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as they say, its always somthin...


coming home from fishing the other day and noticed my port engine would rev up 200-300rpm momentarily, then immediately come back down to the normal cruise of 2300rpm. The engines are cat 3208Ts and the gears are ZF 225A (same as 220A except the down angle is 5degrees instead of 10).

This happened 7 or 8 times on the way home - each time was just a few seconds. When i got to the dock, i dove in and got a look at the props to make sure there was no obvious problem causing cavitation. I then pulled hte dipsticks on the tranny - looked OK, didn't smell burnt at all - but i drained it all anyway. there was a small amount of metal ont eh magnetic plug at the bottom of the pump but being that i'm probably the first guy in 14 years and 1400 hours to look at it, it was really a small amount. I also checked the screen/filter and it was completely clean.

I refilled and ran the boat again, and noticed the same thing happening. Seems like maybe the clutches are slipping - but it always stops after just a few seconds.

I spoke to ZF and they suggested i test the oil for metal, which i am doing now and should hear from the lab today. They also suggested testing the oil pressure to ensure the pump is OK (shold generate at least 300PSI according to ZF). Beyond that, there isn't much to do but pull the gear off and send it out.

Before i do that, i'm worndering if there are any other easy things to check. Could something else be slipping?. Is ther anything in the engine that could cause this?. If i really need to pull the gear, i'm wondering if its not a better idea to replace it with a new gear - pulling the gear in my boat would mean removing the entire engine - so going to all that expense makes me wonder if going with a new gear is a better idea.

...and finally - big geometry question here...if the new 220A gear is the same in all dimensions as the 225 except the down angle is 10 degrees - how far would i have to lower the front of my engine to make the alignment work - i have about 3 inches to play with - about 1.5" in adjusting the mount and another 1.5" could be gained my removing the aluminum shims in the mount itself. i'm estiating that the engine mounts are 47" forward of the shaft /tranmission coupling.


Thansk in advance for any ideas - ..nothing like big trouble in July:(

Joe

The only thing I can think of is to remove the shift cable and manually make sure that it is going completely into gear . . . just a thought.

Is it possible the problem could be in your tachometer.

I would imagine not because I'm sure you heard the engine winding up the 200 or 300 extra RPMs with diesels.

I had a similiar symtoms last week with gassers, where the 200 or 300 delta is not always audibly noticeable, the problem ended being a tachometer.

tapped it the tach on the bezel with the handle of a screw driver and everthing was fine again.

Only wish we could all be that lucky.

good luck

got the analysis back from CAT - unbelieveably, there are NO elevated levels of any metals or particles. I've tested the oil every year for the last 3 yrs and these numbers are right in line with the rest. ...so at least i know there is nothing coming apart inside the gear.

Tomorrow, i'm going to put a mechanical gauge on the gear pump and see if i'm getting the correct pressure -

...as for the tach - i'm pretty sure its not - I'm using the Aetna digital tachs which actually use a magnetic sender (which is inturn driven by a cable coming off the engine) and you can definetly hear it happening when it does.

I'll keep at it and let ya know what i find.


...could it possibly be an issue with the coupling between the engine and transmission?...i think there are various types of couplers - do they make a type that could concievably slip...or break. I've hear about the spring type couplers that make a loud ratt;le when they fail - but i think they still remain in tack - ??? i don't know - this is really wierd

Joe

Joe - the coupling between the gear and the shaft is usually thru a key and likewise on the prop. If the key on either end is sheared, it could maybe do what you describe, but a far fetched proposition.

If you don't feel any shake when this rpm increase happens, I'd eliminate the gear and look to the engine. Most diesels will vary rpm when the filters get clogged, so maybe you want to drain your Racors, change the element, then change the spin on filter on the engine. Might just be a fuel filter issue. I've had my ZF IRM 220A's bust a hose and lose oil, and they shake like crazy as they lose the clutch pressure.

UV