I'm still chasing the tap-tap-tap...... noise I hear at low speed when coasting that I asked about a few months ago. Recently with prop shaft off and spinning rear wheels by hand, the noise seems to be in the differential, but hard to localize. Perhaps right side is slightly louder.

But while doing this I noticed something else that may point to the LSD unit.

The S2000, year 2000 Service Manual lists a "Torsen Type Limited Slip Differential Operational Check." Summarizing steps 1,2,3 it says, with rear of car up on stands, engine off, no handbrake, put the car in first gear. Then, "4. Rotate either rear wheel by hand, and check that the other wheel rotates in the opposite direction. Check both wheels. 5. If the opposite rear wheel does not rotate, or if you cannot spin the rear wheels at all, the limited slip differential is faulty and should be replaced."

On my car I cannot turn a wheel as described using normal force. (If in neutral or prop shaft disconnected, I can easily turn a wheel and opposite wheel turns in same direction. That calibrates what force I expect to turn the wheel.) Back to test set up, if I shove hard, perhaps bouncing at it some, the wheel releases and turns a short distance easily, but I can't keep it spinning. Force required is much more than when spinning with prop shaft off.

So it seems to me that the differential fails the test.

The only potential ambiguity in the procedure is that it doesn't say how hard it should be to turn the wheel by hand.

Does anyone who has done this more disagree that it fails?

 

With a normal car in gear, and off, wheel should turn free with the easiest of effort. Other wheel will turn in opposite direction.

However, I have had cars that were difficult to turn, but the dif still worked properly.

Is your tick-tick while going straight or turned?

 

Thanks. I tried the wheel turn test again with wheel off and bar between the lug bolts to better judge force. Requires 40-50 lbft torque to move and then just turns about 30 degrees before stopping again. Both sides both directions. So I guess it's bad.

Oddly I haven't noticed anything alarming when driving. It's not acting like a fully locked diff or even obviously scuffing one tire around tight corners. Just the persistent tap tap tap tap noise at low speed.

 

I overlooked the question. The answer is yes. Straight or turned is the same. If I didn't say so before, the driveshafts (aka half-shafts or axles) are new.

I went back to the turn test with a torque wrench. Torque required to move the wheel varies as I work through successive resistance points, but the highest was 130 lb-ft, well beyond what I would call "Rotate...by hand" and "...spin the rear wheels..." is impossible.

Fails.

 

The stock LSD will make noise or thump a bit in low speed turns if it is binding, when you are going straight the LSD is in active and should make no noise. If you have a tap, tap, taping sound going straight you may have another issue besides the LSD.

 

Oh, I agree, including a branch stuck under the car.

But regardless of this noise, 130 lb-ft to turn the torsen unit is a problem. A definitive fail.

I'm not sure you read everything I said above, but I've chased down everything else. The drive shafts (axles) were indeed failing, but are now new. Jacked up, prop shaft disconnected, spinning the rear wheels by hand (both in same direction), crawling under the car to listen, we could hear the noise from the diff case.

I haven't changed the wheel bearings, but this is not a typical bad wheel bearing sound and is not affected by side load or turning. And anyway the sound was coming from the diff case not an axle or hub.

While changing the axles I found no play or shifting in any direction in the hubs or wheel bearings. I know all about correct axle nut torque.


In a week or so I'll swap it out and see what I get.

 

I was delayed by slow arrival of new threaded fasteners for the the prop shaft and driveshaft (axle) flanges. I ordered from Majestic (which is in Rhode Island), and in-transit was long, I supposed from the snow and ice storms.

The replacement differential (a low mileage, one-owner, used unit) easily passed the service manual turn test I described originally. Turning one flange required the same pressure as turning a doorknob, and this caused the other side flange to turn in the opposite direction.

Installing the replacement diff also eliminated the repeating tap tap tap ...

I did R&R the prop shaft while I was at it, but since changing prop shafts earlier did not change the tap noise, it seems unlikely that the prop shafts were the source.

Once it was out of the car I examined the old diff a little more. As I discovered while it was in the car, there is strong resistance to turning either side flange with the front flange held in place. I also found that in some positions, there is 20 degrees or more of free play between the two side shafts before they hit hard resistance. In other positions, no free play.

But with the front flange free and the side flanges rotating in the same direction the ring and pinion move easily, smoothly and with minimal backlash. It appears that the ring and pinion are OK, but the differential unit in the center is no longer differentiating. I didn't open the case to look further. I probably won't until I want to use it for something.

It still seems odd that I didn't notice more when driving. The tapping was slowly getting louder, but not really LOUD, and nothing else obvious. Maybe just a strange coincidence that the diff it was about to fail more drastically just as I put it up on stands for the testing that led me to discover the problem. Either that or the differential would become more free once the car was in motion so the unit was constantly rotating with the ring gear and jiggled by road vibration. Just guessing, obviously.