How effective is the FWD fuse?

[nelstomlinson]

'96 Legacy wagon, 2.2L, automatic.

I put the FWD fuse in, and took the guts out of the rear diff, just left the pinion in to hold up the drive line. I put it in drive while up on blocks, and I noticed the drive line is still turning a bit, and the pinion bearing isn't getting lube, of course. Is that drive line turning just a little bit, won't get faster at speed? Or am I going to have to take the drive line out entirely? It's probably going to get driven hundreds of miles before I get a different differential in it.

[GeneralDisorder]

It's going to spin at basically full road speed because you now have no load on it. The transfer clutch regulates power transfer - with the duty-c solenoid engaged all the time it will dump line pressure away from the clutch but that will not prevent it from spinning unless there's a load placed against the driveline. The friction of the plates and fluid will cause it to spin.

Also with the fuse in place running the solenoid at 100% you still run the risk of the duty-c solenoid failing and when that happens the clutch locks with full line pressure. The solenoids job is to dump pressure away from the clutch so if it fails it fails to locked 4WD. 

To disable the rear output safely you should pull the back cover off the transmission and remove the transfer clutch pack.

GD

[idosubaru]

Can you install the diff case without the ring gear attached to it so you can leave the rear axles, keep it sealed snd full of oil?

Or find some stub shafts that fit the rear diff so you can keep it full of oil.  

The rear diffs don’t need much oil so just a little random splashes from driving and fluid being in the carrier will suffice. 

Edited December 26, 2021 by idosubaru

[idosubaru]

Easy fix - strap the driveshaft down so it doesn’t turn. It shouldn’t take much to keep it from turning. I think it just needs some light resistance snd it won’t move if everything is working properly.

Ive run FWD for a lot of miles (over 10,000) on multiple vehicles with zero issues.  Whatever the duty c failure rate is *strictly due to the FWD fuse* is so low that’s it’s probably not even distinguishable from average Duty C failure rates, and likely insignificant for your usage. 

[GeneralDisorder]

2 hours ago, idosubaru said:

Easy fix - strap the driveshaft down so it doesn’t turn. It shouldn’t take much to keep it from turning. I think it just needs some light resistance snd it won’t move if everything is working properly.

Ive run FWD for a lot of miles (over 10,000) on multiple vehicles with zero issues.  Whatever the duty c failure rate is *strictly due to the FWD fuse* is so low that’s it’s probably not even distinguishable from average Duty C failure rates, and likely insignificant for your usage. 

The duty-c failure rate is very high on that year though - use of FWD fuse might push it over the edge. I wouldn't want to risk it for "hundreds" of miles. I would just pull the clutch pack out. Really doesn't take long to do it. 

GD

[nelstomlinson]

Sounds as if I'll pull the driveshaft. That's not terribly hard, and I'll have it back on jack stands in a week or two to repair the exhaust system, so that''s half the work done already!

Edited to add: the carrier bearing got destroyed, and the ring gear carrier was wobbling on just the passenger side bearing. I yanked the ring gear carrier out, so there is nothing there to hold the stub axles. I only shoved the empty diff case back in for in case it's structural, and to hold the drive line.

Edited December 28, 2021 by nelstomlinson

[GeneralDisorder]

The transmission will lose all it's fluid with the driveshaft removed. You need the front of the driveshaft to seal up the tail shaft housing. Some driveline's - most of the non-turbo one's I believe - you can remove just half of the driveline..... as long as you can keep the center carrier bearing and only pull the rear section you should be ok. 

GD