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Rear Differential Differences


December 22, 2016
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City, State
Year, Model & Trim Level
2003 Mercury Mountaineer
A month ago my front pinion bearing went bad (didn't catch it quick enough, thought it was a wheel bearing), lost all the oil, and I had to change out the entire front differential.

Today my daughter took the vehicle to school and on the way back it now appears my rear differential went bad, leaking oil and making horrible noises.

It's a 2003 4.6L AWD Mountaineer, 3.73 limited slip.

I believe in '04 they came with traction control, '03 did not have it.

Can I replace mine with one from an '04?

Hopefully someone else a lot more familiar with this will respond but I was under the belief that the traction control only changed the distribution of the brake fluid to apply more force to the slipping wheel thus forcing the wheel that still had traction to turn. Having said that I cannot see any reason why an "04" differential would not work in an "03" (assuming they are the same). Of course you would also have to replace the shaft seals that caused the original loss of fluid. The only issue you might have is with traction control I would think that each rear wheel would have a speed sensor where without it there was only one on the differential as is the case with my "02". This should only be an issue if you were interchanging the entire rear axle.

Vehicles with IVD (Integrated Vehicle Dynamics I.e. Advance Trac) share the same differential case as the standard model, but you may find a difference on the inside as well as if your speedometer and ABS work correctly.

The IVD equipped used tone rings on the individual axleshafts and may not have a tone ring on the ring gear even though both have a hole for the speed sensor. If you have IVD, you will NOT have a limited slip, no sliding vehicle icon on the lower left of the cluster, no speed sensor at top right of differential (just a black plug) and no traction control button between the cupholders.

Also make sure the axle tear tag (metal tag bolted to rear cover) matches yours or the door code for 'axle' matches. The ratios must match if 4x4 front and rear.

Since you have 3.73 L/S you do NOT have IVD/traction control.

Just to be sure it will work correctly, verify if yours has a speed sensor up top. If it does not, any should work. If it does, you need to pop the cover for the speed sensor (one small bolt) and make sure the tone ring is there.

Why not rebuild what you have? Regardless I would make sure to update to the new Ford axle seals (expensive but good). Would hate to spend $30/seal on a used diff and have it go bad to have to do it again.