Front differential failures: What gives?

[dcarrb]

Seems we're reading much about front diff failures lately. Is this a condition that eventually afflicts all aging Subarus, or is it an SVX-specific problem? And what's the cause... engineering shortcoming, neglect, hard driving, disabling AWD, all of the above? Just curious. Of all the assemblies in a car, seems a differential should be at the bottom of the list of worrisome components.

dcb

[TomsSVX]

Actually it can be many things

1. Early case designs did not hold the side-side torque from the engine well so it could literally push the carrier and flex the case.In this case, break the case or cause damage to the gears itself.

2. Spinning of the front wheels in a FWD or a car with transfer clutch issues will wear out the spider gears and cause failure as well.

3. Many people don't remember to check the front diff oil. So when the axles seals eventually begin to leak, it is not noticed early enough and can cause failure.

4. People may check their oil and notice it a little high, and don't think twice... At this time it is more than likely that the transmission oil is leaking past the pinion seals and contaminating the gear oil and will cause issues not only because of viscosity differences but also the same issues of overfilling a diff.

5. The 4.44 transmissions are a tough bit. The increased amount of torque applied to the diff can cause the case to crack, which has been documented numerous times in the SVX.

More or less many of these are SVX specific simply because of the sheer weight and torque the SVX produces. These in combination with a low gear ratio for the final drive allow the differential to become more susceptible to failures.

Tom

[JDW-SVX]

Originally Posted by TomsSVX

2. Spinning of the front wheels in a FWD or a car with transfer clutch issues will wear out the spider gears and cause failure as well.

That would make sense, given that I have not heard of any front differential failures in SVXs fitted with the VTD AWD system.

Another possibility would be the misconception that the 4EAT is similar to some other transmissions, which use ATF in both the transmission and front differential.

Just after I bought my Vortex (XT), I found that the front differential was filled with ATF - not gear oil. The level was correct, so somebody obviously put ATF in there when the fluid was changed!

Even the mechanic that I use insisted that ATF be used in the front differential! I had to show him the appropriate page in the owner's manual to prove him wrong!

This brings up an interesting question though - why do some transmissions specify ATF for the front differential, and some specify gear oil? Surely the gears are of the same design!



Jason.

[oab_au]

I would say that the main cause of failure is due to the Transfer clutch not working, and allowing the front wheels to spin. If they spun evenly it may not worry it too much, but as there is an open diff in the front, it will let one wheel spin and the other one grips, then it keeps changing sides, this causes a lot of shock loading of the pinion/crown wheel gears, that something has to give.

They did add more ribbing to the front diff casing to strengthen it, this reduced the chances of pushing the crown wheel out the side, but it didn't help the gear teeth survive.

As Jason says; The VTD AWD doesn't seem to have this problem. This is because it can't lose drive to the rear wheels, in fact they will spin the rear wheels instead, and as there is a Viscous center in the rear, both will spin together, so there is no 'spin/grip/spin' to hurt it.

So the message is, if you notice that the front wheels spin a little or chirp on the change, the Transfer clutch is either not working or it is too slow engaging, and the front diff is at risk. The fitting of 4.44:1 gears won't add to the load, it reduces the load on the gears. The higher the gearing the higher the load.

Jason the diff oil has to suit the type of gears used. If the gears are spiral bevel or helical they can use a lower load oil. The gears that are in the front diff are a hypoid bevel set, the pinion is not on the same center line as the crown wheel, so the teeth do a lot of sliding across each other, so the oil has to be a high sulphur oil suitable for this action.

Harvey.

[dcarrb]

Originally Posted by JDW-SVX

Even the mechanic that I use insisted that ATF be used in the front differential! I had to show him the appropriate page in the owner's manual to prove him wrong!

Some time back I took one of mine to my regular mechanic for a transmission fluid change and when I picked it up he said, "You got a free transaxle lube." This guy has serviced my cars for years but had gone and drained the diff by mistake. Of course he realized his error immediately, when he saw what came out. (And yes, I did double-check.)

Thanks for the responses.

dcb