Mazda 6 MPS Differential problems and/or solutions

[kmh001]

No worries Doug, I'm no fan of FWD either. It's just that I've found that turning off the rear diff has a few advantages other than just stopping the vibration. I certainly don't leave the diff off and I only turn it off when I'm in 4th - 6th, which is where the benefits are most evident.

As for the cause of the vibes, I've had my entire drive train replaced piece by piece under warranty. It's not the tail shaft or the diff. We believe we have identified the cause to be excessive wear between 6th gear and the main shaft, which on it's own doesn't cause vibes, but somehow causes a harmonic when a diff coupling operating at around 20hz is added.

Hope these photos work:
Worn


New


The worn gear had only 60,000km on it.

If we accept that the diff coupling is operating on a square wave control signal, but is not a perfect square wave, then it would look something like a Fourier series approximation which could be corresponding with another harmonic source, possibly even the engine, to cause a noticeable vibration. It might even be causing the gear to walk around on the shaft and accelerating the wear we see in the photo.

My vibes are gone now that I have a new gearbox, but I still turn the diff off occasionally.

[LILZLR]

Rule of thumb always go larger tyres than stock on equal basis's (front and back) on an awd car. Cause you a effectively changing the rolling diameter of the original placement plan of the car, thus hurting inbetween mechs.

Suxs though this happened m8

[Doug_MPS6]

KMH001, thanks. Interesting. It seems that the relatively recent trend to giving owners taller gearing, hence longer legs and better cruise economy has some flow-on effects, for I see many gearbox problems stemming from the often quite tall top gears in a few models. Being a tiny gear, it is often working really hard and spalling is common, especially if lubrication to it is inadequate, such as in the old 5 speed Landrover County gearbox where a piss-weak fibre coupling to the internal gearbox oil pump almost always failed, causing premature top gear failure. 5th gear in some earlier Landcruisers is also known to fail occasionally, though I am uncertain as to the reason. 6th in the MPS6 is really tall and must be prone to similar effects, although I haven't seen an example of 6th gear "in the flesh". Your pics didn't work for me - pity.