Why do front diffs blow up?
 

I just pulled the tranny out of my '92 LS-L and the front diff is toast! The case has a huge crack across the bottom and up the left side. I was wondering what usually causes these to fail so catastrophically? Would a bad tranny cause the diff to fail also? I just got this car as a non-runner so I don't know the history, but I'm planning to put a new diff in it. The tranny itself appears to be a rebuild- it's got rebuild markings all over the case and the fluid was clean with no big chunks of anything in it. The car has just over 100k miles.

From what I seen and heard, they go out b/c the seals around the axles go bad and they start leaking gear oil. Once enough gear oil is missing from the diff, the metal ring & pinion gears aren't properly lubricated which leads to failure.

-Chike

This cross-sectional pic...
 

..of an SVX 4EAT trans from Mr. Pockets locker lets you see the front diff.

http://www.subaru-svx.net/photos/fil...ckets/6279.jpg

-Chike

Front diff failure.
 

We have seen a lot of front diff failures. These seem to happen mainly on US transfer AWD models. The US 2WD models don't seem to break them and the Euro models don't.

The way I see it is. Front wheel spin is the major cause. When the front wheels spin, the open differential allows the torque to move back and forth, from one wheel to the other, depending on which has the least grip. This sudden 'grip spin grip' places large shock loadings on the gears. Usually it is the teeth on the crown wheel or pinion that break off to become lodged between the gears, to force them apart, breaking the housing.( they reinforced the caseing on later models)

Front wheel spin should not happen. In the US transfer model when the front wheels start to spin, the TCU applies the Transfer clutch, to move the torque to the rear wheels, so that the front stops spinning. If the Transfer clutch stops working, the TCU can't move torque to the rear, to prevent the front from spinning, so the 'tyres smoke and the diff broke'.

The Euro can't lose the rear drive as it is gear driven, so the front diff won't get the same loadings. The 2WD models use torque reduction, if the front wheels accelerate quicker than the 'set rate' that is written in its look-up tables.

So the thing to watch is, if you get front wheel spin, check to see if the Transfer clutch is driving the rear wheels, if it isn't, get the clutch fixed.

Don't allow the front wheels to spin

Some like to put the FWD fuse in to smoke the tyres, :rolleyes: but you are diceing with 'diffstruction' :eek:

Harvey. ;)

So Harvey, what would be the next thing to check if the fronts are still spinning even after the AWD clutch and solenoid have been replaced?

I take it there are wheels on the back. :D

There are many Dave. The pressure going to the Transfer clutch at the port on the transfer clutch housing would be the first to check. This will rule out all the componets like the TCU, 'C' solenoid, wiring, piping, ect. If it has pressure but still no go, it could be the transfer housing oil seal rings, that pass the oil from the casing port, to the main shaft and the clutch, are leaking. There is the case of the transfer clutch cage breaking off the Transfer gear.

If there is no pressure at the port, you just have to trace back through the series of componets to find the faulty one. Do you have a particular case in mind?

Harvey. ;)

Correct me if I'm wrong. I think that the seals start to go and let ATF leak into the front diffy. This cuts the differential lube (thins it out) and causes some over pressure and heat which eventually leads to catstrofic failure.

I've also seen gear oil leaking out the sides of front diffs where the axles go into them. Let that gear oil level get too low and see how long it'll last! ;)

-Chike

Yes the ATF seals on the front of the box can let ATF into the front diff. But oils are still oils, although not hypoid quality, it will still lube and cool the gears. The rubbing load on the contact patch, on the teeth, will break down the oil film, to burn the teeth, till they wear through the hardening. But it is shock loading that will break the teeth off.

Harvey. ;)

My front diff blew because of front wheel spin ...but it was the transmission that was allowing the wheels to spin.

My 4EAT always seemed like it was in nuetral when in drive. If you didn't baby the gas the car would screech the front tires a tad and take out like a bat outta hell. If you slammed the gas half way or so, the front wheels would spin excessively and then take off.

Everything in the tranny seemed to check out. I think the delayed gear engagement was the fault. It was like you were nuetral dropping the trans every single time ...stupid 4EAT.

I think the MAIN reason the the front diffs are bad to go out, is because so many people replace 2 tires instead of 4 at a time.....mismatched tires will retire a front diff quickly.....my thoughts on it anyway...for what it's worth :rolleyes:

That's what caused mine to go. I wondered where my atf was going?

Funny thing to hang one's hat on, many fwd cars share the same ATF in the A/T and diff.

On my SVX previous owner didn't know about front diff dip stick, so after 318000 kms,run out of fluid and differ broke, see my locker for pictures