Rear Differential Replacement
 

I have a 2009 RX8 R3 with just about 89,000 miles on it. Around 6,000 miles or so ago I started getting what sounded like gear whine (similar to the sound of straight cut gears but nowhere near as intense) coming out of what I thought was the transmission of my car. The noise would only occur while the car was in gear (no noise while coasting in neutral). The noise occurred in every gear and would increase in pitch as I gained RPMs. I took it to two garages both who believed it was the transmission. Well 1 transmission later and the noise remained. The garage I took it to recommended changing the rear differential oil to see if that improved the noise.

When I changed the oil it was definitely shiny with a metallic like paste surrounding the drain plug. I took the car to the dealer to see if they could diagnose something specific or if they would just have to replace the entire rear assembly. They just got back to me and said they would need to replace the entire rear assembly for around $4,200. I took a look online and found an Mazda OEM replacement for about $2,700 so I may do the work myself.

Are you guys familiar with any other OEM replacements that may be cheaper? Would you recommend changing the ratios on the rear to something other than stock? Is there anything else you can think of that would cause such a noise that I should investigate first before replacing the diff. Any help would be appreciated!

I love this car to death and don't want to see it go but these big expenses are getting hard to handle. I have low compression on the rear rotor as well so I'm looking at an engine replacement sometime in the near future that I need to prepare for.

Thanks for the help

How old was the diff fluid you took out? Some metal is normal if that fluid is original.

I'm confused why you're contemplating replacing the diff, if the sound comes from the front, doesn't occur while coasting and varies with rpm, not speed. Those symptoms specifically point to the diff not being the problem.

If you really want to replace it, you can find a used unit online for obviously a lot cheaper.

As for the source of the noise, does it sound the same at the same rpm in every gear? Or does the gear make a difference? Was the same clutch and throw out bearing used when the trans was replaced? If so... I'd start there.

Diff Fluid probably had around 30-40 thousand miles on it.

I've had some trouble pinpointing where exactly the sound is coming from its somewhere along the drive line. I thought it was coming from the front that is why I had the transmission originally replaced but that didn't fix anything.

If none of the symptoms point to the differential any idea what it could be?

It sounds like one of the bearings near transmission input. That's why I asked if clutch and throwout bearing were reused during the trans change. You're saying it varies with rpm and not gear or speed (please confirm). Nothing after the input shaft varies with rpm only.

You say it only occurs when coasting in gear, not in neutral, but does it stop if you hold the clutch pressed while coasting?

I'm thinking input shaft bearing but need more info.

Apologies I'll try to be more clear. The noise only happens when in gear and I'm either accelerating or maintaining a constant speed with the gas pedal. The second I let my foot off the gas, coast in neutral, engage the clutch (press it in) the noise disappears.

I had the clutch replaced prior to any of these noises happening and I don't believe the noise started immediately after that installation. When the new transmission was installed they reused the recently installed clutch.

Going through my history it looks like when I had the clutch replaced they didn't replace the throw out bearing. Is it possible this could be the source of the problem?

A bad throw out bearing is more likely to make noise when you push the clutch pedal in.

Ah I see. Then back to the rear diff it seems.

I called the garage back just to see if they could give me more insight about the repairs and they said they lifted up the car, the noise sounded like it was coming from the rear diff and they confirmed it with their stethoscope thing.

Is the replacement of the rear diff unit feasible for somebody who is relatively mechanically inclined? I'm not finding any used S2 rear diffs on ebay or mazmart. Any other places I should check out?

I'm seeing three separate assemblies? I would imagine it would be a manual trans with limited slip but that doesn't seem to be an option?
RA1327100M - Assembly Auto Trans w/o limited slip
RS0427100R - Assembly manual trans
RS0627100G - Assembly auto trans w/ limited slip

All manual trans RX8s have a limited slip diff.

It's not complicated if you can lift the car and have all the right tools. The part you want to make sure is done right is alignment when everything is reassembled. In fact that could be part of the problem here if the PPF wasn't properly aligned when the clutch was changed.

It doesn't make sense to me that the diff would make a sound that you hear from the trans and that varies with rpm. The diff speed does not vary with rpm. But obviously I'm not with the car.

BTW, the diff your came with is usually the best one available. A number of folks have retrofit it to their earlier year cars.

What is the PPF?

To my untrained ear it sounded like it was coming from the trans area however when it was changed it didn't go away which is what leads me to believe its not coming from that area. The noise fills the entire car which is why I have difficulty pin pointing the exact location. The garage that currently has it said that it is clearly coming from the differential.

Wouldn't as you increase speed that increase the speed within the differential which could explain why the pitch is changing? I was a little confused myself why I would only hear it on acceleration or constant gas and the garage said that it was due to the additional torque the engine applies and that given enough time I would hear the noise constantly as the differential continues to degrade.

Thanks for taking the time to discuss this with me. I really want to cover all my options before I replace something else for lots of $$$ and have it not solve the problem.

The PPF is a metal beam that ties the differential and engine/transmission together.

The entire driveline on this car is effectively suspended as a unit from the body. The engine bolts up to the transmission, the transmission is bolted to the PPF, the PPF is bolted to the diff. The driveline is connected to the chassis by two engine mounts up front and the diff mounting bracket in the rear.

There is vertical play built into this system so getting it aligned on that axis is important.
TRANSMISSION REMOVAL/INSTALLATION [R15M-D]

You did say it varied with rpm, not with speed. That's a subtle but important difference. If they believe its the diff, I have no reason to doubt them, they have the car after all. Just be careful they aren't throwing parts at the problem,on your dime, before conclusive diagnosis. How, for example, did they get a stethoscope on the diff while the car was in gear and moving to recreate the noise? I assume they did it with the car in the air, which could change what they're hearing.

Okay so I'm seeing a couple of options here.
A. When the clutch was replaced the garage didn't align the PPF correctly which could be causing the gears in the differential to not align correctly which would cause this noise
B. The mileage on the car is getting near 100k and its possible that the engine mounts are deteriorating which could throw the driveline alignment off and cause the noise same as the above reason.
C. There are no alignment issues and somehow the differential just started to eat itself which is causing the noise.
D. Something else entirely is causing the noise, maybe a bearing in the differential?

Is there a way to verify if the PPF is aligned?

They said they put the car on a lift and ran it in gear to listen for the noise. Which without the weight of the car on its own wheels could dramatically change how the noise is generated.

That is my concern as well. They just want to throw a brand new assembly at the problem because that will make them the most money. I think I will first investigate the PPF alignment, motor mounts, and bearings within the differential before tossing a brand new assembly at the problem.

<EDIT: Following sentences written with poor end-of-day reading comprehension. Please take the following with a grain of salt.>

What is this insanity? Please tell them to pull the trans and replace the throwout bearing. This is the same issue that my '08 is having. Another potential symptom of the problem is that the clutch pedal is particularly stiff, moreso than it should be.

Basically, what's happening is that when there is no load on the bearing (clutch not depressed) is the tolerances of the bearing cause a whine. When the clutch is depressed, the noise goes away because the bearing is loaded up.

Please don't let them swindle you into replacing things that are not broken.