Knocking sound / Transmission/ flexplate /converter or worse aka engine rod/piston?

[lowman]

ITs a sad day today...
Yesterday this knocking sound appeared out of nowhere
Its a very " dull dubbed /thumping sound coming from the underside of the car.
ITs worse when in reverse and so it makes me think it has to do with either flexplate or converter...most because of the rytmic frequency of the knock...
This only happens after a little minute after cold startup,,,(in reverse and drive for the most part),and can not be replicated when engine is getting passed cold start.
There is no sounds from engine or transmission otherwise....and it does not appear if i apply throttle when the sound is there at idle...

what do we think?
Cracked flexplate?converter?

 

[nocfn]

what about in reverse moving backward - how is that noise...

 

[lowman]

i havent checked that at this point. Like in the video..as soon as you give alittle throttle to " speed" up from walking speed...the sound goes away...
The problem is that the window of possibility to diagnose this, is so small...Man this friggin sucks..i so hope its not engine related..and i cross my fingers its only related to transmission/ flexdisc/converter kinda stuff...There is absolutely no other signs of it beeing engine related ,there is no rattles, no sounds when its starting up, cold or hot....so..its strange this one..

 

[nocfn]

So did the PO change the fluid recently, and not enough went into the TC or maybe some crud is lodged in the ValveBody that passes thru loose at a gear shift. maybe it is just as simple as a drain the tc/pan and if the fluid is clean reinstall what you took out. perhaps there is way too little fluid or way too much...

 

[Alphasud40]

I just had something similar on my 94 SL600. I was in town when it happened and decided to drive straight home without stopping. When I got home I placed the car in park and then tried reverse. Lost reverse so I need to remove the transmission and take it apart. Sounds to me you have a transmission issue.

 

[lowman]

So did the PO change the fluid recently, and not enough went into the TC or maybe some crud is lodged in the ValveBody that passes thru loose at a gear shift. maybe it is just as simple as a drain the tc/pan and if the fluid is clean reinstall what you took out. perhaps there is way too little fluid or way too much...

Oh shit i forgot to mention in my owners thread also,,,i have had the transmission down, replaced reverse discs etc etc...and trans is well maintained through the years, and especially the last 7 years . Ive done actually two fluid changes the last 150km ,since repairing the reverse that was slow engaging...and i also had the flexplate off to do the rear Main seal.. new fly wheel bolts too

 

[Kridre]

To me it sounds like something inside the transmission. Flex plate/piston/lose converter would make sounds not only in reverse or drive, but all the time when the engine is running.

I would put the car on the lift with the guy sitting in the car that would stat the car and put it in reverse and try to locate where the sound i coming form underneath. Maybe use stethoscope or something if you have it.

Also I found this in 722.3/722.4 service manual.

 

[lowman]

To me it sounds like something inside the transmission. Flex plate/piston/lose converter would make sounds not only in reverse or drive, but all the time when the engine is running.

I would put the car on the lift with the guy sitting in the car that would stat the car and put it in reverse and try to locate where the sound i coming form underneath. Maybe use stethoscope or something if you have it.

Also I found this in 722.3/722.4 service manual. View attachment 220004

Thank you
The sound is not comparable with that "flowchart/diagnosis sheet." This is more relevant to those typical reverse issues,
I was hoping someone would reckognize the sound in the video, to give me some directions to any "known" faults that might cause this particular knocking sound...The more i debate the thought of the sound,,the more i feel and hope that it is transmission related. IF there was a engine issue...aka rod /bearing noise,,,it would be prevalent all the time,,and not completely go away after 3minutes run time....?And also,if it was a rod /bearing failure,,,i reckon i would be able to do that "test" of keeping the revs at 2000/2500 revs...steady ..and letting to ,..and there should be knocking sounds...but it is dead quiet...
Arrghh..this sucks hahah

 

[emerydc8]

Is it possible that putting it in R or D causes the engine to move enough to transmit an existing noise into the frame, making it more audible at idle? Could it be the fan hitting the shroud?

The last time I changed transmission fluid I noticed a weird creak with the same frequency as your knock when I put it in reverse. I couldn’t tell where it was coming from so I drove it a few times and the noise disappeared. I forgot about this until I read your post. Maybe @nocfn is onto something with the possibility that not enough oil got to the TQ yet.

 

[lowman]

Is it possible that putting it in R or D causes the engine to move enough to transmit an existing noise into the frame, making it more audible at idle? Could it be the fan hitting the shroud?

The last time I changed transmission fluid I noticed a weird creak with the same frequency as your knock when I put it in reverse. I couldn’t tell where it was coming from so I drove it a few times and the noise disappeared. I forgot about this until I read your post. Maybe @nocfn is onto something with the possibility that not enough oil got to the TQ yet.

The car has been driven 100miles after reverse drive repair....so its not that im afraid... also engine mounts are new...the sound is really pronounced in the cabin,,,and very audible under the car...but i will check more tomorrow at work.. thank you

 

[nocfn]

Don't ghost us on this Stephan, if your flux capacitor is not fluxing properly, we need a proper video worthy of your HELLO People!

 

[lowman]

Don't ghost us on this Stephan, if your flux capacitor is not fluxing properly, we need a proper video worthy of your HELLO People!

I wont And today i have a little " breakthrough"......i will know more when testing the car in the morning at work. If this works out...its definetly worthy a Well Well hello video Cause i need to " wipe out" that sad sad sad video i made of this issue...

 

[nocfn]

Perhaps you found the rodent in your gears then.... HA! I checked my notes, I did NOT have a rodent in the 126 fan motor under the dash. I had cicada carcass that was as long as ones' middle finger. When I removed the dash to replace the entire HVAC System, i fancied some nylon screening from the inside over the 6x12" fresh air inlet to keep large items from finding their way into the car over the rest of its life. The rest of the dash install serves as pressure to hold it steady as well.

 

[lowman]

I just had something similar on my 94 SL600. I was in town when it happened and decided to drive straight home without stopping. When I got home I placed the car in park and then tried reverse. Lost reverse so I need to remove the transmission and take it apart. Sounds to me you have a transmission issue.

Heyy mister.Thank you.You are the one guy repairing LH units etc,right? if so...this thread is going to be just up your alley....actually

 

[rayhennig]

Is it possible that putting it in R or D causes the engine to move enough to transmit an existing noise into the frame

I thought that. Our E280T was rough in many respects, especially when shifting from neutral to reverse or drive. And different noise in D versus R. And plenty of vibration when on the move.

We changed the 2 front engine mounts and the car is now very civilised. Completely different.

80€ for 2 Lemfoerder mounts and a few hours work 100% successful. We also have the rear mount but the existing one looks good but we might change it anyway.

RayH

 

[lowman]

I thought that. Our E280T was rough in many respects, especially when shifting from neutral to reverse or drive. And different noise in D versus R. And plenty of vibration when on the move.

We changed the 2 front engine mounts and the car is now very civilised. Completely different.

80€ for 2 Lemfoerder mounts and a few hours work 100% successful. We also have the rear mount but the existing one looks good but we might change it anyway.

RayH

Changing motormounts is really a night and day experience. I have done both on my car aswell..and its basically a new car afterwards. Hopefully i will get around making a video of the mounts themselves at least.Not the " replacement"...

 

[TerryA]

@lowman,
Stephan, I know you changed the motor mounts but not the transmission mount. I changed all three mounts when I did my 500E. I can envision where it could be possible that the new MMs have shifted the vibrations to the older trans mount and that shift could be causing the trans to be touching the frame somewhere.

The older trans mount might look good visually but still have more ware than you think.

Just a thought

 

[nocfn]

IIRC there are 3 "motor mounts" two front and 1 rear, which is referred to as the transmission mount.

 

[rayhennig]

there are 3 "motor mounts" two front and 1 rear, which is referred to as the transmission mount.

Yes. We'll probably change that too but all seems well without.

R

 

[lowman]

Yes. We'll probably change that too but all seems well without.

R

Transmission mount is not available from mercedes, but a company in Germany makes them

 

[lowman]

@lowman,
Stephan, I know you changed the motor mounts but not the transmission mount. I changed all three mounts when I did my 500E. I can envision where it could be possible that the new MMs have shifted the vibrations to the older trans mount and that shift could be causing the trans to be touching the frame somewhere.

The older trans mount might look good visually but still have more ware than you think.

Just a thought

Thank you But its not engine or transmission mount. The transmission mount is 99.9 percent for sure replaced not many years ago.
I have also found the culprit for the knocking noise

 

[emerydc8]

 

[nocfn]

You had a small turkey wedged under the URO transmission insulation mat you installed....

 

[lowman]

Here is the video SOLVED

 

[gsxr]

Good to hear it will be an easy fix! Need to replace the capacitors in the LH module (or, replace the LH module). That will take care of the clicky relay.