I need advice please
 

So my car developed a terrible noise coming from the rear of the car. Somewhat grinding, kind of growling. So I thought it might have been a wheel bearing. So I jacked it up, the left rear had all sorts of slop in it. So I replaced the hub and bearing today, I was really excited, I thought it would have been the issue. I went out for a drive and the noise is still there.

FUCK!!

So I get it up on the lift, drained the diff fluid, it's black and smells terrible but still full. I looked inside, the ring gear and pinion gear look fine. There is a ton of slop in the pinion though, by that I mean, it seems to turn a lot before it starts to turn the axles (sorry if that doesn't make sense, I'm trying my best to be thorough and detailed) . I pulled the driveshaft, the u joints are pretty tight. Meaning I think they are binding up.

So for shits and giggles I drained the transmission fluid. It's still purple, no metal in it.

So can someone give me some pointers on what to fix? This is my list;
1) pinion bearing
2) drive shaft u joints
3) right side wheel bearing
4) somehow I fucked up the transmission

I am stumped.

Thanks for any replies

Driveshaft U-Joints if its a repetitive sound/squeak/grind that occurs about 4x as quickly as your wheels are turning. They should be very easy to move by hand with the driveshaft unbolted from the diff input shaft.

Except the U-Joints aren't serviceable.

I've replaced two driveshafts in different cars with good used driveshafts because of this. The first was my '94. The second driveshaft had completely failed in a car that I bought. The guy was selling the car for $1200 because he didn't know what was wrong. After starting the car and running it through the gears, I told him I thought the driveshaft had failed. I looked under the car real quick, saw my evidence and convinced him to help me push it forward. The moved car revealed the front "pinion" of the driveshaft laying on the ground. I parted with $1200 for the car, taking a gamble on the condition of the transmission tail (I've seen them grenaded after a front U-Joint failed). I put a used driveshaft in it and that car still drives like a champ today. Blake@isellmiataparts.com might be able to get you a good used driveshaft for under $100. Alternatively FM sells new driveshafts with serviceable U-Joints for something north of $300.

If a U-Joint fails, there will most likely not be a catastrophic "ground strike", as the PPF will hold the fallen driveshaft against something and off of the ground, I believe it's the exhaust, but it might just pin it to the side of the transmission tunnel. A failure of the front U-Joint though, as I've said, can have catastrophic consequences to the tail of the transmission. This transmission came from a different car after a failed front U-Joint.
https://www.miataturbo.net/attachmen...ine=1313899830

For the record, when my front U-Joint started going out on the '94, I was convinced it was the rear U-Joint by the sound. Once I had the driveshaft removed, I found that the rear U-Joint was about as smooth and easy to manipulate as a new joint, but the front joint seemed damn near welded in place. Best bet is to have someone listen from outside your car while you drive around as slow as you can go while still creating the sound.

You mentioned play in the input shaft of the differential, but does it turn the wheels smoothly by hand?

dunno if you tried this: get the wheels up in the air and try to replicate the sound/occurence while listening/looking at it all close up. see where EXACTLY the sound is coming from.

I dunno...probably blew the diff or something

Grinding/growling is a classic U-joint noise. Driveshaft U-joints should move easily and smoothly. Be aware that used Miata driveshaft U-joints will almost always have what feels like a "detent" at center but can still run smooth. The "detent" is wear and happens because our driveshafts don't move much. A lot of people use these without issue. Personally, I spring for new with HD, serviceable U-joints. I've used "Driveshaft Specialists of Texas" out of San Antonio for new driveshafts on older Benzes and Miatas. Less expensive than FM.

Diff oil should not be black and smelly. Any metal? I'd be concerned about it. Might be worth dropping the Diff and inspecting the pumpkin. Could be a bearing.

Sounds like you blew out your rear end.

Did you run a small magnet on a sick into the bottom of the diff? Or did you take it all apart?
I've always been able to feel U joints "vibrate" a little bit on a hard take off, or starting from a dead stop. Then again, i didn't know U joint failure on Miatas was very common.

Well I dug into it some more. After convincing myself it wasn't the pinion bearing I went to the right rear hub. The pinion turns really smooth without any binding or noise by hand. I pulled the wheel off and that hub was flopping around. Go to pull the axle nut, which seems like an easy job wound up costing me an axle! Either the nut galled when it went on the first time and I thought it was tight but it never was, or it galled coming off. Either way the threads are fucked. So I ordered a bearing/hub/ and axle.

And that is how I would describe the driveshaft. It feels like at the center it has a detent. But piss on it, I'm already spending more than I want so I'll run it for now.

I'd at least keep checking the diff lube condition periodically for a while. Diff lube should not be black.

I think I was being a cheap ass and used some O'Rielly's brand GL5. Which...well we all know what that means. I put some Royal Purple in it yesterday and no more noise, but I will definitely keep an eye on it. The axle I got was a remanufactured unit, which someone must have messed up the axle nut threads because they machined the threads off, built up some weld then tapped it and put the groove for peening the nut. It was $65 after the core charge and it has some sort of warranty, so I no care. I will address the driveshaft issue later.

Thanks for the help guys :makeout: