Have there been any cases of rear differential failures in new ND’s and what have been the root causes?
I have a 2.0 Litre Sports Soft top ND that was purchased new in the summer of 2017 and now has 3300 miles on the clock (rural and motorways only). I have owned earlier versions, including the NC. From the start I thought that the ND was slightly skittish but put it down to a lighter car and only having a factory alignment whereas my other cars where professionally aligned to suit my local roads.
Over the last few months the handling has got progressively worse - to the point where I was wary of going above 50mph and felt downright uneasy driving the car. When I accelerated the nose ducked slightly to the left and on taking my foot off the accelerator the tail twitched back and the car came back to the right. The movement became pronounced if I accelerated /de-accelerated several times at higher speeds - the car almost felt like its back end was trying to come round. I had one instance where the car skidded in a normal right hand turn at low speed. There were no noises or sounds of mechanical failure.
Having checked all the tyres - all new from factory and inflated to same pressure I then had the geometry calibrated. At the geometry check the engineer noticed that the right hand rear brake disc had a centimetre of rust on the outside edge where the calliper was not coming into contact with the disc - all the other discs were clean and shiny as expected.
As the handling was now unnerving me, I took the car back to the Mazda garage. The engineers agreed the car was all over the place. They reported the results of their investigation to Mazda and the conclusion was that the rear differential had failed. Mazda replaced the rear differential under warranty and skimmed the two rear brake discs - whilst they could not conclusively state the disc rust was due to the differential they thought it may be connected and in essence it was just wait and see what happens…
However when I went to collect the repaired car it felt totally normal at low speeds but the same handling problems remained at 65/70 mph - slightly reduced but still there. I then compared the handling of the car with an identical car from the Mazda garage and there is definitely something still wrong with the car. The car is back into the garage for more investigations.
Has anyone experienced anything similar? Could the differential failure be a symptom of something else rather than the primary cause? I’d be very grateful for your thoughts.
Question - does your offside rear wheel get hot after a drive?
LSD on a 2.0 Sport presumably?
The fact that the diff has been changed and the problem persists suggests it is something else, whether or not that did actually cause a diff problem.
The most obvious ‘something else’, disregarding geometry or unwanted suspension movement causing rear wheel steering, is the DSC/TCS.
If the car is going left, that would be consistent with unwanted braking of the left rear, causing the car to rotate around that corner.
The right rear disc might have rusted because it is doing very little. I tend to suffer rusty rear discs on my cars (not specifically the MX-5) I think because I am a very light braker - it isn’t unusual on my everyday cars for front pads to last 50,000 miles or more and the rear discs to rust early. So check if the left rear is getting hotter than the right.
Even as I type it, this is sounding vaguely ridiculous. DSC is well developed, computers don’t make mistakes, it’s a new car…but I am recalling Rodders’s racing experience of being chucked off the track at least twice IIRC by suspected unwanted interventions from DSC/TCS.
What happens if you switch off the DSC? Bearing in mind that ‘off’ probably doesn’t mean completely off. This could be a software issue or a mechanical problem in the ABS/TCS/DSC valve block.
That is just a stream of thought by the way, not any kind of analysis based on actual experience or technical knowledge, which I do not have. It might well be worth asking Rodders what he thinks as he has some experience of unwanted self-steering!
Thanks for the very quick and helpful replies. I haven’t noticed any excessive heat from either rear disk or binding noise/smell. Will get the garage to check on Monday as well as for bad bushings that could cause movement. Garage is doing a thorough job - they are as perplexed as I am.
Thanks for the suggestion to contact Rodders - have pm’d him.
This could be many things…
a front shocker is causing the front to dive hence DSC pulling at the back to straighten it up…possible but unlikely
rear shocks? Presumably already checked.
tyres? Presumably already checked.
i would also guess they have checked roll bar links so would rule them out…this leaves the electronics!
This WILL show as a diff fault to many people as it uses the brakes to pull on the back wheels, in extreme cases resulting in loss of control (see video posted previously) I would suggest/ask the dealer if he knows about this issue?
the way around this is to disconnect the rear brakes…clamp off the rear flexi pipes and CAREFULLY try it and see if there is any improvement
keep us informed and I’ll help all I can…ps don’t mention my name to Mazda as they’ll have the vo-do doll out again…lol
I had dinking one way on acceleration and flicking back when lifting plus ‘all over the place’ handling. Successfully diagnosed to one replacement Hankook rear among a set of Kumhos.
A replacement matching Kumho sorted it but also I was advised to move front to back and vice versa to get matching rear wear to not upset the LSD.
Although the tyres are original and pressures checked it just might be worth trying swapping them around. If the rears are in some way unmatched that could cause problems. Different batch or faulty compound maybe.
My Mk 3.5 would what I call torque steer, it would veer to one side when the power was on and veer back again when power was off. Tried all sorts to cure it. Laser alignment helped but it wasn’t as good as it should be and also when it was nearly new.
Only done about 20k but the OH had nicked the sidewall of the NSR Bridgestone Potenza, I replaced it with another Bridgestone Potenza unknown at the time but it was made in a different country. Changed all four tyres to Kumho Ecsta. Totally transformed the handling to what it should be, not as good as my Mk1 though
Many thanks for all the helpful suggestions and thanks to Rodders for the comment on electronics! I’ll run through the list with the garage and see how we get on… will report back on what happens. Have also pulled Maxda UK into the loop as having a new diff at 3,300 miles has not given me that warm loving feeling about the car!
As said was the diff faulty or was it “It must be the diff” and it was changed.
The rear diff had failed - the mechanic identified that it had mechanically failed when he had the car on the ramp. Could see some ‘play’ and slight noise. When they drained the diff oil they could also see particulates in the oil.
Again not surprised when we have seen temps close to 200c and remember Mazda say it doesn’t need an oil change for 5yrs/60k
I know it sounds unlikely but have you checked the physical size of each tyre.
I ordered 4 new tyres for my Mk3 and when ATS came to fit them one was smaller despite being correctly labelled. Luckily the tyre fitter spotted it before fitting to the car.
A smaller tyre on one side would cause the handling issue and would wreck the diff.
Unlikely as it does sound, having reflected on that as a non-expert it could reasonably explain both a handling problem under acceleration and damage to the diff.
The handbook actually says
(With limited-slip differential)
Do not use the following:
-Tyres not of the designated size
-Tyres of different sizes or types at the
same time
-Tyres not sufficiently inflated
If these instructions are not followed,
the rotation of the left and right wheels
will be different and will thus apply
a constant load on the limited-slip
differential.
This will cause a malfunction.
It will take very little time and zero technical skill to put a tape measure around the circumference of each tyre to confirm or dismiss it as a factor.
Sorted!!
With a lot of thanks to the Mazda Dealer and to Mazda UK my car is back to how it should be and kudos to many of you on this thread - the culprit was a rear tyre.
All the tyres were Bridgestone Potenza’s - straight from the factory…or so we thought. It appears that one of the rear tyres was replaced with another Potenza - possibly at point of entry when the car came off the ship. That tyre was identical to the other three in terms of ID marks etc but had a different tread pattern that was not immediately apparent (probably due to it being manufactured in a different country). This was sufficient to affect the handling and to damage the diff. Garage rotated the front to the back and put two new tyre on the front, rebalanced and she drives like a dream.
Had a look at the guilty tyre and it looked identical - only on very close examination could you see the tread pattern difference - this also impacted the wear depth too.
Just goes to show how sensitive the car is to the tyres and to be very observant when it comes to replacing the tyres - don’t just trust the manufacturer marks - check the patterns closely.
Big thanks for all of your help on this one - much appreciated.
Glad you got it sorted and the reason established!
Unmatched tyres can kill the diff…?
Glad to have helped.
As I said I bought 4 Kumho tyres from Blackcircles and fitted by ATS. All labelled right size and looked identical but tyre fitter spotted tyre wall marking on one was wrong size. Apparently this was not the first time he had seen wrongly labelled tyres. As you found out only a slight difference in tyres will wreck a limited slip diff.
That’s me and a few more on here feeling a bit smug with ourselves now.
Would be interesting to know for sure if the odd tyre was swapped at import or supplying dealer. Is there anything in place at the factory to stop odd tyres being fitted?
Whatever the case it’s frightening that such a minor point could kill a diff in such a short time.