Borg Warner 72CR on Perkins T6-354M

[NotQuiteLost]

Ok, so the above transmission has been flawless for me for about ten years. No problems whatsoever. A couple of months ago, there was a pronounced growling and accompanying vibration at higher RPM's that has persisted until today. At first I thought it was a bad bushing or maybe even a bent prop (the noise originally sounded like it was coming from near the bushing/hull area), but when I was motoring for a few hours the other day (at low RPM's, getting maybe 4.5knots), I accidentally grounded on mud. I put it into reverse, revved it up to maybe 2,000 RPMS and I got myself unstuck.

The catch is that the vibration and noise seemed to disappear while I was in reverse. I wasn't positive that it had disappeared, because I was acting a bit hastily to get off the ground, but the same thing happened a few days later when I got myself stuck in a bit of mud in a local bay with which I was unfamiliar. So, I put it into reverse and backed off, and again..no growling or vibration that I could notice.

So far, the problem only seems to occur in forward gear, and it gets progressively worse the higher the RPM's go, starting as an angry growl and becoming a full-on vibration around 1800 RPM. I'm stupid on motors, so any advice is appreciated. Things I've done so far:

-Visually inspected the prop underwater. It doesn't look like it's bent, to my eyes.

-Checked fluid levels on the transmission. The fluid on the stick is the proper clearish/red color that it is when new, and the level is right where it should be at, according to the stick.

I'm not convinced it is the transmission for certain, and I'm also not 100% sure that the problem disappears entirely in reverse (although I'm between 80-90% sure that it does) because my total time in reverse has been less than one minute since noticing the problem. Sorry for such a vague question/post, but I'm simply no expert on diesel motors and/or their transmissions, so it casts all kinds of doubt.

[Laidback]

First , if you can disconnect the propeller shaft coupling from the gearbox output shaft flange. Then move the prop-shaft back an inch or so out of the way - then clamp the shaft.
Now start the engine, engage forward and rev up & listen and feeling for unusual vibration. Then reverse gear and compare sound and vibration.

The above tests may eliminate shaft and prop problems.
Take some pictures of coupling and shaft.

Engine mountings OK ??

[NotQuiteLost]

Engine mountings are good. I was thinking about just taking the whole thing out and having a go-over, but disconnecting the prop shaft for a test makes all kinds of sense. Good suggestion.

Anyone have specific issues with this particular drive unit? From what I've read online here and elsewhere, the main issue people have is in shifting in and out of gear. Not reading much about grinding gears.

[Laidback]

Hi,
Here is a small pix of the flange it is attached tp the coupling'd studs by 4 nuts.
The test may give a general indication where to look next. The main thing is to eliminate a major component.
Later it might be necessary to check the BW's thrust bearing

[NotQuiteLost]

Ok, so uncoupled the tranny from the driveshaft, and aside from the often-discussed *clunk* (although really, it's quite minor) when going into forward gear, there's no vibration of any kind on the body of the transmission. So I think we can eliminate it as the prime suspect, or at least knock it back down the list.

Still interesting why it didn't vibrate at all in reverse, but I'm pretty darned happy that the transmission seems to be fine.

Thanks for the idea, Laidback I get tunnelvision when working on things like this. It takes me days and days to come up with ideas for testing, especially when I'm sweating a gallon an hour down in the engine 'room.'

[DeepFrz]

I think Laidback's suggestion that you check the thrust bearing is a good one. In reverse or with no load on the shaft the thrust bearing wouldn't be doing much of anything. Put a forward load on it and it will be working. If bad, that is when the problem will show up.

[NotQuiteLost]

Thrust bearing. Thanks DeepFrz One more thing to add to the list, and sounds like it's a good fit for my problem.

[Laidback]

"Notquiteoutofthewoods" Need to know set up of the propshaft and the coupling.
1. while it is still disconnected, turn the shaft by hand observing any 'out of alignment', 'wobble', 'noise' etc.
2. Is the shaft stopped from moving FORWARD by being held in a pillow block. (Your pictures please) a pillow block looks like this :-

[Laidback]

And, there is yet another test to perform, before a reasonable diagnosis can be arrived.
The reason for that is simply because that this engine is understood to have been operating for some 10 years. (The Engine and the gearbox's combined weight and rotational force - gradually over time compress the cushioning material in at least 2 of the engine mountings)
That test will involve returning the prop shaft and coupling to face up to the gearbox's output flange - Each Face must line up vertically and horizontally so that the gap on any point of the compass is no more than 3/1,000"