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  1. #1
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    Differential upgrade - Truetrac for the front D-30

    Trutrac.jpg
    I installed a Truetrac LSD (part# 912A585 ) in the front D30 differential with the help of some friends in the Washington 5.9 group. It was an easy setup since I wasn't changing gears (yet), and my gears were in good shape - I was able to use the original factory shims, with adding only one 0.010 shim to the passenger side to bring the lash to the same .007 measured before disassembly, and give the bearings their proper tight fit for preload. The Truetrac is designed that way, but they really nailed it - not everything made in Taiwan is crap! Truetrac is a high-bias, clutch free helical planetary gear LSD. Here's a cool video that shows how it works: https://www.youtube.com/watch?v=lZmsY2YvVsc

    Truetrac performance report: so far it's awesome. My main object was to keep the front left tire from spinning off red lights on rainy Seattle streets, but it would also give traction advantages off road, and on the drag strip. It does everything I hoped it would do, and nothing I was worried it might do (yet).

    There was enough rain to do a launch test after the install last night. As I'm sure most have experienced on slick roads, normally almost all the torque gets transferred to the front left spinning wheel due to the open carrier front diff, giving very weak acceleration - with Truetrac installed in the front both rear wheels spin (maybe all 4?), and there's much harder acceleration since all 4 tires are doing work.


    I was worried the front would behave as front wheel drive cars do, and my friend's hoity toity BMW X5 does in tight turns under power, where you can feel the torque shifting back and forth, unsettling the chassis and pulling on the steering wheel. If Truetrac did that I would have put the open diff back in immediately, because I hate cars that do that. Fortunately it doesn't do that at all, and I tried all different types of cornering on dry and wet pavement. The ONLY difference in feel I noticed was steering felt slightly stiffer, and it tends to track straighter on the freeway, but there was absolutely no pulling or pushing, or instability. If anything, it feels more stable. When wet-cornering at high power, the 60% rear bias of the 249 transfer case pushes it into a predictable oversteer.


    The only remaining test to be done is on ice/snow. In 4WD with locked diffs turns can be scary because it tends to go straight - Truetrac is designed to operate like an open diff when there's no input torque, so I'm hoping it won't have that problem in the snow. I'll return and report when there's snow for the test, but for now I see no negatives to this mod, and I think it's easily worth the cost (especially when you have friends help you install it!). It absolutely exceeds my expectations so far.

    Diff cover 1.jpg
    Also, installed at the same time, an LPW Ultra diff cover for the D30 (they have one for the 44A also!). I installed it with a Lube Locker gasket - gets rid of silicone, and it's reusable. It's similar construction to Felpro head gaskets with silicone ribs printed on a rubberized metal core. Lube Locker says they'll have one for the 44A in 6 weeks also.

    The LPW Ultra cover has load bolts that press down on the bearing caps to increase the torque capacity, and it's built very strong, with thick cast aluminum and reinforcing ribs to the load bolts. It's really made to handle bigger tires for off-road, but would do the same for high power engines. They don't make a polished version - I did the polishing because I can't help myself when I see bare cast aluminum!

    I also replaced the bearing cap bolts with ARP studs (also available from LPW for D30 and D44A).

  2. #2
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    Thanks for the review, very interesting.
    98 ZG 318 limited: np 242 swap,dana30hp swap,aussie locker, shift kit, optima red, 170lbs reduction, 5.9 vents, kn air filter, air ram, 1.7 HS RRs, SCT 93 oct tune, magnaflow muffler 12255, magnaflow hi-flo cat, spectre air hat, 2x52mm tb, 5.9 efan swap, ngk fr5-1 plugs, mopar perf wires, new cap 'n rot, maxxperf coil, iat relocation, roof lights, pirelli scorpions on masitaly 16x7 rims ET=0, ome HD coils +2",rubic exp +2"shocks, skyjacker ss, rubic exp adjustable front arms, jks rear+procomp front trackbar, prothane engine n tranny mounts, prothane sway bars bushings, drilled n slotted rotors, aeronautical front brake lines, purple led into front grill, hella h4s 100/55w, osarm h3s 55w.
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  3. #3
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    Very nice, Ive always wanted a TT in the rear axle of my other Jeeps that have an open diff.

    Just out of curiosity, I guess since you were spinning the front left tire then both back tires were spinning? Assuming that your rear LSD is still working.

    The 96-98 249 is directly coupled to the rear diff and only sends more torque to the front axle when the rear is slipping.
    1998 Jeep 5.9- Slate Black


  4. #4
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    Quote Originally Posted by Jeepjeepster View Post
    Just out of curiosity, I guess since you were spinning the front left tire then both back tires were spinning? Assuming that your rear LSD is still working.

    The 96-98 249 is directly coupled to the rear diff and only sends more torque to the front axle when the rear is slipping.
    Hmm... good question. It depends on how much traction there is.
    Before TT: On wet pavement both rears will spin, and front left, so there's no weight shift, and very little power to the ground. With moderate traction I can't spin the rear wheels (yet), but the front left can still spin, but doesn't matter as much because there's good weight transfer to the rear. I guess the FL tire isn't actually spinning in this situation (I don't think it can go faster than the rear), but it squeals.

    I figure once the new motor goes in it will behave on dry pavement as it does on wet without TT, so this, and the fact that roads are wet in Seattle 364 days a year, are the reasons I installed the TT.

    Also, in corners when I hit the power and it shifts to oversteer, both rear and either front wheel can spin (without TT), which reduces its power to keep the front ahead of the rear. This is very noticeable on wet pavement- it used to get really squirrely, but now it's much more controllable, almost anticlimactic. Even when I intentionally induce the back end to slide out it won't do it as much as I expect- the front does a much better job pulling it through the corner now. I haven't pushed it that hard on dry pavement yet, partly because it's not time for new tires yet!

  5. #5
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    Just got the TrueTrac in the front of my wife's 5.9L. She's running an NP231HD, but when winter actually hits and we have some snow/ice build-up on the streets, it'll stay in 4-high essentially until we see asphalt again. We've BARELY had any snow so far, so we're mostly still in 2WD, but there's a fair amount of ice in our neighborhood so we'll pop it in and our of 4-high on our side streets each day. It DEFINITELY feels more sure-footed with the TrueTrac on the ice, especially in corners. I ran it in 4-high on the highway for gear break-in (more or less on straight stretches, or with gentle curves) and concur that steering "feels" tighter and it tracks straighter.

  6. #6
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    Quote Originally Posted by AK-RWC View Post
    It DEFINITELY feels more sure-footed with the TrueTrac on the ice, especially in corners. I ran it in 4-high on the highway for gear break-in (more or less on straight stretches, or with gentle curves) and concur that steering "feels" tighter and it tracks straighter.
    Glad to hear it's an improvement on ice and snow. This was the one thing I was concerned about, but hadn't tested. I'm sticking with the 249, so always 4WD. There doesn't seem to be any downside, except cost, which isn't really too bad.

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    Just put 500 highway miles on the TrueTrac, probably 70% of which was 4-high (again, NP231HD rather than 249). I definitely felt more stable, and will describe the steering as "heavy." The drive down was ~97% in 4-high due to icy roads, and I barely noticed. At one point I popped into 2WD and punched it to verify the roads since I hadn't been slipping--immediately started to spin out. Back into 4-high and did it again--launched cleanly. Drove back in daylight and pushed some higher speeds sinc it was easier to make out the road conditions. Zero concerns.

  8. #8
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    Thanks for the additional feedback. How did it to in tight turns on ice? That's where my 79 Jeep would push through the turns when it was locked. It could be a little scary if you weren't expecting it.
    "Heavy" steering is a good way to describe it. I don't think it's a bad thing, just the LSD giving resistance to the steering. Especially on bumpy turns I think it's more positive steering control, just like it tracks straighter on straight roads - just takes a little more steering input force.

  9. #9
    RallyJeep GO
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    Truetrac shouldn't make it push much more than an open diff when you're off the throttle in 4wd (it doesn't have much preload). In 4wd, it'll definitely push more when you're on the throttle though. And I'd expect no noticeable effect when in 2wd either.
    1998 ZJ 5.9 Limited - Deep Slate
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    Almost 4 months later, and nearing the end of tax season for me, I have more experience with the Truetrac, including snow.

    Short version- as AK-RCW said, it's no problem in the snow (it's great actually!), and as Rob said it does push more in corners when you're on the throttle, but not such that it's an issue in any situation.

    Long version:
    The pushing in corners on throttle is very subtle, but it does happen, but unlike the push from a fully locked transfer case of diff lockers, it does not induce understeer because it only happens on power, and with the 249 throttle induces oversteer. The pushing is hard to notice because the small amount is easily corrected with a little more steering input, which is instinctive. It took me quite a bit of testing before I was sure it was happening at all.

    On Christmas eve I drove a friend and his family to Leavenworth WA, including over a mountain pass and LOTS of snow and terrible icy road conditions. We lost count, but saw at least a dozen vehicles that lost control and slammed in snowbanks or other vehicles- I saw three as they happened. Jeep had no trouble. It did get slightly sideways a couple times, but the Truetrac keeping both front wheels powered makes it extremely easy to tame oversteer.

    The next weekend, a bunch of guys in 4 niners from the WA 5.9 group drove up the same highway over Stevens pass, and we took turns screwing around in an ice-covered parking lot. The Truetrac almost took the fun out of it. I could still make it do tornados, and I could make it crab-walk the whole length of the lot. I'd induce oversteer, stay on the gas, counter steer, and it would take a 30-degree sideways posture and hold it, completely controlled all day if there was enough room. No car is idiot proof, but Truetrac and ABS come pretty close.
    Sorry, my computer with photos and a video crapped out just this afternoon, or I would have posted them. For now this is the best I can do:

    Oh - on my drive to Leavenworth I noticed a wheel bearing starting to make noise, and while crab-walking in the parking lot it was making a lot of noises. At the time I thought it might be the Truetrac working hard, and maybe it had a problem, but fortunately when I finally replaced the wheel bearing all the noise stopped.

    If I had three thumbs I'd put them up for Truetrac, but I only have two

  11. #11
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    Very nice review!! I've been wanting truetracs for a while now but whenever I can afford stronger axles/regear. My rear lsd is shot, so on rainy days I could spin the back right and front left at a launch lol. Running without front driveshaft since the 249 is stuck in 4wd. Launching is comical. Burns out one wheel all 1st and 2nd, tiny bit in 3rd. Pretty sure I'll need new tires once engine build is done, even though these tires are only a couple months old haha.

  12. #12
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    If your traction problems are that bad, it's probably time for stickier tires. I need to rebuild my trac-lok in the rear as well (and I'm running a 231, so no 4wd on the street) but unless I get really stupid with the throttle in a hard corner or something, the lack of LSD has been surprisingly not much of a problem. Then again, it's pretty rare that I launch the thing. But coming out of a corner at 30 at WOT? That happens more often than it probably should...
    1998 ZJ 5.9 Limited - Deep Slate
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  13. #13
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    Tires are Firestone Destination A/T's. Pretty happy with them, don't think I can gain any more street traction without giving up off road grip. Maybe 265 in the future vs the 235's I have now.

  14. #14
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    Yeah, gaining more street traction involves having more sets of tires. I'm up to 3 for the ZJ... A set of snows for winter, sticky street tires for summer and a set of grooved M/Ts for when I need off-road grip.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

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