Ok, I've got a '79 SA with an LSD rear diff/axle from an '85 GSL. It was a used axle from a non-driveable car, so I didn't really know the condition. I've put maybe 5k miles on it since I put it on, and it seems to work ok. I've been having a few issues with it lately, though, and would like some opinions...

1 - When the car is loaded down (heavy passenger, lots of cargo, etc) I get a 'thruming' sound under acceleration.... THRUMmmmmmmTHRUMmmmmTHRUMmmmmm

2 - When I jack both rear wheels off the ground, I can rotate the passenger side wheel without a lot of drag, and the driver's side wheel will not rotate at all. When I rotate the driver's side wheel, though, there is a good bit of drag, and the passenger wheel rotates in sync.

Both of these issues have me slightly stumped. I haven't spent a lot of time troubleshooting, but #2 really has me confused.... anybody else experience this?

Also, just put Redline in the diff, rear springs are stock with one coil cut, shocks are unknown and ok (not great) condition. Overall it really doesn't seem to 'Limit' the slip a whole lot on cornering, especially on the 'lift-the-passenger-tire-and-spin' corners. Better than the open diff, but it's no locker. Thoughts?

 

did you change the whole rearend,housing,axles,3rd member ?? or just 3rd member and axles . i'm hoping to hear you say you have disk's on the rear. you might try taking the axles out and reinstall on the other side. the noise you hear are the wheel / axle bearings when under a heavy load, more presure on the bearings. if you replace bearings do both sides. BTW a locker would drive you nuts always clunking and clicking.check the pad wear on the rear too.

 

Yeah, I'm aware that a locker would be unuseable for me... sigh.

Yes, actually the whole suspension/brake/hydraulic setup for the whole car has been converted to '85 GSL hardware. So yes, discs on the back. Good call on the wheel bearings, that's what I was wondering about, just haven't had rear wheel bearings fail on an RX-7 before, so I don't know what the sound is like.

What am I looking for with the pad wear?

 

Not sure about #1 unless I hear it myself.
#2.. jack the rar of the car up...hold one wheel to keep it from spinning...spin the opposite wheel, are you able to turn it or not? Do the same with the other. If it spins freely then my guess is that the LSD discs/plates are worn out.
*just a note..I have worn out/ broke causing lots of noise in 3 rear differentials in 12 years of owning 1st gens...this has usually occured after purchasing the car, then routinely draining what original fluid was in there and refilling with a synthetic lube. Puzzled, I spoken an RX-7 rally car builder, a GT-3 car builder and the mechanic who recently rebuilt the 79 Daytona winner.. each one has told me NO to changing to synthetic AFTER it has been running on dino oil from the factory. It has something to do with the properties of synthetic that accelerate wear of parts after the diff has been 'used' to a mineral based lube after so many years. Most racercars have rebuilt diffs and run fine straight with a synthetic put in. Your experiences may have better results than I have..So being your car, you can put in whichever you please to. Im happy to say that the diff in my current SE has been running smoothly with cheapo Valvoline for over a year now.

 

That's what has me confused. I'd think that the clutch plates were worn out if I could turn either wheel freely, but one side is tight while the other side is free. Is that possible just from wear?

Thanks for the warning on the synthetic lube, I've heard that about auto trannys as well. I actually just put the Redline in last night, these issues have been around for a while...since it looks like it may need to be rebuilt soon anyway, I figured it couldn't hurt to try.

 

Is the noise in #1 coming from right behing the tranny? My 85 was doing that a little everyonce and a while and I thing the griveshaft is rubbing the top of the exhaust heat shield. Just my .02.

 

bad news .....the 85 3rd member uses larger axles... the 79 axes are too small...

 

simple question yall, whats a synthetic lube and what does it do?

 

cveklund - No, the noise is coming from the rear axle assembly.

680RWHP - Huh? I know, I'm using an entire '85 rear axle assembly (housing, brakes, diff, axles) as I said.

neoboi - 'Synthetic lube' (in this case Redline MT-90) serves the same purpose as a regular gear oil, it's just formulated differently and is supposed to be better at high temperatures, last longer, and be more lubricative than the regular 'dino' gear oil.