NCT fail on rear suspension imbalance

honda boi · 17-02-2025 03:28PM #1

Hi all,

2014 Bmw 520d f11 failed NCT this morning on an imbalance on the rear axle.

Most cars I've had I've just changed the rear shocks and that usually does the trick.

But I see this car has air suspension in the back.

My question is could the imbalance be something from the airbags in the rear? Or do these not affect the imbalance readings?

Priced a set of Sachs rear shock absorbers at about 300€ so don't want to waste it if it could be an issue on the airbag side of things.

Car drives great , no noticable issues. No sagging In the rear.

17-02-2025 03:37PM

The imbalance is most likely due to a difference in braking force between both rear wheels.

You might have worn brake pads or discs/may be uneven. Thats the first thing I’d check and might well account for the imbalance.

You could also get the brake calipers checked, and if your brake fluid hasnt been changed in a while, have that changed to bleed any air out.

Edit: Just notice that its suspension, in that case, perhaps a leak in the air springs.

Hope you get to the soln 🙂

honda boi · 17-02-2025 05:09PM

https://www.boards.ie/discussion/comment/123181016#Comment_123181016

Thanks fish, I'm assuming a leak in the air bags would result in the car riding low?

GSBellew · 17-02-2025 05:12PM

Replace the two rear shock absorbers.

landmarkjohn · 17-02-2025 09:32PM

For reference attached is our 2015 F11 pass sheet with readings of 36 Mm and 30 Mm, imbalance 17%.

Yours is just 5% outside the limit. A quick trawl on google suggests there is a calibration option in diagnostic software for these cars. I would be tempted to run a calibration, check tyre pressures are equal on the back, measure distance from floor to top of wheel arch on both sides and if all looks OK put it through again. Good luck.

landmarkjohn · 17-02-2025 09:33PM

https://www.boards.ie/discussion/comment/123182176#Comment_123182176

Also I would try and do the calibration as close to the retest as possible.

M3CS · 17-02-2025 09:37PM

I had this issue before with the NCT. Failed on an apparent suspension imbalance and I was quoted in excess of €1k to get it sorted.

I did nothing, booked it in for another test a couple of months later and it passed with no issues whatsoever. That's the magic of the NCT.

17-02-2025 09:41PM

https://www.boards.ie/discussion/comment/123181307#Comment_123181307

Yes, that’s possible. Could be down to a no. of possible factors. A diagnostic tool should read any fault codes to help pinpoint issues with the air bags, compressor, or height sensors.

honda boi · 17-02-2025 10:34PM

https://www.boards.ie/discussion/comment/123182176#Comment_123182176

Interesting, I just googled that and I see there is software calibration for the airbags. I currently have bimmerlink and bimmercode and it seems it's not doable on these and looks like I need ISTA software and a laptop. But it seems that's only really for when replacing or working on the airbags on my quick research?

Went and checked the tyre pressures and there was only a difference of 1 psi between the 2 rear tyres. Will check with the measuring tape to see if there's a difference in height between each side in the morning , but eyeing it up there doesn't look to be any difference.

It is really annoying that it's only 5% imbalance, id like to chance it but now its €40 retest fee.

Tho it's a lot cheaper then the 300€ in parts to replace!

User1998 · 18-02-2025 02:49AM

They already top up all your tyres before the test. Might be worth chancing it again but getting a calibration done by someone with that software probably won’t be cheap

Birka · 18-02-2025 09:44AM

If you're only 5% out it might be worth trying another test centre as the machine calibration will vary somewhat

mk7r · 18-02-2025 10:43AM

The calibration can be done with any decent scanner (Autel, Launch etc) but it is a bit of a faff involving measuring tapes and is only to set the ride height generally. In reality you are 35% out not 5% and you are going to be needing new shocks, that kind of imbalance will definitely be noticeable on the road

honda boi · 18-02-2025 01:44PM

https://www.boards.ie/discussion/comment/123183214#Comment_123183214

Your right mk7r, Ordered the sachs shocks , mounts and dust covers.

Wanted to make sure that it couldn't be something else before I splash out 300€+.

At the minute I don't notice anything odd in the back, but I'll prob notice the difference when I get the new shocks on :D:D

Will update once I fit them and retest.

BrianD3 · 18-02-2025 07:07PM

What do you think the reading should be, 28 or 43, neither? Have you got the readings for previous tests?

I have a hunch here that if you bring the car for a retest without doing anything, it may pass. If one or both shocks or suspension were bad I'd have thought that the Mm readings would be much higher with a bigger imbalance and the car would drive like a pig.

mk7r · 18-02-2025 07:52PM

https://www.boards.ie/discussion/comment/123184985#Comment_123184985

You want the mm to be as low as possible and equal ideally, those readings are a mile out and very indicative of a bad shock

BrianD3 · 18-02-2025 08:00PM

https://www.boards.ie/discussion/comment/123185106#Comment_123185106

I've never seen a reading as low as 28 on a rear axle which is why I'm questioning whether the 28 or 43 is the right reading. Then again I know very little about BMW so maybe 28 is normal and 43 = fcuked but I know that in my own FWD cars, 43 would be a normal and common reading for the rear.

mk7r · 18-02-2025 08:54PM

https://www.boards.ie/discussion/comment/123185138#Comment_123185138

The measurement is done in maha meters or mm and can be thought of as how well the suspension recovers after a bump, my car with sports suspension typically reads between 3 and 4 mm, a higher number would be an indication of softer or worn suspension although the NCT doesn't actually care how worn your shocks are, just that they match within 30% which is a huge margin

honda boi · 19-02-2025 01:35PM

https://www.boards.ie/discussion/comment/123184985#Comment_123184985

Just had a look at its previous NCT results and the rear readings were rear Nearside 24Mm and Offside 22Mm, so definitely seems the Driver shock is travelling more then the passenger side.

https://www.boards.ie/discussion/comment/123185295#Comment_123185295

Thanks mk7r, that actually explains what this test actually means, before i had no idea what the readings meant. Thanks!!

Must mean my rear suspension was much stiffer then my front.

honda boi · 06-03-2025 11:52PM

Update:

Fitted new shocks and strut mounts last Saturday.

Brought it for the test this evening. And it passed.

Happy days.

Looking at the test sheet it seems there is a 10% imbalance.

Would of assumed 2 brand new shocks would have less imbalance ?

mk7r · 07-03-2025 12:26AM

https://www.boards.ie/discussion/comment/123239408#Comment_123239408

That's pretty close to be fair, it's not an exact test which is why the allow so much margin before a fail (30%) what brand were the shocks?

padraig.od · 07-03-2025 12:29AM

https://www.boards.ie/discussion/comment/123182211#Comment_123182211

Literally did the exact same thing! Alternative was to replace entire back axle

honda boi · 07-03-2025 12:30AM

https://www.boards.ie/discussion/comment/123239500#Comment_123239500

Ah ok no problem.

Shocks and mounts are "Sachs".

Google seemed to point me In there direction.

mk7r · 07-03-2025 12:54AM

https://www.boards.ie/discussion/comment/123239509#Comment_123239509

Sachs are good quality parts, you have nothing to worry about

vincent92 · 19-04-2025 08:34AM

Hey, I have the exact same problem 2012 520d f11. Second time failing now, just replaced rear shocks with Sachs but the exact same figures came out. NCT guy did the test on 3 different machines and the same result on each. Car feels and drives fine. No issues with leaking airbags or what not. Very similar figures to you Honda boi. Any one have any idea where to go now, I’m at a loss on this one.