Twin Disc MG506 erratic oil pressure

[brettportzer]

I've got 6-71Ns with Twin Disc MG506 gears.

The SB gear has started to have swings in oil pressure that it hasn't previously had. It usually runs at 360psi according to my helm gauge, which is mechanical.

I changed the oil, replaced the screen, and replaced the screen gasket. That didn't help anything.

Symptoms: At 1300 rpm cruise, it runs pretty steady at 360, although usually the needle wiggles ever so slightly.

After running at 1300 rpm for 3 hrs, when I back down to 800 rpm the pressure drops to 270 and fluctuates somewhat wildly, and then will pop back up to 360.

It goes back and forth between jumpy around 270 and sticking up at 360.

It does not do this in reverse.

Wondering if this seems like it could possibly be the selector valve assembly? Or the pump?

Thanks!

 

[brettportzer]

Oh, I made a short video of normal behavior and the erratic behavior:

https://www.dropbox.com/scl/fo/4za3...gWwV1hQE?rlkey=3uk9wv278wqdsf1bjbgtec9e6&dl=0

 

[Captain Ralph]

The 506 is one heck of a bomb proof gear.

Any debris in the oil or screen? Your using the same CF-2 oil as in your engine?

I would check the cable linkage to ensure it is pushing the selector completely to the forward position.
Then figure if you want to reseal the selector valve or send it out.
I don't think the regulator would bounce around past idle but that would be my #3 thought.

 

[brettportzer]

Yea, it is the same Delvac 1240 oil as the engines.

The screen had a couple little things on it, but hardly anything. It broke when I was taking it out, so, I had to get a new one.

I did verify the linkage was good and that it was engaging all the way.

It does jump around a little more at idle, sometimes, it seems like it jumps around more when it is warmed up, vs starting out the trip.

Not a lot of marine know how for this type of stuff where I am, but I have a marine mechanic that I can bring stuff to, so, I can bring him water pumps to rebuild, etc...

I thought of maybe pulling the selector valve and bringing it to him, if I wasn't going to end up getting in over my head with that project.

You never know though!

 

[drburke]

Not to doubt you, so please don't be offended if I ask that you are 100% certain it is a mechanical gauge with a hydraulic line running all the way back to the transmission.

I say this because that looks suspiciously like a bad sender for an electrical gauge.

 

[brettportzer]

Not at all, but they are definitely mechanical. They are the only gauge on the helm panel that are not electric.

 

[Captain Ralph]

How hard is it to swap the dash pressure gauges?

 

[brettportzer]

Well, I do have an extra gauge actually, from the flybridge that I removed... Probably easy enough to do.

I was under the impression that that isn't really a failure mode for mechanical gauges though.

 

[Captain Ralph]

In 20 years on on my 3rd set of mechanical temp gauges that come up to the lower helm.

I'm fixing to rotate my mechanical engine oil gauges. Last year they started reading differently. I noticed one of them settles to zero, the other below zero. Just the same error when the engines are running.
These are SWs also.

I was in the river today, heading back to Jax for a haul tomorrow morning. So after my first reply, I started watching my TD514 pressure gauges, Yes, just a lil wiggle (224-225PSI).

Then, drburke commented about the gauge. An issue I had on my friends ZF clutch, electric sender.
So I had to reply again,, offer at least to rotate the gauges.

Mechanical gauges work fine when they work, but as simple as they are, they do fail,, or do silly things.

So, drburke may have been on something that I stink is still worth pursuing,, and it wont cost you anything; move some gauges around.