Front differ damage

PaulSVX · #1 12-14-2005, 12:12 PM

I'm confused here, Does driving in FWD mode or with solenoid C stuck open damage the front differ, what about the actual FWD cars, they only have a front differ which is probably the same as AWD. Is it not?. So driving in the FWD should not damage the front differ. Right?

trying to make some logic, from some of the posts.

Thanks

Boxer6SVX · #2 12-14-2005, 02:17 PM

Um... I sure screwed mine up driving FWD with an AWD car... Still paying for it...

PaulSVX · #3 12-14-2005, 02:59 PM

Was your front differ replaced before?

oab_au · #4 12-14-2005, 03:02 PM

The short answer is yes, you can damage the front diff by driving with the FWD fuse in.
The longer answer is, that although the diff is the same in the FWD model and the AWD, the way the TCU operates it, to prevent wheelspin, is different.

In the FWD model, the TCU reads the rate of acceleration of the front wheels, if it exceedes the rate that is written in its memory look-up tables, it means that the front wheels are going to spin, so the TCU tells the ECU to reduce the engine power to prevent the wheels from spinning.

In the AWD model the TCU compares the front wheel speed to the rear wheel speed. If in exceedes 20%, it decides that the front wheels are going to spin. To prevent this it applies the Transfer clutch to send more of the torque to the rear wheels.

You can see that both systems are designed to prevent the front wheels from spinning, if the FWD fuse is in, or the Transfer clutch is not working, there is no way it can stop the front wheels from spinning.

The damage is caused when the front wheels are spinning, the grip varies from one side to the other, depending on which tyre has the best grip. So the load on the differential gears changes from full load to no load as this happens. The shock loading on the diff, will eventually break the ring/pinion gear teeth.

Harvey.

Boxer6SVX · #5 12-14-2005, 05:00 PM

Which is exactly what my front dif did. I noticed some time before the dif broke that I could feel it tugging the wheel left or right transferring power... it broke 3 days later... sheared a pin, chewed some teeth, lucky thats all it broke...

No my dif hadnt been replaced before that. 120k miles on it. I left it FWD via removing half the rear driveshaft because with it in I felt bad vibration from the rear with it in... Take the rear shaft out and it was fine, just the ABS light is always on. Would that have to be the rear dif or could that be in the transmission as well?

SVXer95 · #6 12-15-2005, 01:38 AM

Quote:

Originally Posted by oab_au

The short answer is yes, you can damage the front diff by driving with the FWD fuse in.
The longer answer is, that although the diff is the same in the FWD model and the AWD, the way the TCU operates it, to prevent wheelspin, is different.

In the FWD model, the TCU reads the rate of acceleration of the front wheels, if it exceedes the rate that is written in its memory look-up tables, it means that the front wheels are going to spin, so the TCU tells the ECU to reduce the engine power to prevent the wheels from spinning.

In the AWD model the TCU compares the front wheel speed to the rear wheel speed. If in exceedes 20%, it decides that the front wheels are going to spin. To prevent this it applies the Transfer clutch to send more of the torque to the rear wheels.

You can see that both systems are designed to prevent the front wheels from spinning, if the FWD fuse is in, or the Transfer clutch is not working, there is no way it can stop the front wheels from spinning.

The damage is caused when the front wheels are spinning, the grip varies from one side to the other, depending on which tyre has the best grip. So the load on the differential gears changes from full load to no load as this happens. The shock loading on the diff, will eventually break the ring/pinion gear teeth.

Harvey.

Not to mention destroy the transfer clutches!

PaulSVX · #7 12-15-2005, 09:29 AM

That explains a lot, however, what about cars that don't have a "track control"
i.e. retard timing when wheel are spinning. it is the same idea right?
If all the cars do that we will have a lot of broken diffs. I can only see that happening is when parts of the differ where replaced NOT as a set i.e. pin and crown. differ will definitely brake after 2k-3k km

So what U R saying is, you can drive in the FWD mode as long as your wheels are not spinning.

oab_au · #8 12-15-2005, 03:06 PM

Quote:

Originally Posted by PaulSVX

That explains a lot, however, what about cars that don't have a "track control"
i.e. retard timing when wheel are spinning. it is the same idea right?
If all the cars do that we will have a lot of broken diffs. I can only see that happening is when parts of the differ where replaced NOT as a set i.e. pin and crown. differ will definitely brake after 2k-3k km

So what U R saying is, you can drive in the FWD mode as long as your wheels are not spinning.

Yes Paul, As long as the wheels do not spin.

Though from what I have read, this can be a bit hard to prevent.

Track Control = Torque Control ? All the models, have and use Torque control during the gear changes to reduce the engine power by retarding the ignition timing, Its just the FWD models that use it to prevent wheelspin.

Harvey.

NikFu S. · #9 12-15-2005, 03:25 PM

Quote:

Originally Posted by PaulSVX

So what U R saying is, you can drive in the FWD mode as long as your wheels are not spinning.

A short time before my AWD started dying I burned first gear in FWD mode showing off.

It's important to never do this.

PaulSVX · #10 12-16-2005, 10:46 AM

I will not argue anymore, I got scared after all the "broken diffs" comments and I fixed my AWD yesterday, (after driving for 3k KM with solenoid C stuck open) I replaced solenoid C assembly and everything seems to be in good order (for now). The cost was minimal, but look at all the benefits. "AWD-The way it should be"

Thanks for all your comments.

Boxer6SVX · #11 12-16-2005, 06:37 PM

I broke my front dif pin at 5mph coasting... killed the bearings all the way up the shaft to the trans itself... And I can say I may have spun the tires once inthe 2000 miles that I had the rear shaft out.

PaulSVX · #12 12-17-2005, 08:08 PM

I know how that feels, when i got my SVX in June this year, front diff was broken too. see my locker for pictures.

Boxer6SVX · #13 12-17-2005, 08:22 PM

yeah it chewed up the bearings in the lower shaft going to the dif too... ugh...

TomsSVX · #14 12-17-2005, 08:39 PM

Ok... just my 2 cents...

Since all Subaru Front wheel drive differentials have the same gear ratio... 3.70 I feel that maybe Subaru treated these gear a little more to make them more resistant to wear so they could handle 100% of the power put out. WHile Harvey is correct about the TCU... there is no doubt in my mind that a FWD SVX can spin tires all day if it wanted to. SO... THe 3.54 gears in the AWD auto SHOULD be able to handle 100% power that is put out but I would not recommend this over extended periods of time in fear of destroying gears. I have also found that with every front diff I have rebuilt in an SVX auto the pinion bearing has crapped out. I feel it's lack of exposure to gear lub. is the cause. So in closing... Make sure your AWD works and you have a much better chance at keeping your gears and bearing is one piece... also keep up on your front diff fluid to make sure it is still good and the level is where it wants to be

Tom

Boxer6SVX · #15 12-17-2005, 08:43 PM

ie: dont drive it 2 thousand miles FWD because you feel its too cold to swap out the rear dif for one thats sittin in your trunk like me

it'll save you $2178.50... trust me.... I paid the bill last tuesday.