Propeller Shaft Necessity

[kensei]

This spring I am planning on thoroughly going over the Baja (NA EJ251, 5MT) to see if I can isolate and mitigate a minor vibration. It seems to have increased marginally since I had to install a new propeller shaft last fall due to the failure of the original's rear u-joint. It is not so bad as to make the Baja un-driveable, but can be felt and is speed sensitive. The engine mounts seem to be in order, I replaced the transmission mounts last summer and all fasteners that I have noted are secure. In the interest of thriftiness, when replacing the propeller shaft I used the Dorman shaft over the OEM Subaru shaft. The Dorman shaft has a little different design, but is also hundreds cheaper. Part of me now wishes I would have just spend the extra money...

I am thinking that one of the easiest ways to determine how responsible the propeller shaft is would be remove it and drive the vehicle without it. I have been wondering if this would be damaging to any of the systems. It seems that, aside from not having the rear wheels driving, there should not be any ill effects given that the Baja is old enough to lack the electronic control over the AWD system that a newer Subaru may have, as far as I know.

There is a shop about an hour away from me that works on OTR trucks that advertises driveshaft balancing, so that may be an option if the vibration can be attributed to it. There is probably a lesson about the false economy of aftermarket parts in here somewhere.

Any thoughts? Thanks!

-John

[kensei]

In response to my own query, I can report that the consensus across a search of other Subaru forums is that the vehicle should not be driven without the propeller shaft in place, as the front of the shaft is an integral part of providing the seal at the rear of the transmission. Therefore, driving any distance without the shaft in place would result in loss of the lubricant and subsequent badness in the transmission...

[kamesama980]

DO NOT drive a manual Baja (turbo or NA) without the rear driveshaft.

It has nothing to do with electronics. You can drive a 4wd truck in 4wd without a driveshaft because there's no center differential. You can drive most automatic subarus by pulling the AWD fuse that engages the rear wheels. In a full-time AWD with a center diff, removing a driveshaft results in either no vehicle movement if you have an open diff or some movement and heavy limited slip wear/damage if it has a limited slip diff. The MT Bajas have a viscous limited slip center diff that will be very quickly damaged by overheating without the driveshaft.

Without the rear shaft, that side of the diff has no resistance and will spin to the limit of the viscous coupler. you'll move the vehicle but it'll feel like the clutch is slipping badly (lots more revs for the same speed). The fluid in the coupler heats up from internal friction trying to bind the front and rear together (like stalling a torque converter) causing chemical breakdown. This can cause it to either act like an open diff or a locked diff (from what I hear, usually locked in subaru centers)

[kensei]

DO NOT drive a manual Baja (turbo or NA) without the rear driveshaft.

Done and done. Thanks for the excellent technical explanation as well!