Differential Mystery
 

Hey guys whats up? I got a little predicament for all of you that I officially need to reach out for help for because i cant find an answer anywhere on the internet. So here we go, i own a 89 1.6 miata and it had a nasty diff whine above or at 25-30 mph. i decided to take out the crappy open diff and throw in a torsen from a 94 miata in it. A week or two ago after a couple weeks of driving it, i started top hear the same kind of whine/howl. I could hear the straight cut gear sounding moan only on acceleration at first and then if i put it in neutral or wasn't gassing it it was gone. Now i can hear the whine both on acceleration, deceleration (while in gear) but when im in neutral i still cannot hear it. Hearing the whine on deceleration leads me to believe that the crush bearing is garbage now but something else ive found out is the feeling you get from the gear selector when driving. Miatas are known to be noisy cars, I get that but when i throw my hand on the gear shifter to change a gear it feels extremely rough and almost like i can feel each individual gear going into each other creating not exactly a rumble but thats the best word i can think of.. ive checked the diff fluid, ive checked the tranny fluid and if there were metal specs they were way to small to see even with the flashlight. The drive shaft is balanced, axles are fine, wheel bearings are good, it has a good enough alingment for it to drive straight. The biggest reason why I'm so worries about it though is because on my old miata the same exact things were happening until one night it started grinding and boom went the diff at 2 in the morning on the side of a highway. So, what do you guys think? R&P gears too tight or loose, diff service, new diff, anything else or am I just smoked?

If you replaced the diff and the noise did not change, I'd be suspicious that it was another part. Driveshaft joint, maybe?

Put the entire car up on 4 jack stands and crank it and put it in gear. Use a stethoscope or long screwdriver with your ear against it to listen to the differential and transmission. Be careful not to get any clothing caught in the drive shaft or CV axles or you will quickly lose body parts. Try the transmission in different gears and listen to it. If the transmission is failing it won't necessarily make noise in every gear. With the engine turned off you can turn the rear wheels by hand and feel any grinding that may be occurring in the wheel hub bearings, if the other components check out.

This thing can get you about 3' away from whatever you're listening to for $4 if you're worried about being killed by death:

https://m.harborfreight.com/mechanic...ope-63296.html

The only other obvious thing I can think of to check is your PPF alignment. Throw a 2x4 across your frame rails back by the diff & check the height of the bottom of the ppf relative to the rails. Should be ~60-72mm higher than the rails IIRC. Not sure how that could be potentially contributing to your issues, but it is another drive train variable you can eliminate without tearing into anything.

​​​​​​Sorry about the no reply. Broke my back and had to chill for a while. I haven't tried using a stethoscope yet so I'll give that a shot. Any idea of what I should hear if its a bad tranny/diff or a good one? Also, a quick update is that the feeling has gotten twice as bad. I told my buddy to put his hand on the gear selecter while driving home from Jimmie John's and he said it felt like he was feeling up the throat of a guy screaming in a death metal band. And when I went to feel it I almost want to say it feels like gears are grinding. I tried to do more research online and I'm wondering if it could have something to do with 3rd gear, wheel bearings or the clearances between the pinion gear and ring gear are too tight or loose causing them to grind. Has anybody had issues with those parts in their miatas before?

I didn't think about that, never thought the PPF could get fucked enough for that to happen aha. I'll check that out in the next few days. Thanks for the advice

Did you buy the new diff complete in a housing, or did you put it all together in a housing yourself?

Id say since you're familiar with pulling the diff already, pull the new one and take a look at it for anything obvious. If you dont see anything take it someone who can set the backlash and pinion preload for you.

Okay so after two blown up differential and a couple.months later, finally figured it out. It was the ppf alignment as someone previously stated above. The ppf was hanging way too low, so I had to adjust it a lot higher so it was 2 inches above the frame rails. Thank you for all the help you guys, I would have never expected the ppf to be the problem.

i had a diff run dry that blew, which also blew my trans, which also blew my block...