New Mark 4 MX-5 rear diff backlash - why does this happen?

Technical Area Engine, Transmission, Exhaust etc
2

 

 

Hi everyone, this is my first post on the website as I’ve only recently become the proud owner of a new Mark 4 MX-5 as my daily drive to and from work. I must admit that I’m really enjoying it. I also have a TVR Tamora which I’ve had for more than 12 years, but she can try to kill you if you’re not careful. The MX-5 is different, clearly, but doesn’t try to kill me, which is good for a daily drive.

I have a technical question regarding the MX-5 drive train. Shortly after collecting the car, I noticed a sound, like a ‘ch’ noise, when engaging the clutch. I thought it may have been the clutch release bearing. However, this didn’t happen when the car was cold, only when warm, and even then it was intermittent. The noise was always there after coming off the motorway onto local roads, and was more noticeable in the low gears (particularly 2nd and 3rd). I have driven probably hundreds of cars in my lifetime (lots of them hire cars), and none has ever made this noise. I therefore thought it was a fault, because why would it be intermittent otherwise? Surely Mazda’s flagship car wouldn’t have such a noise? To try and clarify, it is not a hard mechanical sound of two metallic parts hitting each other hard.

I took the MX-5 to my trusted TVR mechanic, who drove the car and confirmed that the noise came from behind the clutch, probably backlash from the rear diff. He commented that it sounded like a 50,000 mile car (ie with a very worn drive train). As a result of this, I actually rejected the car under the Consumer Rights Act.

The dealer has been very good. They invited me to take the car back so they could confirm the noise again, and asked me to drive their demonstrator. They even had a brand new MX-5 with the same spec as mine which they said I could have if I wasn’t satisfied. After confirming again that the noise was there in my car, I drove the demonstrator, which was warm because it had just come back from a test drive. I was gobsmacked to find that the demonstrator had the same noise when engaging the clutch. I also drove the demonstrator with the Service Manager, who confirmed it was rear diff backlash, but that this was a feature of the Mark 4, and even the Mark 3, MX-5. On the basis of this, I withdrew my rejection of the car, which of course I therefore still have.

However, as I don’t understand why this intermittent rear diff backlash is there when warm, and being a physicist and petrolhead who needed to repair my very first car myself (an MG Midget) on a regular basis, I would like to understand the technical details of this rear diff backlash and why it is normal/acceptable.

Can anyone please explain this?

I hope everyone has a great Christmas and New Year!

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Hello and welcome.

I can’t comment on the reasons for the lash you are noticing, it is documented in the MX-5 history though, the thread linked below highlights a discussion on Miata.net.

http://forum.miata.net/vb/showthread.php?t=586739&highlight=Lash

Hope you find a definitive answer somewhere.

4

This might help you a little:

http://www.bing.com/videos/search?q=rear+diff+backlash&view=detail&mid=A3524CDF699E59B24E87A3524CDF699E59B24E87&FORM=VIRE2

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I have a MK3.5 NC 2012 from new and never had the problem your experiencing.

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Which model is it?

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Must admit both of mine a previous 3.5 and now a 3.75 experience backlash, very noticeable,off throttle, decelerating. The uprated clutch I’ve had on both cars because of FI probably doesn’t help matters either. I would expect with a lighter flywheel for this to increase.

8

When I first took over my NC 3.75 (1 Year old /11,000 m) I was disappointed and disturbed by the noisy transmission/diff, particularly at low engine revs.  However I have adjusted my driving style, and tend to use  the gears more to keep the revs up. The noise is now much reduced, presumably because there is little or no torque reversal input to the drive train.

As to why the issue arises, Mazda has done a very good job of producing a competent sports car at a keen price, but compromises have been made.  Tight clearances, over spec’ing and preloading can reduce backlash and noise but are expensive in production terms (the law of diminishing returns applies with a vengeance). It should be no surprise that ‘higher quality’ sports cars come at premium prices.

Don’t panic - accept the MX5 package as a whole  - and enjoy!   

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Many thanks to all who have replied so far. Mine is the 2 litre 160ps Sport Nav.

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Thanks for the info.  I can’t hear any noise from mine, but it’s a  1.5l and therefore doesn’t have LSD.  Hope your’s is not a serious problem.

I wish you a very Happy Christmas and an enjoyable, trouble-free motoring New Year.

 

11

Not noticed mine do this, will listen out in case it happens as miles increase 

12

Me also,  new from 2010 never had that problem, in fact I 've never had any problem with Cariad.

Gales

 

13

I have exactly the same model and quite a lot of transmission noise gets through to the cabin. Not only the occasional chink that you refer to but also a high pitched continuous hum or buzz at certain speeds and loads. I have tended to accept this as the price for a very communicative driving experience.

 

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This isn’t to say that you don’t have a problem with your specific car however I read a lot of “issues” that people have with their cars when they compare them or other vehicles. In general those same problems are present in many other vehicles just they are masked. There has been diff noise in every MX-5 I have driven if I care to listen for it. Part of the reason for this is that my head is far closer to the diff than if I were in a Focus or RWD saloon, added to which Mazda put far less insulation in the car and less rubber components between the driving bits and you than most other cars. The MK3 became most like a “normal” car, but with the back to the roots approach with the MK4 I expect insulation is minimal again.

You will always get more noise when the oil is warm after use and certainly when decelerating you will get a different noise when the the gears are driving rather than being driven. 

If you are particularly worried, ask the dealer to check the diff oil level, it is not impossible for there not to have been enough put in, but I suspect it is just the noise they make, like all the other MX-5’s. I expect your TVR would make similar noises, but with that there will be a louder noise source 

 

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I dont want to add ‘fuel to the fire’ but we have already found that due to the ratio of the mk4 diff it is paramount that good quality high temp oil is used.

 

we are already seeing diff temps of above 150 deg C on road cars

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Again, thanks to everyone who has replied. Interesting the range of responses!

I understand why backlash would increase on deceleration (or in reverse) given that tolerances in the drive train would be designed to minimise this on forward motion. Although the TVR is significantly noisier than the MX-5, and the TVR also has a LSD, it doesn’t have a similar noise. The TVR guy I mentioned in my original post has worked on TVRs of every model for years, and from his perspective the noise sounded like that on a high mileage car. However, the noise only occurs when the clutch is operated, not on deceleration. Your comment on the diff oil level sounds worth following up.

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That’s interesting - not doubting what you say, but could you please explain how do you know that the diff temp is so high? That might explain why the noise seems particularly apparent after driving on the motorway for a while.

Do you think the standard diff oil used from the factory is not of sufficient quality? If not, do you have any suggestions for a suitable oil?

18

We are already logging all oil temps. We have fitted calibrated sensors to the drain plugs on engine,box & diff. We are doing just as we did with the mk3’s…

 

we work with an oil company and any oils we use are drained and sent back to the labs. The oil we use (I can’t disclose) will run to 200degC 

what you are seeing is what we have been saying for years with the n/c. The n/d has just exaggerating it with a 2.8-1 diff ratio

we have said for quite some time when the diff holds so little oil .7l iirc changing it annually is paramount.

 

 

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Fascinating stuff. Thanks for posting, please keep us updated on your results.