MSP Differential swap issues

I have a 2003 Mazdaspeed protege which comes stock with a helical LSD. Recently, the differential sheared (common problem on these cars). I took apart the trans and replaced it with a used LSD unit that is welded for reinforcement (internals are not welded, just the housing to the ring gear). After re-assembling the trans, I spun the input shaft by hand and could feel the outputs moving. All gears seemed to work.

Last night, after getting the car put back together, it has the same problem as when the first diff broke. I can shift into any gear and the clutch pedal has pressure, BUT the car will not move. If I jack it up in the air the wheels will spin but I can stop them by hand. Basically, any load prevents them from moving. I can shift into any gear including reverse. Clutch action feels fine.

I'm not sure what the problem is. The clutch disk is not in backwards and a new throw out bearing was installed. Is there an internal problem with the used diff? Anything I could have gotten incorrect in reassembly that could be causing this?

This is the write-up I used. https://www.mazdas247.com/forum/showthread.php?123791691-G-series-5sp-Transmission-Complete-Teardown-amp-Rebuild-w-LSD-amp-5th-Gear-Swap

Is the clutch actually any good?

I've experienced vehicles that had good clutch action but the disk was so burnt out that you could idle them in 5th gear and it'd just sit there.

The clutch was replaced before I got the car. It only has 12k miles on it (SPEC stage 2+)

Forgive me for assuming that aftermarket clutches are garbage and being 12k old doesn't mean anything. It's an assumption borne by much painful experience - if it's not an OE application, the quality is usually remarkably shoddy for what you spend. (Fortunately, a lot of aftermarket clutches are OE clutches given a respray and a "Stage" number along with an inflated price tag)

If you don't KNOW it's good, you don't know it's good, you know?

Re-reading your commentary, I am assuming the ring gear is welded to the differential? Meaning the ring gear on the diff is not the ring gear from your transmission? Besides the mental shudder I get from mixing gears from different gear pairs, I wonder if the tooth count is the same.

A thought. Are the transmission outputs actually connected to the hubs, meaning is it possible you have a blown CV joint? Don't guess, check. Throw it in gear, jack one wheel off the ground, spin it and make sure the inner joint is also spinning.

Uhh, the clutch worked fine before. SPEC is a very good brand. Yes, it has 12k miles on it and it still looks new. The diff I bought is a used OE diff with an OE ring gear. Tooth count is exactly the same.

I will check the CV joint and update.

Just checked the axles. They are fine.

Generally speaking, you don't want to mix and match gears. Gear pairs are "married for life" so to speak. It's great that it's an OE gear but that doesn't mean it's the right tooth count.

Not that this would cause any immediate concerns if it were the correct tooth count, but the ring and pinion would wear out rather quickly.

And no, just because they are a large name brand doesn't mean squat to me. I've seen many manufacturers on all different kinds of component have big names because they advertise a lot, sell shamefully bad garbage, that people will vehemently defend because... well, they advertise a lot, so they must be good right? I don't remember if SPEC is one of the brands that sells garbage junk to the Audi community. Either way, point stands, if you aren't sure then you don't know, and if you don't test then you aren't sure.

Measure your Clutch Disk. 12k means nothing as does who made it. Do you know it is with in spec or not. If you have not actually inspected it and measured it you are just assuming it is good.

I'll measure it when I take it back apart BUT the disc looked brand new, flywheel and pressure plate barely looked used. Worked fine until the diff broke.

Sounds like you replaced one failed differential with another failed unit. I very much doubt the clutch is the issue. If the disc was in backwards you would have no disengagement and the car would mve whenever in gear.

How does a helical diff fail? Is it possible that the gears inside are stripped so bad that it doesn't make any noises?

Nothing else has failed in my opinion. Without being there to personally inspect the parts and your work, that leaves the diff you just swapped in.

If the clutch cheacks out I would agree that the replacement dif is bad. That really sucks.

The clutch disc measures 1.94 mm on each side. The minimum thickness is .3mm

Thought I'd report back and close this thread with success!!

After a lot of head scratching and sleepless nights we worked our way backwards. Took the drain plug out of the tranny and while rotating the wheels could see the diff spinning. Then took the 5th gear/ reverse cover off and could see the main shaft spinning. Leads me to the clutch. Turns out that the throw out bearing that was purchased from Rockauto was incorrect. It was only releasing halfway not allowing the pressure plate to fully engage the clutch disc. The bearing had a flat area for clearance at 10:30 and 4:30. These should have been at 9 and 3. So, we had everything right to begin with and had it not been for the bearing we could have skipped the multiple installations and sleepless nights. In retrospect I should have figured this out much sooner knowing that the only 2 things that changed were the differential AND the throw out bearing. Live and learn.

Rockauto at first refused a refund due to the fact that the purchase was over 30 days ago. I understood their position and didn't argue. Not sure what went on behind the scenes but a day later received an email saying that the part was indeed incorrect and they'd issue a full refund. Kudos. Not sure if it was their fault or the manufacturer.