Possible problem with transmission / c.diff??
 

I've got an MY04 Spec C, so has the adjustable c.diff control although I always run the car with the diff 'open' (?), (the bottom green setting).

Since owning the car it has made a very slight 'clunk' from the back of the car when changing gear, particularly noticeable in gears 1,2,3, which I have always assumed was a rear suspension / top mount knocking. It sounds like there's slight play in something 'mechanical' and clunks when I press the accelerator, or take my foot off, so as the car moves forward or backwards (if that makes sense :wonder:)

Recently it seems to have got worse and so I've tried a few things like;

1. putting the car in neutral while travelling at low speed and dabbing the brakes - no clunck

2. today I played around with the c.diff settings to see if it made any difference - it does. When the c.diff is in any setting other than fully open, (including the second green setting) the car doesn't clunk.

From this I'm assuming that it's a problem with the transmission and/or diff and that there's some slack in the mechanics somewhere.

Does anyone know what this is and more importantly, how to fix it?

There doesn't appear to be any detrimental affect on gear changing, etc but it's annoying and I'm sure it can't be doing any good.......

Thanks :confused:

I get this on my my99 type r..
I know exactly what you mean about dabbing accelerator on and off. its like if your crawling in traffic and touch acc then let off thats when i can hear it.. it also sounds central.. when mine was on the ramp last thursday he mentioned something about it being open and showed there is play but from what i can gather thats how its meant to be !! sorry its not much help. am interested to see what other responses are now though !

That's exactly what it's like, dabbing the throttle or pulling away it clunks most of the time.....

:thumb:

anyone?

This is one of those things that is very difficult to diagnose without actually seeing the car.

What you're describing sounds generally like the well known rear subframe/rear beam knock issues. The fact that it reduces in severity when you turn the centre diff up is consistent with the increase in diff lock pressure sending more torque to the front (and therefore, reducing the amount going to the back).

If it's been getting worse gradually over time it could simply be the result of the bushes in the subframe or beam wearing a little more. One thing to check, especially if it's got worse quite suddenly, might be the strengthening plates that go between the bottom of the beam bushes and the sills. If these get hit by stones (or someone clumsily sites an axle stand under them) they can get bent up closer to the beam, making contact more likely.

That's interesting info, it did seem more noticable after my recent MOT, when it was jacked up with a hydraulic jack and they had a problem with where to jack it up (dodgy jacking points IIRC).

Are the strengthening plates easy to sort out / straighten out?

Thanks, Rich.

Hmmm, yes, if you can associate a worsening of the problem with the car being lifted. suspended or whatever, then definitely worth looking at them.

The most practical way of "sorting out" the plates is to take them off (axle stand under the beam, two small bolts and one large one) and bash/bend them back down. Take a peek under the car and you'll soon see what's what.

Sounds like a good start, thanks. Would you expect it to only do this when dabbing the accelerator though, would it clunk when going over bumps, etc?

The reason dabbing the throttle seems to cause/make it worse is that putting torque through propshaft into the rear diff makes the entire rear subframe, and the crossbeam in particular, twist a little bit. As the beam twists against its bushes, one side will push up towards the body, the other will push down towards the stiffener plate. If that plate is already bent upwards enough, you get contact.

If it's really bad it'd do it in response to bumps too, but, in practice, before it got to that stage, it would have got so bad under acceleration and braking that you'd already have got it seen to.

Thanks splitpin. It doesn't do it over bumps, (although I may have heard the occasional clunk - hard to tell really).

It is pretty bad and getting really annoying so I'm now planning to take it somewhere to get it checked......I'll post back the results when it's sorted.....