Transmission down shift clunk, FIXED (2 Viewers)

[Campbell]

I had a loud clunk pulling away from curb really slow thought I was rear-ended. There was a diagnostic saying see dealer with an engine alert. Before I could get to dealer next day see dealer disappeared. The inspector said there was a diagnostic but just must have been stuck. Really an $85k car with 10o miles just got stuck and clunked. Any more info would be helpful

 

[tig]

I have less than 400 miles on my new 2025 OT.

The clunk has happened at least 3 times so far. Pfffft!

 

[tig]

For what it’s worth I now have 600 miles on it and the clunk happens almost every time I come to a stop if I try and stop smoothly.

I called Lexus and Albuquerque spoke to the service manager and was told that Lexus is aware of this issue and is working on a fix but there’s no fix or additional information available now,

 

[GMan93]

So I had the same thing on brand new car <500 miles and it was clunking 2-1. I wound up disconnecting the negative on the battery for 15-20 minutes and it did help a good deal resetting the ECU. It went away completely for a bit then came back at probably 25% of the time. I've since been very deliberate at slowly coming to a stop and doing so by evenly depressing the brake and not letting up all the way to a complete stop as i've found if applying the break and going all the way to a dead stop the clunk doesn't happen for me as much. Might be all in my head but i've been trying to "train" the transmission to not shift as hard down into 1st as it starts to pick up that i'm not needing to accelerate and it has stopped happening as much now (~1800 miles). If I really slam on the break then quickly go to accelerate around that 12mph zone where it shifts down ~1k rpm it will often clunk but the smoother I am and the more I try to only go below 12mph if i'm coming to an absolute complete stop it's seemed to be getting better and "learning" to be smoother.

Can't wait for an actual fix but worth a shot to try different things in the meantime for what it's worth...

 

[Pottsy]

FWIW my OT+ also clunks when coming to a full stop right as the inertia is about to pivot from in motion to at rest. I've always tried to feather my brake pedal to make that moment as unnoticeable as possible in all my cars. In this car I simply cannot do that. I've found if you have light pressure on the brake pedal or try to feather it as its coming to a stop the clunk happens, but if you have more pressure on the brake pedal (enough that it results in a more abrupt stop) and just keep it there the transmission downshift clunk doesn't happen, its just a less graceful stop. It 100% seems programming related to me. It feels like when you're feathering the brake the transmission isn't quite sure what to do and makes a poor decision on when to downshift into first. I'd bet money if you could just tell it to not shift down to first until you're at a complete stop the issue would go away completely. There is something in the programming involving input from the brake pedal, acceleration, etc., and I think the nature of the direct drive transmission, that isn't set up correctly.

My transmission is butter smooth in almost every other shift scenario. Because I'd rather have a un-smooth stop than a messed up tranny, I've been braking harder than I normally would when coming up to red lights and stop signs. It rarely clunks if I follow this, and really only does it when I need to feather it for some reason and then it will catch me off guard and clunk (and royally piss me off).

 

[Garauld]

I wonder if adjusting the idle speed a bit might have an effect.

 

[jclaffee]

I wonder if adjusting the idle speed a bit might have an effect.

I'm guessing you're thinking that dialing down idle rpms would result in less clunk? I'd think working on the trans programming makes more sense than adjusting idle speed. Seems like a bunch of other considerations go into idle speed. But I have no real idea with all of the drive by wire crap happening in these vehicles.

 

[incognito.gx]

Dealer transmission reset followed by normal driving helps alot

 

[CitizenEarthling]

I had a loud clunk pulling away from curb really slow thought I was rear-ended. There was a diagnostic saying see dealer with an engine alert. Before I could get to dealer next day see dealer disappeared. The inspector said there was a diagnostic but just must have been stuck. Really an $85k car with 10o miles just got stuck and clunked. Any more info would be helpful

Transmission Clunk TSB L-SB-0042-25

I started at 16, right after I was around 20-21, settled around 18.6. Every refill now shows >310 miles? When before it was like 298. I’ll take every mile 🤣 Still impressive, I haven't had enough driving since the update to notice myself... If it does work, I'll take it! :D

www.lexusgxforum.com

Get the ECM reprogramed..... asap!

 

[jaytee]

In a previous post, I described a harsh downshift clunk when slowly coming to a stop. Most noticeable in stop and go traffic. Sometimes felt like two bumps, one around 3-5 mph, and the second just when I come to a full stop. I was not sure if this normal for that type of transmission or if something else was wrong.

I was at the dealership for 10k service, and went for test drive with tech to demonstrate the issue. The tech said no mechanical issue, no problem codes, and no software update (yet) but cleared the transmission memory.

Now the shifting is absolutely smooth, no harsh downshift at all.

While I was there I also had the break squeal repair (known issue).

I told my service guy I felt clunking when I came to a stop and he told me that was normal. Is there a different way for me to explain this or do I just suggest what’s being said here to clear the transmission history?

 

[cmfhsu]

I told my service guy I felt clunking when I came to a stop and he told me that was normal. Is there a different way for me to explain this or do I just suggest what’s being said here to clear the transmission history?

I suggest asking your dealer (or any local shop, as long as you always go to the same shop) to lubricate your driveshaft slip yoke and u-joints. This has been in my experience the most reliable way to reduce the clunky downshifts, with the bonus of making our cars drive so luxuriously smooth.

Just make sure you go to the same shop every time for the lubrication, as mixing different formulations of chassis grease is a pretty big no-no; it can turn the mixed grease into watery, unprotective goo. You might have to get it re-lubricated every 5-10k miles thereafter.

You can also ask your dealer to apply the downshift clunk recalibration - tell them to apply TSB number "L-SB-0042-25". There's been pretty mixed feedback from this forum about the efficacy of that recalibration, though. Personally, I kinda wish I didn't have it done. It didn't solve my downshift clunking, and now my transmission has learned a pretty harsh torque converter clutch engagement that annoys me. Maybe I'll finally spring the $500 for a Techstream OBDII adapter so I can reset adaptations myself.

 

[cmfhsu]

FWIW my OT+ also clunks when coming to a full stop right as the inertia is about to pivot from in motion to at rest. I've always tried to feather my brake pedal to make that moment as unnoticeable as possible in all my cars. In this car I simply cannot do that. I've found if you have light pressure on the brake pedal or try to feather it as its coming to a stop the clunk happens, but if you have more pressure on the brake pedal (enough that it results in a more abrupt stop) and just keep it there the transmission downshift clunk doesn't happen, its just a less graceful stop. It 100% seems programming related to me. It feels like when you're feathering the brake the transmission isn't quite sure what to do and makes a poor decision on when to downshift into first. I'd bet money if you could just tell it to not shift down to first until you're at a complete stop the issue would go away completely. There is something in the programming involving input from the brake pedal, acceleration, etc., and I think the nature of the direct drive transmission, that isn't set up correctly.

My transmission is butter smooth in almost every other shift scenario. Because I'd rather have a un-smooth stop than a messed up tranny, I've been braking harder than I normally would when coming up to red lights and stop signs. It rarely clunks if I follow this, and really only does it when I need to feather it for some reason and then it will catch me off guard and clunk (and royally piss me off).

I think you might be surprised how much of this might be related to mechanical design & manufacturing tolerances.

I can't find the link anymore, but I was reading the unsealed internal GM 8 speed communications from their class action lawsuit, where they described a very similar thunk downshifting into first / second gear - almost like being rear ended.

It turns out a combination of a leaky internal seal plus unaccounted manufacturing tolerance was causing one of the clutches to have varying fill time (i.e. how much time it takes the clutch applying piston to fill with transmission fluid so it would start to move), leading the clutch to suddenly engage at an uncontrolled speed and thunk the car forward. Because the fill time was variable every time the car starts, it's really hard to "learn" it or program around it.

I wonder if Toyota is also facing manufacturing tolerance challenges that are hard to calibrate around.

 

[CCTX]

I suggest asking your dealer (or any local shop, as long as you always go to the same shop) to lubricate your driveshaft slip yoke and u-joints. This has been in my experience the most reliable way to reduce the clunky downshifts, with the bonus of making our cars drive so luxuriously smooth.

Just make sure you go to the same shop every time for the lubrication, as mixing different formulations of chassis grease is a pretty big no-no; it can turn the mixed grease into watery, unprotective goo. You might have to get it re-lubricated every 5-10k miles thereafter.

You can also ask your dealer to apply the downshift clunk recalibration - tell them to apply TSB number "L-SB-0042-25". There's been pretty mixed feedback from this forum about the efficacy of that recalibration, though. Personally, I kinda wish I didn't have it done. It didn't solve my downshift clunking, and now my transmission has learned a pretty harsh torque converter clutch engagement that annoys me. Maybe I'll finally spring the $500 for a Techstream OBDII adapter so I can reset adaptations myself.

@cmfhsu Do you think a Toyota dealership would use the same grease as that of a Lexus? Thinking of stopping by my Toyota dealership rather than taking a trip to the Lexus one. I usually use my Lexus dealership for regular maintenance so I may ask them in the future to re-grease.

 

[cmfhsu]

There's no way to tell. I don't think Toyota makes a genuine grease, so I think the dealers will probably use whatever brand is their preference.

If you can put up with it for a little while longer, maybe hang tight until you can have the Lexus dealer grease it. There's probably hundreds of thousands of un-greased driveshafts rolling around out there without failure.

Alternatively, this is a pretty DIY-able job if you're willing to crawl around under the car. That way, you can guarantee you're always using the same stuff.