Summary: I installed a Detroit TrueTrac by Eaton in the front diff of my 99 4Runner which has also had a manual hub conversion.

There are two fitment issues that had to be corrected which I detail in this writeup

(1) The CV shaft support bearings are missing in the Eaton design and are necessary for proper operation especially in an ADD-equipped truck.

(2) Because I have manual hubs the stock Tacoma CVs had to be modified to allow snap ring engagement; this problem is unique to the non-ADD Tacoma CVs. I will also review the differences between various CV shafts.



I was interested in installing a Detroit TrueTrac limited slip in the front diff of my 4Runner to further improve its traction. My old 1st gen had a TrueTrac in the front and back and it was a great all-purpose combination. Others may be better served with selectable lockers but this setup works well for me. A big shoutout to @mtbtim for helping me R&R my front diff *twice* as I worked through the install quirks.

Information on the fitment of the TrueTrac is scattered and incomplete. Eaton has updated the part # for the 7.5” Toyota clamshell TrueTrac a few times over the years and they have yet to perfect the design. From what I have read their product, despite being an excellent limited slip device, has poor application development support. There are a number of forum threads related to 3rd gen 4Runners and 1st gen Tacomas with unresolved questions about installation problems with the TrueTrac. The lack of information about this device made this an interesting project for me — I wanted to figure it out and make it work and share my findings with the board here.

The TrueTrac replaces the stock open differential carrier. The ring gear bolts to the TT which then has to be setup in the differential housing. I opted to have ECGS order and modify a TT and ship it to Zuk for install. Zuk did a writeup of this install with a lot of pictures for your enjoyment — http://gearinstalls.com/Jordan75clamTT.htm. (Side note: Zuk is a master of his trade and a pleasure to work with. I highly recommend him if you have any diff setup needs.)

CV support bearings in differential carrier:

The factory differential carrier has needle bearings on both sides to support the CV shafts. When the unit “differentiates” (that is, when the CVs are turning at different speeds than each other) there is a relative motion between the CV and the diff carrier. This is because the side gears (which the CVs plug into) are moving with respect to the carrier. The needle bearings support the end of the CV which would otherwise be sliding against the carrier. In the picture below you can see the needle bearing on one side of a factory diff (I borrowed this pic from Zuk’s writeup).



For some reason, Eaton decided to just leave these bearings out. So when the CVs are installed they are supported directly by the carrier. Steel-on-steel sliding, even immersed in gear oil, is not a recipe for longevity. Some people have reported CV shaft ends galling against the carrier over time.

This is especially important if your truck is equipped with ADD (like all 4Runners and most Tacomas). The CVs are running all the time. When in 2WD the passenger side CV is disconnected from the diff by the ADD system but the driver’s side CV is still spinning and connected to the diff. In fact, the stub shaft on the passenger side actually has to run the same speed *in the opposite direction* to prevent the front prop shaft from spinning.

East Coast Gear Supply developed a nice countermeasure to this problem which is a solid bronze bearing. This is a special kind of bronze alloy used in bronze bearings/bushings that can handle large loads and heavy wear conditions. They are as good or better than Toyota’s factory needle bearings. In fact, ECGS sells this as a replacement to the needle bearings in case the factory needle bearings get damaged or wear out.

ECGS bored and installed these bronze bearings on both sides of the TT carrier for me. Their install was clean and very high quality.
You can see the bushing installed in the picture below. It’s grey/silver in color:



Note: I recommend that if you are installing a TT into a truck with ADD that you have ECGS install these bushings. That is the only modification you need to do and the TT unit will operate fine. As you’ll see later my issues with the CV shaft fitment was because I have manual hubs. If you do not have ADD then there’s really no need for these bearings as your CVs simply don’t spin very much. But, if you have manual hubs you will run into issues with the CV snap ring fitment.

CV end snap ring fitment issues:

Now for the fun part. As mentioned previously I did a manual hub conversion on my 4Runner. It is still equipped with ADD.

There’s a stub shaft that installs into the passenger side of the differential. It is part of the ADD tube. The passenger side CV plug into the ADD tube — there was no issue with all this. Everything fit as it should.

However, I soon found out that the stock Tacoma CVs did not fit into the driver’s side of the differential all the way. It would go in 90% of the way but the snag ring would not engage. Without the snap ring engaged the CV can be pulled right back out and does not engage with the oil seal properly.

It was a puzzle to determine the offending feature preventing the CV end from plugging into the diff but it turned out to be that the splines were not long enough. The CV splines would run out just before the snap ring was able to get beyond the internal splines of the side gear where there is clearance for the snap ring to “snap” out and lock.

Interestingly enough ECGS had tested and confirmed fitment of a CV for me to help me confirm the TrueTrac would work for my application. But it turned out they were using a normal ADD CV — it had never occurred to ECGS or myself that the CV ends could be different.

In troubleshooting the issue I got access to three CV shafts to experiment with: 1) a stock Tacoma non-ADD 2) stock 4Runner ADD and 3) an aftermarket Cardone CV for non-ADD applications.

Here are a series of pictures showing the inner CV ends and how they fit into the driver’s side of the TT-equipped differential.

Stock non-ADD Tacoma manual hub CV:





As you can see, the dust flange on the CV end is quite far from the oil seal. At this point the snap ring is also not engaging.

Stock ADD 4Runner CV:





At this point the snap ring has just engaged. The dust flange still has a small gap from the seal but we could always cheat the seal outboard a bit to compensate. The splines are clearly longer on the 4Runner CV than the Tacoma CV. Why? Who knows.

Aftermarket Cardone CV:





Here you can see that the aftermarket CV actually fit very well. Its splines are longer allowing the CV to install deeper. The dust seal is properly located with respect to the seal.

One solution would have been to run this aftermarket CV on the driver’s side and call it a day. Unfortunately, as many of you know, aftermarket CVs are quite inferior to stock ones with respect to strength. I have personally had two wheeling trips prematurely interrupted by a busted aftermarket CV and I’d rather not repeat it.

Note: It had occurred to me that perhaps the stock 4Runner inner CV end could be “grafted” onto the stock Tacoma CV. This is not possible — the inner splines are different sizes and the tulip joints are also not interchangeable. The 4Runner stuff is all just a little bit bigger than the Tacoma stuff. This would have been a nice solution but alas.

With some help from a friend with a lathe we came up with a solution. The idea was to cut a small relief groove at the base of the splines on the Tacoma non-ADD CV to allow it to insert further into the differential. I removed the outer bell and it popped into the lathe nicely:



We removed a very small amount of material about 60 thousandths long. We took a few passes making the cut deeper until it just fit into the differential. At the thinnest point the diameter of the axle was reduced by a total of 0.030” (0.015” deep plunge). Final product:



Here’s a before and after picture of the CV end:



I’ll spare you the math but I calculated that the removal of this material should reduce the strength of the shaft by a little less than 10%. It’s a compromise solution I still feel much better about using a quality axle with a slight weakening than an aftermarket axle with substandard metal or heat treatment.

After the relief groove was cut the CV popped into the differential successfully, just like the ADD 4Runner CV. Success!!



At the final install we ended up installing a new driver’s side oil seal and seating it a bit shallower to allow it to mate with the CV end better. Side note... when doing seals on a diff it sure is nice to have a big ass vise



The install of the TrueTrac turned out to be a lot more convoluted than originally expected. I hope that these details will help others know what modifications need to happen to ensure a successful install.

Excellent tech on the Truetrac issue

I am interested in installing the Tru Trac in my front diff as well. I have a 99 with lockout hubs and when I contacted ECGS they replied “Unfortunately, at this time we can only do the true trac in the rear axle.” If I understand you correctly, it can be done if you also:

(1) Have ECGS bore and install the bronze bearings to replace the needle
bearings on both sides to support the CV shafts

(2) run aftermarket CV on the driver’s (but, I agree this is not desirable
because aftermarket CVs are notorious for being inferior to stock ones
with respect to strength. OR machine a small relief groove at the base
of the splines on the Tacoma non-ADD CV to allow it to insert further
into the differential.

Is this correct? Those are the only additional steps required? It is the machining part that may present an obstacle, I am not a machinist nor do I have access to a lathe if I were foolish enough to try it myself.

First, I want to second that ZUK is awesome to work with. He did the rear on my Camry Alltrac. That one has a 'huge' 6.7" ring gear.

Do you mind letting us know how much ECGS charged for installing the bronze bushings?

This might put a damper on my plans for my front end... though maybe not, since I'm kinda thinking about going full time 4WD...

-Charlie

Charlie,

I'd rather not post anything about pricing here but it was reasonable. I think ECGS is a stand-up operation and will be fair.

Yes, that is correct. BTW, the machining was totally not a big deal.

Also, if you already have manual hubs and don't intend on driving around with your hubs locked a whole lot or using 4WD at highway speeds then you could easily forgo the bushing install.

Also, another thought would be to see if a company such as ECGS might be able to machine the driver's side side gear of the TrueTrac to make the snap ring engagement point a bit outwards, to allow the factory non-ADD CV to fit. This was not really an option for me.

Doesn't the issue come up with the hubs are unlocked or the ADD is disconnected? If the carrier is spinning much slower than the driver's axle, both CVs/stubs are then spinning in the carrier at high speed.

-Charlie

edit: totally understand about the machining price.

If you have manual hubs and they are unlocked then nothing up front is spinning (as long as the shifter is in 2WD).

Of course! I ready it backwards.

-Charlie

Update: I had an opportunity to address the CV fitment issue when I had the front diff apart to regear. ECGS machined the side gear a bit to allow the OEM non-ADD CV axle end to fit in and allow the snap ring to engage. Kudos to ECGS for being willing to take care of this for me. They now have all the information necessary to help fit a TT to the 3d gen front clam diffs regardless of whether or not have ADD, manual hubs, etc.

Here's the front diff driver's side fitment with OEM CV now:

@JZiggy -- Is the following summation correct:

1. If using ADD CV-axles, highly recommend having ECGS install their bronze bushings on the side of the carrier to address lack of needle bearings provided by Eaton with their TrueTrac LSD.
2. If using OEM Tacoma non-ADD CV-axles in a manual locking hubs conversion/modifciation, have ECGS machine the side gear to enable proper seating of the OEM non-add CV Axle in the differential such that the snap ring properly engages (thus avoiding oil leaks).

Question #1: if one has ADD CV-axles now, but wants to eventually do a manual locking hub conversion down the road, will having ECGS machine the side gear cause problems with the (initial) ADD setup?

Question #2: In #2 above, with the OEM (Tacoma) non-ADD CV-axles, if ECGS machines the side gear, then does that mean one does NOT need to lathe the relief cut as you had stated in post #4 above?

Many thanks for your writeup on this.