Eaton Detroit Truetrac Differential 27 Spline 1.15in Axle Shaft Diameter 3.73 & Up Ratio...

The Eaton Detroit Truetrac 912A588 is a helical-gear limited-slip for the Dana 30 with 27-spline, 1.15 in. axle shafts and ring and pinion ratios of 3.73 and numerically higher. Most buyers have a Dana 30 front axle under a Jeep or a light 4x4 that loses forward progress the moment one front tire lifts or hits ice, and they want more pull without the steering fight of a front locker. It is also a common pick when someone is regearing a Dana 30 to 4.10, 4.56 or 4.88 for bigger tires and wants the traction upgrade done in the same teardown.

What the Truetrac does in a Dana 30

A stock open Dana 30 sends torque to whichever tire is easiest to turn. The Truetrac replaces the spider gears with helical pinions running in pockets around two helical side gears. While both tires grip, it rotates like an open carrier, so turning in 4WD stays smooth. When one tire starts to spin, the angled teeth push the gears into the case and that friction shifts torque toward the tire that still has grip. There are no clutch packs or springs inside, and Eaton describes it as maintenance free, which matters in a small axle where a worn clutch-type unit can chatter or simply go open.

Symptoms that lead people here

• In 4WD one front tire spins on a rock step, snowbank or muddy rut and the rig stops.
• The truck climbs well in the rear but the open front contributes almost nothing on loose ground.
• Chipped or broken spider gears found while doing a regear, carrier bearing job or axle seal replacement.
• Whine or howl from the front axle that turns out to be worn ring and pinion or bearings, the Dana 30's most common complaints, often fixed in the same job.

How to confirm what you need

Read the ratio from the axle tag or count ring and pinion teeth, count the inner shaft splines, and measure the shaft at the spline. This case is the 3.73-and-up carrier, so it takes 3.73 through deeper ratios; 3.54 and lower gears mount on the Dana 30 Truetrac 3.54-down. Our carrier break guide covers why, and Dana 30 front axle problems lists what else to inspect while it is apart.

Fitment

Eaton catalogs 912A588 as a Dana 30, 27-spline, 3.73-and-up Truetrac with a 1.15 in. axle shaft diameter and a 7.2 in. ring gear, and the online listing we checked describes it as a front-axle unit. Eaton also builds 912A585, a Dana 30 27-spline 3.73-up case that lists a 1.16 in. shaft and the catalog note applies to non-axle disconnect style housing only. Because the two look alike, match your shaft size and housing style rather than ordering by spline count alone, and if your axle uses a center axle disconnect, ask us before buying. Eaton does not publish a vehicle list for 912A588, so send us your year, model, axle tag and VIN and we will confirm. For non-disconnect Jeep fronts, compare the Truetrac 912A585 Dana 30 3.73-up.

Install notes

Pull the inner shafts or at least slide them out of the carrier, mark the bearing caps, and spread the housing slightly with a Dana 30-80 housing spreader so the case comes out without prying on the bearings. Move the ring gear to the Truetrac with clean threads and thread locker, press on new carrier bearings, and set backlash and pattern. A Dana 30 front master bearing kit gives you bearings, races and seals in one box, and new inner axle seals are cheap insurance. Refill with a quality petroleum-mineral GL-5 gear oil and leave out friction modifier, which the Truetrac does not need.

What is the difference between 912A588 and 912A585?

Both are Dana 30, 27-spline, 3.73-and-up Truetrac cases with a 7.2 in. ring gear. Eaton lists 912A588 with a 1.15 in. shaft and 912A585 with a 1.16 in. shaft and a note that it applies to non-axle disconnect housings only.

Will it work with 4.56 or 4.88 gears?

Yes. It is the 3.73-and-up carrier, so it fits 3.73 and deeper Dana 30 ratios. Ratios of 3.54 and lower need the 3.54-down case.

Does it need friction modifier?

No. The Truetrac has no clutch packs. Use a quality petroleum-mineral GL-5 gear oil without friction modifier.

Will it make steering harder in 4WD?

Only slightly compared to an open carrier. It stays unlocked until a tire slips, so it does not fight the steering like a front locker.

Which vehicles does it fit?

Eaton does not publish a vehicle list for 912A588. Send us your year, model, axle tag and VIN and we will confirm it against your Dana 30 before it ships.