TwoXJS

Hey, all. I've started to recently notice the rear end on my XJ making some noise, at least I think it's from the rear end. For those who don't know, Jeep in question is a '93 XJ 2.5L, AX-5, NP231, 30 front and 8.25 rear. Best way to describe the noise is kinda like a loud roaring sound when it gets up to highway speeds, like 55-60+. Below that, it sounds fine.

I did change the rear diff fluid recently and refilled it with thishttp://www.walmart.com/ip/Super-Tech...allon/16795067. I know it's probably not the BEST stuff I could get, but it was for the money at the time about a month ago. I strongly believe I remember it making the same noise before I change the fluid, but I'm not 100% sure.

I didn't put any friction modifier since I don't know if I have an LSD or not. I can't find any other tag on the diff cover bolts other than "4.1", which I assume is a 4.10 gear ratio? There is one other tag, but it seems blank, which is kinda weird to me....Other than searching more, what would be the best way to determine if I do have an LSD or not? I do remember that when I drained the old crud out, there wasn't any metal particles that I saw, just what looked like clumps of carbon and it smelled HORRID!

Roler

diff oil smells a bit iffy, thats normal.
You can tell if you have an LSD, by jacking up the rear and spin one wheel and observe what the opposite wheel does.
in case of LSD, you will have to add the friction modifier, if thats not included in the oil. I recall some synthetics here have the stuff in it, the 80w-90 doesnt and will require adding.

TwoXJS

What should the opposite wheel do with or without an LSD?

Roler

opposite wheel spinning same direction: limited slip diff
opposite wheel spinning other direction: non LSD

TwoXJS

Ah, okay. I'll check it out tomorrow when I've got some time and report back! Before I forget, could you confirm for me if that "4.1" tag means a 4.10 ratio or something else?

Roler

mine are missing, always have, so cant check for ye....but yeah, the ratio is one of the tags on that diff cover. Dont know much about the available ratios though. Sure somebody else can chime on on that.

djb383

IF u do have a LSD, it's doing nothing at 55-60+ mph highway speeds, so the noise u hear is likely an axle bearing on it's way out.

The only time u need LSD additive is IF the LSD clutches make "chatter" noise while making a sharp slow 90 degree turn. The LSD clutches r doing nothing when the vehicle is going straight, which is 99% of driving (going staright). The only time the clutches do anything, as in "slip", is in a sharp slow turn......the axles need to differentiate in a turn. If the diff is quiet in a turn, then the clutches r slipping normally and there's no need to make the clutches slip more by using the additive. More "slip" just means less torque to both wheels.

Roler

That's some nice info there djb, I was under the impression the additive always had to be in there.

s346k

you had the cover off and you can't tell if it has a limited slip dif in it?

TwoXJS

Okay, that makes sense! Now that you mention a chatter noise, there is something that sounds kinda like that. It normally always happens after starting out in a turn, like out of a parking lot. It almost sounds more like it's coming from the front end, but I'm not sure. It's like a metallic rattling or chatter sound is the best way I can describe it.

I wouldn't know what an LSD looks like, to be honest. I didn't really look for anything that might suggest I have one, I just inspected the gears and cleaned the old oil out before sealing and refilling it.

djb383

Too much additive can turn a perfectly good LSD into a "open -one tire fire" diff. I've seen folks refill clutch type limited slip diffs with gear oil that already has the additive in it and then they add a tube of the additive (they bought from the dealer) to the diff. They've now converted their "traction enhanced" diff into worthless "open" diff because the clutches that were designed to "grip" 99% of the time now "slip" 99% of the time. I'd much rather have a little clutch "chatter" (only in a tight turn) and also have max torque to both wheels. I would never use a gear oil, with the additive already in it, in a clutch type LSD. If the diff chatters too much (in a tight turn) I would add no more than 1/2 of a tube of the additive and go from there. Works for me.....YMMV.