, it wasn’t totally ready to go, as the gears inside had to settle in before we went hog-wild out on the trail. Eaton’s website states the Truetrac does not require breaking in, but it does need “a little time (depending on driving conditions) to ‘seat.’ Afterwards, there will be a slight improvement in its performance.” To be on the safe side, we decided to reset the tripometer and go for 300 miles before we gave the Truetrac a real test.

IMG_4445Going Farther In The 4Runner

David's 4Runner (New LSD) - 46 of 95

We can’t wait to get the Truetrac all situated and see what the Toyota can really do, now that it has an LSD to keep the rear wheels rocking out when the hammer drops. It was definitely useful as we headed out to King of the Hammers, where deep sand had a way of trapping the 4Runner last year. Airing down the tires and choosing the right lines, we were able to navigate through soft-sand portions of Johnson Valley that were previously all but forbidden.

Whether it was out at King of the Hammers (left) or romping around town (right), the 4Runner's traction has markedly improved.

Even outside of pure off-roading, the Truetrac has the added benefit of allowing an open differential style of performance when on the road and making turns. That, along with a one-year warranty, will add peace of mind while driving the 4Runner in the coming months.

You’ve got to have traction to have action, so if you feel your rig is losing its grip on off-roading, head to Eaton’s website and Facebook page to learn more. Stay safe, folks!

David's 4Runner (New LSD) - 92 of 95

Article Sources

About the author

David Chick

David Chick comes to us ready for adventure. With passions that span clean and fast Corvettes all the way to down and dirty off-road vehicles (just ask him about his dream Jurassic Park Explorer), David's eclectic tastes lend well to his multiple automotive writing passions.
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