1987 190D 5 speed vibration in 4th at 35ish MPH
 

ok... I Really love the car, it's slightly underpowered, but not a problem


HOWEVER...

when I'm driving in economy mode, and shift into 4th below 40mph, there is a drone from the driveline...

it's ONLY in 4th gear.

and only below 40mph, when I'm not trying to accelerate. if I give more acceleration, the noise goes away.

I'm not sure if it's in the transmission, or the driveshaft, or the differential... or the axles, or what... just that if I'm in third, and keep pushing, it's quiet...

if I put it into 4th, and just coast on the edge of acceleration, the car will DRONE... HMMMMMMMM
center bearing and mount are new. flex discs are in great shape. as all mounts appear solid, and full height...

I would inspect the front flex disk. With the rear end jacked up off the ground and rotate the front half of the DS back and forth. Look for play in the front flex disk. It takes the most abuse on manual diesels. I put in a 2024101215 front flex disk. Its stronger than the one I had in there before.

Seems like from your description it has to be in the transmission.....

Hmm. Both discs looked pristine. ..

If you guys think trans, I'll start there. A good drain and refill seems in order at any rate... no idea if it's ever been done.
I'll too in some m1-0w20...ought to clean it out nicely

You said only in 4th gear...
if you replicate the speed the driveline is turning in some other gear..... as a tesst... ..and load condition...and it does not show up.... it seems like that narrows it to the trans....

I agree w/leathermang, it's very simple to prove this, and I think you already did.

35 mph is the same number of revs in the driveshaft no matter what gear you are in.

Only thing that is changing is the transmission and the engine speeds.

Not related to this thread.... but in the paper copy of the FSM... there is a lot of stuff related to checking the flex discs which seldom gets mentioned... and I do not think it some of it can be checked without removing it from the drive train...
AND ... some situations call for TWO DIFFERENT densities of flex discs on the same drive line.... so if one has vibration problems and no other obvious cause is found... one needs to really research how to check the flex discs...

And torque and vibration. 4th gear is the only 1:1 ratio meaning power travels straight through 1 shaft in and out of the gearbox. There is no slop in 4th gear to absorb engine vibration like all the other gears so any weaknesses you have in the drivetrain after the gearbox are going to show up in 4th. Check the front flex disk very carefully.

One day I noticed a very distinct ticking noise in 4th gear, guess what I found?
http://i.imgur.com/GqYOlYJ.jpg?1

Wow...

What's the engine rpm when you do this?

(Have you got the factory fitted differential or has it been changed to try and lower revs on motorways?)

How old are the engine mounts and the gearbox mount?

I don't think Mobil 0W-20 would be a good choice for transmission oil...

That oil will not have hardly any anti-wear additives like transmission fluid does. The ad-pack is totally different. The viscosity will be wonky too. Multi-grade oils were made with the assumption they'd be used in a specific application. The OE Dex3 fluid will behave quite differently than a multi-grade 0W-20 which is going to behave like a 0 grade oil when cold (way too thin). The synchros and bearings were designed with Dex3 ATF fluid properties in mind, and anything else will deliver abnormal performance. How do I know? BTDT experience... :(

If you wanted, they do make full synthetic base stock Dex3 ATF. Valvoline Maxlife comes to mind. You could also try Dex6 since it is SUPPOSED to be backward compatible with Dex3 specifications.

Honda transmissions used to call for 10W-30 motor oil at one point in time. The difference is that they were designed to use that from the start, and the ZDDP content of oil was higher back then adding to the oil's usefulness in extreme pressure scenarios.

I have a similar sounding problem with an '87 turbo that was converted to a manual, but its in 3rd and 4th gears when the rpm's are below 2000 and the drive line is under some amount of torque (doesn't happen when sort of coasting down hill). No drone if rpm's are kept above 2000.

New flex discs were installed with the tranny conversion. After racking my brain and checking a bunch of stuff, I've decided to check for worn bushings on the shifter as the next possibility as those weren't addressed with the swap. Donor car for the tranny had 180K and the tranny has a bit of noise when in neutral which appears to be well documented in this forum.

I've only had this car two months, and I know little about it yet...

The rpm is between 1500 and 2000. Below that there's not enough power to hold any speed in 4th, above it, it's quiet.

This sounds EXACTLY like my symptoms. Neutral noisy as well. Sitting at a drive through, pop in neutral, and the noise is noticeable, step on clutch, noise gone.

My cars will always buzz if I shift them into too high a gear for the speed. Engine likes a little more rpm.....shift at a higher speed.;)