To me, Torsens are evil voodoo Pandora's boxes and I have no idea how to diagnose one; the most I know is they use helical gears and if one wheel leaves the ground they act like an open diff.
A buddy has one in a Spitfire and it's doing something weird: the car backs up just fine but when you go forward he hears crunching noises and it won't go after moving a few feet. He originally thought it was a problem with a CV axle, that's been ruled out, both have been disassembled checked and reinstalled. He has had the differential out, pulled the rear housing and inspected it; no metal fragments etc and there's no visible damage to the helical gears so he reinstalled it only to have the same exact problem.
He's pulling it back out to look at it again, does anybody here know what to look for?
Is this Keith? Damn that dude can break stuff. 
Yep, it's Keith. That boy either is a human hammer reaping the results of no mechanical sympathy or he has the world's worst luck. He was loading the car to go to Roebling when this happened.
He was over for New Years and we had a discussion about not driving at 11/10ths, all the time. Has he already missed the event or is it next weekend? He's going to be seriously pissed if a breakage makes him miss another one.
It's next weekend but any parts would have to come from Blighty it seems. Not many Spitfire Quaifes sold here in the Colonies. He was loading the car when he discovered this problem.
Have you tried with the driveshaft disconnected?
The symptom to me sounds like the differential itself has nothing to do with it. More like one of the pinion bearings is done (cause problems driving in reverse/coasting forward) or drum brakes with failed springs. But if the rear was inspected already, I'd start looking at the transmission as a failure point.
The car moves in reverse but not in forward. First thing I thought was transmission as well so I asked where were the noises coming from and he said the rear of the car. The transmission was just gone through as well, I can't think of a reason to condemn it. But the worst part is I'm 150 miles away, trying to diagnose this over the phone.
EDIT: Don't know if this is a clue, I forgot to mention what brought all this on. He recently converted the rear axles from the stock Triumph stuff to early Miata, had custom CV shafts made by Moser. It all went together fine, he took the car to Roebling for its first outing with the new stuff. In his first session, after about three or four laps he was entering a corner and that's when the noise/lack of drive started. He says there was no warning beforehand, it was like flipping a switch.
I have no input, but this video is pretty good for understanding how a torsen works:
https://www.youtube.com/watch?v=JEiSTzK-A2A
The thing with the differential is, a failed differential can only have two modes: zero drive (no power transmission from ring gear to axles) or locked up (Murphy's Locker, like a Lincoln Locker but by breakage) but in both cases the car will still roll, or at least, if sufficiently busticated, like a chunk of something is hanging out of the carrier and catching on something, the car will not want to roll in either direction.
The only other option is that possibly something broke loose in there (bolt head?) and is rattling around on top of the pinion gear. That will allow you to go backwards, but trying to go forwards will just jam in the gears.
Yeah, I can understand that and it's happened to me before. He says the car quits moving but you can hear something grinding in the rear. The car can still roll.
He's on the wife's E36 M3 list ('you spend too much on that damn race car...'
) so he's knocked off for the day. My advice to him was to raise the rear of the car, start the engine, have someone inside hold the brakes put it in gear then engage the clutch while he watches underneath to see what is/isn't turning and to try to pinpoint the source of the grind noise and only then make the decision whether to pull the diff and disassemble. It would be bad to go through all that trouble only to find it was something else, like maybe stripped inner CV splines...
In reply to Curmudgeon:
Didn't he just go through a busted CV clip or something like that?
I would think a diff problem would be fairly obvious (chunky) after you pulled it, but I have no experience with a torsen. Can you see the inner working of it when it's pulled?
I bet his wife is like:
The outer circlip for the left CV axle came out in two pieces (so did the right, for that matter) and we both ASSumed this allowed the CV shaft to float out of the splines. It now appears this may not have been the case. My advice to him last night was to check the left inner CV (he's already disassembled and checked the right inner) before dropping the diff.
Every time I've seen a diff fail it's been chunky and clunky. Or the R&P has howled like a whole tribe of Indians in the trunk. But like I say I'm not even faintly familiar with the guts of a Torsen, for all I know when the damn thing barfs it could send out sonic vibrations that kill small rodents.
And yes his wife is ready to go ballistic; what's new about that?
Does it make the grinding noise while trying to move or will it literally not move at all in forward? As in, is it jammed trying to move forward but will roll backwards or does it just not move under it's own power?
I wonder if it's still trying to spit the CV. He might have wiped most of the splines last time and it's kicking the joint as soon as it sees load.
Don't be hung up on the fact that it's a Torsen. Torsens are just open diffs that use worm gears instead of bevel gears. (A worm gear can be driven but can't backdrive, so someone decided to make a differential with them)
The problem is going to be unrelated.
What Knurled said. I'm betting it's not in the diff.
I lean that way as well. From what I see, a Torsen will break teeth but just lock up like a spool if that happens.
As long as the wheels are turning at the same speed, the Torsen isn't really doing anything inside. So yeah, look elsewhere. Like missing teeth on the trans. What happens in second?
The helical gears on the Torsen just create a thrust load on the two side gears. They're either pushed together or pushed apart and thrust washers keep everything in place. You can change the thrust washer material to change the torque bias ratio (IE how much limited slip).
So yeah, the Torsen has nothing to do with what you're describing - its something else in the driveline. I'd start by taking the axles apart and making sure nothing in there is broken and allowing the axle to move somewhere it shouldn't.
Agggg... trying to diagnose over the phone. He stuck his Go Pro under the car with both rear wheels on the ground (a very important point) started it, held the brake and put it in gear. The right CV axle did NOT turn, but the left DID. Keep in mind this is with the tire ON THE GROUND, meaning something in the left side is screwed up. That pretty much rules out the diff. i asked him to do the test again and look for the point where the left CV axle stops turning; there's the trouble area. As in:
If the left inner cv housing turns but the shaft does not, it's inside the left inner CV joint.
If the inner housing and the shaft turn but the outer joint does not, the problem's inside the outer CV joint.
If BOTH cv's and the axle shaft turn but the wheel hub does not, the problem is in the outer cv/wheel hub interface. Really long shot: if the hub fractured just outboard of the splined part that could definitely cause this weirdness.
Instead, he took the damn shaft out and started taking it apart, can't find anything wrong. Damn. I advised him to put it back together and do the 'look for where it quits turning' spot again.
Gottas love it when people ask for help and then make things more difficult for themselves by ignoring your advice.
Sometimes it works out for the best, but usually not.
Well, turns out he was blaming the wrong thing. He called me late last night, said he went over the Go Pro video again and saw that the left axle shaft was spinning inside the outer boot, i.e. the shaft spins and the boot/joint don't. Looks like the left outer joint's inner race spline is too big. At least we both learned a lot about Torsens!
