Seeking advice regarding hard clunk from transmission

[Faheem_23]

I'm looking for advice on my 2018 Mazda CX-5 (FWD, 129k miles) regarding a concerning 'hard clunk' I feel during gear changes at low RPMs (1,000–1,500 range), particularly in stop-and-go traffic. Additionally, during cold starts in the morning, the car violently jerks/lunges forward the very first time I apply the brakes while moving. The transmission fluid is likely original, as the dealer confirmed they did not change it during their recent service. I'm trying to determine if this is a known TCM software/logic issue (like TSB 05-006/24), a sign of worn transmission mounts, or if I should risk a drain-and-fill on original fluid at this high mileage to resolve the shift shock. Has anyone else with a high-mileage Gen 2 CX-5 dealt with this combination of symptoms, and did a software flash or fluid refresh actually fix it?

 

[dan25man]

I mean i have a 2014 with 131k miles and the original transmission fluid and it did have a slow/jerky 1-2 and 2-1 shift. Especially in cold weather. I updated the Transmission Computer (TCU) and that seams to have made the shifting better. I will eventually get around to replacing the trans filter and fluid but for now it's perfectly fine. Still on original trans mounts as well. While i was there i also updated the Power Control Module (PCM) and Body Control Module (BCM) just to make sure all the firmware was up to date.

 

[N7turbo]

Additionally, during cold starts in the morning, the car violently jerks/lunges forward the very first time I apply the brakes while moving.

To be clear, are you throwing it into drive during the 30-second high idle?

concerning 'hard clunk' I feel during gear changes at low RPMs (1,000–1,500 range), particularly in stop-and-go traffic

Can you confirm which gears you are using?

 

[Faheem_23]

I mean i have a 2014 with 131k miles and the original transmission fluid and it did have a slow/jerky 1-2 and 2-1 shift. Especially in cold weather. I updated the Transmission Computer (TCU) and that seams to have made the shifting better. I will eventually get around to replacing the trans filter and fluid but for now it's perfectly fine. Still on original trans mounts as well. While i was there i also updated the Power Control Module (PCM) and Body Control Module (BCM) just to make sure all the firmware was up to date.

Hi! Thank you for your reply. It’s a great relief to know that the issue is fixable. Can you share where you got the update from and how much were you charged for the transmission computer, pcm and bcm update? And are you worried about changing the transmission fluid at a high mileage as i have read that it can do more harm than good?

 

[Faheem_23]

To be clear, you are throwing it into drive during the 30-second high idle?


Can you confirm which gears you are using?

No i wait for atleast 2-3 minutes every time i start the car and then attempt to drive. I just put it into drive and but i think the jerk happens when it shifts from 2nd gear to 1st gear. It only happens that one time after each start.

 

[dan25man]

Hi! Thank you for your reply. It’s a great relief to know that the issue is fixable. Can you share where you got the update from and how much were you charged for the transmission computer, pcm and bcm update? And are you worried about changing the transmission fluid at a high mileage as i have read that it can do more harm than good?

So I got a Mazda/Ford IDS adapter from VX Diag and followed their process to install IDS v136 on a old laptop. (because possibly computer viruses) Then I had to source the calibration files which I found posted online from Mazda South Africa OneDrive download. It was kind of a pain to download all of the calibration files, I'd suggest just downloading what you need, not the whole repository. Also Mazda IDS is somewhat difficult to use so be warned. When you try to use IDS to scan the modules it'll tell you what calibration files it needs. I think the adapter was $80-90?

I'd suggest finding a 3rd party repair shop that has the official licensed Mazda IDS tool and they should be able to do it fairly easily. Or have a Mazda dealer do it. Also be warned that if you mess up updating a module it'll likely be bricked and then you'd need to find another TCU.

Yeah so I've done drain+filter+fill for the transmission fluid on all of the vehicles I've owned, I've also head those arguments. A drain, trans filter change+ cleaning the internal magnet, and then refill gets ~40-50% of the old fluid in the transmission. Less of a shock than doing a 100% flush, I would not recommend a 100% flush personally. If you perform a TCU relearn after replacing the fluid the computer with automatically adjust the engagement points to shift safely. My understanding is that you need to replace the filter+clean the magnets as those elements are catching the wear/debris. If you just do a 100% fluid flush you have the possibility of disturbing the same wear particles which can cause issues. That's just my 2'cents. I disagree that transmission fluid is lifetime fluid but 2 of my cars had over 180,000 miles when I bough them and they shifted fine (Lexus), so if the software update fixes your issues then up to you.

 

[N7turbo]

Yeah so I've done drain+filter+fill for the transmission fluid on all of the vehicles I've owned, I've also head those arguments. A drain, trans filter change+ cleaning the internal magnet, and then refill gets ~40-50% of the old fluid in the transmission. Less of a shock than doing a 100% flush, I would not recommend a 100% flush personally. If you perform a TCU relearn after replacing the fluid the computer with automatically adjust the engagement points to shift safely. My understanding is that you need to replace the filter+clean the magnets as those elements are catching the wear/debris. If you just do a 100% fluid flush you have the possibility of disturbing the same wear particles which can cause issues.

According to these guys' experience, it's fine:

 

[Lazy2.5]

I'm trying to determine if I should risk a drain-and-fill on original fluid at this high mileage to resolve the shift shock.

There is no risk here, a drain and fill can be done at higher mileage than yours without risking any damage.

However, just original fluid on it's own, while possible is unlikely to be the culprit. The first step is to make sure your transmission fluid is not low. (It probably is)

While you're in there, you may as well do the drain and fill. I would have the pan dropped to assess the transmissions conditions and have the filter changed.

I'm confident this should resolve your hard shifts.

 

[Lazy2.5]

To be clear, are you throwing it into drive during the 30-second high idle?

Are you suggesting this might be the source of the issue?

Also highly unlikely. I have been skipping the high-idle, most of the time, for well over 200,000 miles now. Never any issues.

No i wait for atleast 2-3 minutes every time i start the car and then attempt to drive.

Unrelated, but the owners manual says to put the car into drive after the 30 second high-idle is finished. Waiting any longer (Again, nothing to do with your transmission issue) is detrimental to your drivetrain.

I just put it into drive and but i think the jerk happens when it shifts from 2nd gear to 1st gear. It only happens that one time after each start.

Does it happen only when it is cold out? Or regardless of ambient temperatures?

I strongly recommend to start with this transmission reset:

The process is a little finicky and may take some tries to get right. Do it on a full warm transmission (20min of driving or more) Hopefully that offers some lasting solution, but I would still get that pan dropped, fluid, filter changed, have the bottom of the pan, filter and fluid inspected for signs of internal wear, Asap.

 

[N7turbo]

Are you suggesting this might be the source of the issue?

It's just a question.

One asked because someone either here or on Mazda Reddit (I can't remember which) was doing exactly that. Then asking why their car was lurching forward.

 

[cz5gt]

Yes. There was a post that its best to wait for the first start (cold car/engine/transm) high-idle to settle (20-30secs) especially if going in reverse first.

Apart fron the transmission, waiting 20-30secs is also good in general. Lets fluids go around where they need, CAT gets prewarmed, oil flows in crannies, etc

 

[Lazy2.5]

Yes. There was a post that its best to wait for the first start (cold car/engine/transm) high-idle to settle (20-30secs) especially if going in reverse first.

Apart fron the transmission, waiting 20-30secs is also good in general. Lets fluids go around where they need, CAT gets prewarmed, oil flows in crannies, etc

I would much prefer to have the oil circulate without the AWS or whatever it's called, the accelerated warm-up cycle. The only way to circumvent it is by rolling forward in neutral... Lol. Not very practical.

Oil pressure builds and begins to circulate through the engine in seconds. I just don't like how the engine also reaches 100% load and significantly retards timing just to warm-up the cat faster when I can do that in a couple minutes of normal driving.

Waiting a few seconds, and simply taking off in gear gently is also perfectly fine. It's your choice, but don't wait longer than that 30 sec mark.

 

[dunhillmc]

Are you suggesting this might be the source of the issue?

Also highly unlikely. I have been skipping the high-idle, most of the time, for well over 200,000 miles now. Never any issues.

Unrelated, but the owners manual says to put the car into drive after the 30 second high-idle is finished. Waiting any longer (Again, nothing to do with your transmission issue) is detrimental to your drivetrain.


Does it happen only when it is cold out? Or regardless of ambient temperatures?

I strongly recommend to start with this transmission reset:

The process is a little finicky and may take some tries to get right. Do it on a full warm transmission (20min of driving or more) Hopefully that offers some lasting solution, but I would still get that pan dropped, fluid, filter changed, have the bottom of the pan, filter and fluid inspected for signs of internal wear, Asap.

OP, do the relearning process in this video and see if that helps