Once an hour transmission thunk at highway speeds only

[TruckFirst]

Searching the forum, there's lots of complaints about transmission clunks or thunks when slowing down to a stop or getting off the line. I have the opposite problem - it only happens at steady highway speeds.

I do a lot of long roadtrips in my truck (2019 limited, 5.7 hemi, bone stock) and only get clunks when at steady highway speeds. Almost like clockwork too, happens once per hour, plus or minus 10 minutes! Steady 70-85 mph, just cruising along and then a thunk that I can feel and hear. For those who grew up driving manuals, it's as if I quickly tapped a clutch pedal - feels like it drops out of gear for split second and then re-engages with a sharp clunk. Then all fine until about an hour later.

No way to replicate this with my dealer as it doesn't happen in town and no service rep is going to go for an hour long ride with me! The service guy at my local dealer says he's heard of something similar with Mercedes but not on the 5.7 Ram.

Sorry if this has been posted before. I searched but didn't find anything. Any ideas much appreciated.

 

[Richard320]

Ideas, you say?

Maybe some kind of self test within the transmission?

That's all I got.

 

[Scap]

Maybe the 4A is engaging or binding?

 

[TruckFirst]

BTW, I'm always in 2WD when on the highway (obviously). Most of the time it happens, cruise control is activated but it also happens without cruise control. Other than resetting the computer and changing out the trans fluid, I can't think of anything else I can try.

 

[GKIII]

How many miles on the truck?

 

[TruckFirst]

44k miles. I bought it used in 2020 with 19k on the clock. I'd say 80% of the miles I added have been highway.

 

[GKIII]

I'd suspect a faulty valvebody or a leaking clutch pack drum, but the regularity/predictability would throw me off for sure. No codes?

 

[TruckFirst]

Thanks for the ideas. No codes last time I checked, but that was about 5 months ago.

 

[GKIII]

Yea sorry I can't be of more help. This sounds like one of those things that's impossible to diagnose without a code...that is unless you want to open the trans a take a look.

Maybe change the fluid and see if that helps (if it does, it might just be a temporary fix masking the issue). Use the OEM fluid/Pan/Filter, just in case there's a failure while you're under the powertrain warranty.

 

[TruckFirst]

I'll have the shop run a diagnostic next time I take it in. I've got the 8 year MOPAR extended warranty on it...so part of me just wants to ignore it rather than chase gremlins!

 

[GKIII]

I'll have the shop run a diagnostic next time I take it in. I've got the 8 year MOPAR extended warranty on it...so part of me just wants to ignore it rather than chase gremlins!

In that case, I would do exactly that. If it grenades you're covered hah.

 

[Fatherof3]

This is just a guess but are you sure it's in the transmission ?
Reason I ask is because when MDS kicks on and off it can feel like a thunk and being that it only happens on the highway . MDS usually won't come on around the city but being between 70 and 85 that seems to be when MDS would kick on and off .

 

[HemiDude]

Possibly the A/C compressor? Are you running with the A/C or Defroster on?

 

[GKIII]

This is just a guess but are you sure it's in the transmission ?
Reason I ask is because when MDS kicks on and off it can feel like a thunk and being that it only happens on the highway . MDS usually won't come on around the city but being between 70 and 85 that seems to be when MDS would kick on and off .

The only difference I notice when MDS kicks on is the change in exhaust tone.

 

[TruckFirst]

This is just a guess but are you sure it's in the transmission ?
Reason I ask is because when MDS kicks on and off it can feel like a thunk and being that it only happens on the highway . MDS usually won't come on around the city but being between 70 and 85 that seems to be when MDS would kick on and off .

Interesting - I had no idea our Hemis had this feature! Just googled it and read this article: What Is Hemi MDS?

According to the article, shutting down the 4 cylinders happens sequentially per revolution so at highway speeds it would feel like it happened all at once. But wouldn't I feel it more often than once an hour on long road trips only?

 

[TruckFirst]

Possibly the A/C compressor? Are you running with the A/C or Defroster on?

Probably not - it happens whether I'm driving in the summer or winter. Defrost not on. AC sometimes on, heater sometimes running, sometimes neither.

 

[Fatherof3]

The only difference I notice when MDS kicks on is the change in exhaust tone.

Interesting - I had no idea our Hemis had this feature! Just googled it and read this article: What Is Hemi MDS?

According to the article, shutting down the 4 cylinders happens sequentially per revolution so at highway speeds it would feel like it happened all at once. But wouldn't I feel it more often than once an hour on long road trips only?

I can feel mine when I forget to turn it off . Like I said though I am just guessing .
It's possible to only feel it once , I would think if it was mechanical you would feel it all the time not just on the highway .

 

[TruckFirst]

Thanks everyone for the brainstorming. I'll take the truck in for service in a few months and run the diagnostic. Maybe something will surface and I'll update this forum. In the meantime, if it grenades, I just hope it's within an easy tow of a Ram dealer and not in the mountains somewhere!

 

[Darksteel165]

The only difference I notice when MDS kicks on is the change in exhaust tone.

Today I noticed if MDS kicks on at the same point I take my foot on the pedal it sometimes makes a weird loud thunking noise not sure if it's similar to the OP but I noticed it between switching from cruise control, to regular foot driving to triggering the MDS (remove foot from pedal for 1-2 seconds to trigger MDS and then put foot lightly back on)
When I triggered MDS I took my foot off the pedal for whatever reason and I got that very loud thunk which scared the crap out of me.
I turned off MDS and I didn't notice the same problem.

Have you tried turning off MDS and seeing if you get the problem?
Push the manual shifting button and set it to 8, MDS will stay off (need to do every time you engage drive)

 

[sparky2shoes]

I've had the exact same thing happen several times, only on long hwy trips as well. Tried the gear selector to eliminate the mds and it still happened. Haven't taken it in to see what they say yet as they will not be able to duplicate it and will not do anything without confirming the issue.