Diff grinding/slipping?

[the_eskimo_wonder]

Just swapped in a used 3.73 LSD into my eta, along with a rebuilt driveshaft, guibo, trans mounts, and SSK while I was down there. Went to take it for a test drive and am having a pretty major issue. If I drive very smoothly and gently everything is fine. As soon as I get on it even a little bit there's a loud grinding coming from the driveline. Feels like it's lacking the pep I expected too, almost like it's slipping. No chirp into second gear (though I was a little afraid to truly be aggressively testing while it was making these noises). This will happen in a dead straight line so I'm inclined to think it's not LSD/clutch related.

Any idea what's fucked up? I did the output seals and fresh fluid on the diff before I put it in. Everything looked kosher when I peeked in from the side. If my ring and pinion were trashed it wouldn't drive decent at all would it?

[TobyB]

Have you checked for things hitting the driveshaft, rear subframe, etc?

Did the diff input shaft feel smooth?

t

[the_eskimo_wonder]

It all looked okay last time I was under there but I'l have to put it back on stands and check for contact again. Input shaft felt perfectly smooth, LSD smooth and with what felt like an appropriate amount of lockup trying to spin one jacked up tire in neutral.

The driveshaft was refurbished but used. Everything felt good but is it possible that the center support bearing is doing this somehow? It seemed good but was used

[jeffnhiscars]

Did you insure your fluid was spec ed for an LSD

[Wanganstyle]

Any idea what's fucked up? I did the output seals and fresh fluid on the diff before I put it in. Everything looked kosher when I peeked in from the side. If my ring and pinion were trashed it wouldn't drive decent at all would it?

if ring and pinion are fubar it will be either very sloppy or you wont move.


do you know if the pinion bearings are good inside the diff? random used diffs are hellllla old

the #1 job of a differential is to ROLL SMOOTHLY; if you are not rolling smoothly there is a problem. Rolling smoothly requires good bearings; after 30+ years of being r3vtarded most e30 diffs are just that.


and no, there is no snake oil to make noises go away.



I run NON-LSD oil on purpose in my cars; additive for LSD is only good for whole foods parallel parking purposes; it does not do shit for speeds over 3-4 mph.
tons of idiots will tell you that LSD oil is marvelous etc- the additives gunk up the OEM bmw ZF clutch plates over time and actually reduces the lock up in clutch plate differentials - its a friction modifier i.e. something that makes things slip more.

[the_eskimo_wonder]

Found the issue and figured I'd post for anyone that finds this thread later or is just curious. Rear subframe bushings are beyond toast (which I already knew but was playing the ignorance is bliss card on). The car is a beater I just picked up and started building as a racecar project so I think all the suspension work and hard driving made the bushings finally let go.

Luckily I already had a full set of IE rear poly bushings on my workbench. Looks like I'm just doing this job a little earlier than planned.

Anything else to replace while I've got the subframe out? Already did springs, shocks, brake pads/rotors. Planning on doing brake lines and diff mounting point reinforcement. Anything else?