Yanmar gearbox problem

[f2bullit]

I have a Yanmar 3GMD engine on my 32ft yacht that is nearing 30 yrs old. Some time ago (after overfilling gearbox with oil) gearbox was reluctant in forward; a very minimum amount of forward propulsion occurs until giving it approx 2500 revs, the drive suddenly comes in (engages) fully and off it goes. The throttle can then be reduced and everything works fine until neutral is selected. Then have to go through same process when engaging forward again.

I reduced oil in box and after not using boat for some months, thought perhaps problem gone but no, it's still happening.

Doesn't happen in reverse by the way.

Anyone with similar experience or some idea as to best cure? Any & all observations would be appreciated

 

[Bosun Higgs]

What is happening is that the cone clutch after gradual wear over the years is not engaging first time. You might have made the problem worse by thinking you would give the gearbox the best oil you could - maybe even gear oil. Did you?

In an identical situation, I had the gearbox re-shimmed to take up the wear since the cone clutch is an expensive item to replace. I then filled the box with SAE 10 engine oil ( and not synthetic or even semi) and had no further problems until I sold the boat

You can fiddle about with the adjustment of the gearbox lever but all you are likely to achieve is engagement ahead and transferring the problem of slip to the reverse gear. Got the T shirt on that one.

 

[f2bullit]

Yanmar gearbox

Spot on Bosun Higgs; I gave it good quality gearbox oil and overfilled it to boot, so your theory looks right. So many thanks for your help.

Excuse my ignorance but what exactly is "gearbox re-shimmed to take up the wear", is it easy to do? Is it something average boat yard mechanic can sort? What sort of cost are we looking at?

 

[wotayottie]

The average boatyard mechanic would go for a new bronze cone clutch - guaranteed result.

I took my gearbox to Cellar marine at porthallow in cornwall where they have to take a more frugal approach for the inshore fishermen they mainly service. I never took the box apart after that so I dont know where they put the shims buy it certainly worked.

If you have been " so kind" as to use hypoid gear oil you might get away with just cleaning that out and inserting engine oil.

Incidentally, I ruined a newish Yanmar engine by feeding it on semi synthetic oil and then running it continuously at steady revs for a long trip. The bores were so polished you could have used them as a mirror and no amount of honing would recover them. Both Yanmar and Castrol tech dept ( I didnt believe Yanmar so I rang Castrol who said exactly the same thing) told me I should use old style mineral oil of the sort used on old trucks and buses in the Yanmar engine. Its not a modern design but a good old one.

 

[Jabamusic]

After a trip to Cellar Marine, i too now have a fully functioning gearbox. Exactly the same problem as you describe with slipping engagement and adjustment of the gearbox lever simply shifted the problem for forward to reverse.
Necessitated the cone being re-glazed (once it was determined to still be within tolerances- so don't leave it hoping it will get better) and replacing the shims.

Not particularly cheap - but certainly affordable and heaps better than replacing the box.
The biggest PITA was having to remove the engine to get the g/box out - but then discovered a broken engine mount so it was probably worth it overall.

Made one hell of a difference as I now have hull speed under engine again - certainly didn't realise just how much it was slipping until now!

Good Luck

 

[f2bullit]

Thanks for all your information. Very useful.
I think 1st effort will be your suggestion to drain out oil, flush and refill with mineral oil.

Unfortunately I'ts a 4 or 5 hour drive from home to west coast Scotland location of my boat so it might have to wait a bit before I get a chance.

Again, thanks

 

[tudorsailor]

I know that this is an old thread, but I need help with a similar problem

I have a Yanmar JH Turbo engine with KM4A gearbox. I was having a problem engaging forwards gear having been in reverse (going into a berth stern to in the Med). This was accompanied by a horrible grinding noise until the gear engaged. There was a possibility that incorrect oil had been put in the box. The gearbox was removed and instead of a repair I was offered a new box that was taken off an engine that was having a sail drive fitted. Imagine by feelings when the horrible grinding noise on engaging forwards has returned. The oil in the box is Yanmar geabox oil, which I changed after 50 hours.

Not only am I gutted to have bought a new box and still have the problem, but I am anxious that one day I will reverse into a concrete quay when unable to engage forwards at a critical moment

Any suggestions?

TudorSailor