1987 325E LSD Whistle/Grind

[Pro1914]

Hey folks, just decided to do some preventative maintenance last week by changing the differential fluid, but now I regret even trying haha. Everything went smooth, let the oil drain all out, then filled it back up with 75W90 RedLine. For the first day, I heard no noises whatsoever. Then the second day, when backing out of my parking spot at a low-speed turn, I heard a clunking/grinding noise. Not sure what it was, I just figured it was a random noise and went about my day. On the third day, anytime I went over 35MPH, the diff would scream for its life! Even with the windows up and the radio on, I could hear clear as day something was making a very high-pitched whine from the rear. And every time I would take a corner at low speeds it sounded like the titanic was sinking... Maybe it just had to break in? So I went one more day before I knew something was wrong. Jacked the car up, and checked the level, everything looked good. So I figured I'd put the old stuff back in since it wasn't making noise before. If the noise went away, it was the diff, if not, then it was something else, or I had broken the diff lol. But sure enough, old stuff back in, and it was dead silent for a couple of days, but now the whistle is slowly coming back. The new stuff I had pulled out was charcoal black and did not look healthy at all. Does anyone have any idea what I did wrong? Should I run a thicker oil like 80W90, or 75W140?


TLDR; Changed differential fluid, started grinding, went back to the old stuff, it doesn't grind (as much if at all). What should I do?

Thanks!

[jbontke]

Did the 75w90 have the friction modifier for limited slip differentials? If it did not, you may have burnt up the LSD clutches (hence the very dirty newer fluid; dirty was clutch material).

Use 75w140 that has the LSD friction modifier or add some friction modifier if it does not have it. 75w90 is fine, but 75w140 is suggested as the replacement weight.