Eaton Detroit Truetrac Diff 35 Spline 1.50in Axle Shaft Dia Rear 10.25in/10.5in (Full Float Only)
The Eaton Detroit Truetrac 915A550 is a helical-gear limited-slip carrier for the Ford 10.25 in. and 10.5 in. full-float rear axle with 35-spline, 1.50 in. shafts. This is the rear axle under three-quarter and one-ton Ford pickups, vans and cab-and-chassis trucks, and owners usually go looking for it when the factory open carrier spins the inside tire pulling a loaded trailer out of a wet field, or when a worn factory limited-slip no longer does anything. It drives like an open differential until a tire starts to slip, so it suits a daily-driven tow rig that still needs to behave on the highway.
Inside the case, the Truetrac swaps the usual spider gears for sets of helical pinions that run in pockets around two helical side gears. In normal driving the gears turn freely and the truck corners like it has an open axle. When one wheel loses grip, the helical teeth thrust against the case walls and the gear sets load up, which biases torque to the tire that still has traction instead of letting it all go to the spinning side. Eaton states it has no wearable clutch parts and needs no maintenance, so unlike a clutch-type Trac Lok or factory limited-slip there is no friction pack to wear out under a heavy truck.
Jack up one rear wheel with the truck in neutral and turn it by hand. If the opposite wheel turns backward, you have an open carrier. Next, confirm it is a full-float axle: the hub has a ring of bolts holding the axle flange and the shafts slide out without pulling a C-clip. Our full-float vs semi-float guide shows how to tell. For common wear and what a rebuild costs, read Ford Sterling 10.5 rear axle problems, and if you are deciding between this and a full locker, see Detroit Locker vs Truetrac.
Eaton catalogs 915A550 for Ford three-quarter-ton and one-ton trucks, vans, wagons and cab-and-chassis models from 1985 to 2019 with the Ford 10.25 or 10.5 rear, 35-spline axles, and it carries the note full float applications only. There is no carrier break: one case covers all ring gear ratios, so you can keep your gears or regear at the same time. The 10.25 and 10.5 share this carrier, but Ford changed gear and housing details over the years; if you are swapping an older 10.25 ring and pinion into a 2008 or newer 10.5 housing, the 10.25 to 08-up 10.5 conversion spacer is the piece that makes it work. Semi-float versions do not apply. Read the axle tag or door code and send us the year and VIN if you want a second check. Prefer a full locker? The Grizzly Locker for Ford 10.25 and 10.5 fits the same axle.
Pull the full-float shafts by removing the flange bolts, then unbolt the bearing caps and lift the old case out. Move the ring gear to the Truetrac with new ring gear bolts, press on new carrier bearings, and set preload and backlash, then check the pattern. The Ford 10.25 and 10.5 carrier installation kit covers the bearings and shims for this step, and the Ford 10.25 master bearing kit adds pinion bearings and seal if the pinion side is worn. Eaton calls for a quality petroleum-mineral based GL-5 80W-90 gear oil, and says synthetic oils and friction modifiers should not be used, so drain any limited-slip additive from a previous setup. Drive gently for the first few hundred miles while the new gears and bearings seat.
| Brand | Eaton |
| Part number | 915A550 |
| Fits | Eaton Detroit Truetrac Diff 35 Spline 1.50in Axle Shaft Dia Rear 10.25in/10.5in (Full Float Only) |
| Type | Differentials |
| Condition | New |
| Ships | Same business day (in stock), from the USA |
| Warranty | 1-year |
Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
Eaton lists Ford three-quarter-ton and one-ton trucks, vans, wagons and cab-and-chassis from 1985 to 2019 with the 10.25 or 10.5 full-float rear and 35-spline axles.
No. This carrier has no ratio break, so it works with all factory and aftermarket ratios for the Ford 10.25 and 10.5.
Eaton recommends a petroleum-mineral based GL-5 80W-90. Do not use synthetic oil or friction modifier with the Truetrac.
No. Eaton notes it is for full-float applications only. Check that your axle shafts bolt to the hubs rather than being held by C-clips.
A helical limited-slip needs some resistance at both wheels to bias torque. With one tire fully off the ground, light brake pressure can help it load up. A locker drives both wheels regardless.
New carrier bearings and races, ring gear bolts and a gasket or sealant at minimum. Replace pinion bearings and the pinion seal too if the axle has high miles or whines.
