Grinding at higher RPMs 1979 300TD - PeachParts Mercedes-Benz Forum

BenCohen-300TD · #1 08-03-2013, 04:15 PM

Well, I've searched and searched, and there are lots of posts about grinding noises, but nothing that describes my situation:

1979 300TD non-tubo, 139,000mi. New (rebuilt) transmission shifts great.

When accelerating, at higher rpms, I SOMETIMES get a grinding noise that sounds, to me, like a bad bearing. The sound is correlated with the engine RPM, so I also hear it at highways speeds, but not all the time. I haven't heard it in a while, but my GF reportedly heard it driving on the highway earlier this week.

If it's a bad bearing, where could it be? Since it's correlated with engine speed and not wheel speed, it's sounding scary to me.

Any thoughts? Thanks everyone,

B

cooljjay · #2 08-03-2013, 05:11 PM

Where's it coming from? I don't think anyone can tell you where its coming from, just by the description.....you should be able to pinpoint a general area where its coming from....can you make a video of the noise?

BenCohen-300TD · #3 08-03-2013, 05:18 PM

Oh good point!

Well, it sounds to me like it's coming from down where the transmission is.- forward of the shifter. I've only heard it while driving - I've never been able to reproduce it with the hood open.

cooljjay · #4 08-03-2013, 05:32 PM

Has it happened since the transmission was rebuilt?

Tubularfab · #5 08-03-2013, 05:38 PM

Are you sure it's grinding? An exhaust pipe or anything else attached to the engine/tranny can create a similar sound hitting the chassis at certain frequencies. In other words, hard acceleration the drivetrain torques, clearances go away, and something hits with each power pulse from the engine. Can give the impression of a grinding noise but isn't... Just a thought...

whunter · #6 08-03-2013, 08:58 PM

Quote:

Originally Posted by BenCohen-300TD

Well, I've searched and searched, and there are lots of posts about grinding noises, but nothing that describes my situation:

1979 300TD non-tubo, 139,000mi. New (rebuilt) transmission shifts great.

When accelerating, at higher rpms, I SOMETIMES get a grinding noise that sounds, to me, like a bad bearing. The sound is correlated with the engine RPM, so I also hear it at highways speeds, but not all the time. I haven't heard it in a while, but my GF reportedly heard it driving on the highway earlier this week.

If it's a bad bearing, where could it be? Since it's correlated with engine speed and not wheel speed, it's sounding scary to me.

Any thoughts? Thanks everyone,

B

You need to mention that this is an automatic transmission.

My best guess:
* Exhaust making random hard contact.
* Failed mounts
* Loose torque converter bolts

.

BenCohen-300TD · #7 08-03-2013, 09:36 PM

This is an automatic transmission.

Thanks everybody.

Whunter: Those all sound like good guesses. The transmission mount is new but the motor mounts are definitely not in great shape. After the car is warmed up for a while the motor flops around a bit at idle - I've been meaning to replace them but have had a hard time with those bolts. AND the exhaust is all getting replaced next week as it's rusted through in a bunch of spots.

Hopefully the exhaust replacement will eliminate that possibilitiy.

Are you referring to the 3 (4? I can't remember) bolts that attach the flywheel to the torque converter? Those should be easy enough to check on. I'll have a look tomorrow.

Cooljjay: yes this has only happened since I put in the new rebuilt transmission

Tubular Fab: No I'm not sure it's grinding, but that is the best way to describe the sound. Any good ideas on how to eliminate the possibilities?

whunter · #8 08-03-2013, 09:39 PM

Quote:

Originally Posted by BenCohen-300TD

This is an automatic transmission.

Thanks everybody.

Whunter: Those all sound like good guesses. The transmission mount is new but the motor mounts are definitely not in great shape. After the car is warmed up for a while the motor flops around a bit at idle - I've been meaning to replace them but have had a hard time with those bolts. AND the exhaust is all getting replaced next week as it's rusted through in a bunch of spots.

Hopefully the exhaust replacement will eliminate that possibilitiy.

Are you referring to the 3 (4? I can't remember) bolts that attach the flywheel to the torque converter? Those should be easy enough to check on. I'll have a look tomorrow.

Cooljjay: yes this has only happened since I put in the new rebuilt transmission

Tubular Fab: No I'm not sure it's grinding, but that is the best way to describe the sound. Any good ideas on how to eliminate the possibilities?

There are six 13 MM bolts holding the torque converter to the flex plate.

.

BenCohen-300TD · #9 08-03-2013, 09:47 PM

Thanks whunter. Any idea what the torque spec is on these? I put the new transmission in last year but can't remember if I ever had torque specs for these.

Tubularfab · #10 08-03-2013, 11:36 PM

Basically what I said and what Hunter said were the same thing except for the flex plate bolts.

t walgamuth · #11 08-04-2013, 05:55 AM

I'd start with the motor mounts If they are suspect and age not known.

BenCohen-300TD · #12 08-05-2013, 08:57 PM

Well I checked the torque converter bolts and they're pretty tight. Definitely not loose. There was one that I got to turn a tiny bit but mostly they're locked up.

Next I'll try and replace those motor mounts... I remember last time I tried this I gave up - maybe they're a little looser now...