Why do front diffs blow up?

davew833 · #1 09-08-2005, 12:38 PM

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.

Chiketkd · #2 09-08-2005, 01:34 PM

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

Chiketkd · #3 09-08-2005, 01:37 PM

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

-Chike

oab_au · #4 09-08-2005, 05:36 PM

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,

but you are diceing with 'diffstruction'

Harvey.

Earthworm · #5 09-08-2005, 05:43 PM

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?

oab_au · #6 09-08-2005, 06:40 PM

Quote:

Originally Posted by Earthworm

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.

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.