My starboard transmission oil pressure has started dropping significantly when I shift, then recovering. It also takes longer to come up to pressure than the port. What do I need to fix ? Something is off, this isn’t normal behavior for it. Boat is a 47 with standard 8v53 detroits and gears.

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My port engine oil gauge is acting in a similar way. It was explained to me that the manual pressure gauges have some type of backflow valve in them so the oil doesn't run back down into the engine when it's not running. If this isn't working, and the oil flows back, then upon startup it takes some time for the oil to make it's way up the copper pipe to the gauge, before you see a reading. 

I'm not sure that would explain the large pressure loss during a shift, I suppose it depends how fast the oil could flow back? My boat drops to 250 when I shift (both engines, either direction) and I recall a post where Jim R said that is normal operation.

Jim Rivas once told me that pressure drop when shifting indicates an imminent failure of something or another I just don’t remember what it was. I’d rather find out before it goes bang.

Ron made the comment that if the pressures are different in F / N / R, then that is an issue - I'm hoping that's what the reference was about.

        "The important question is if the pressure is the same in neutral, forward, and reverse.

         Impending clutch pack failure will usually show itself as a pressure drop in forward or

         reverse in comparison to the other positions."

The usual failure mode in a Twin Disc transmission is for one of the two seal rings to fail on a clutch pack piston. This will cause a leakage flow that will be observable as a reduced pressure while the clutch pack is  fully actuated in comparison to the neutral running pressure or a healthy clutch pack, all observations at the same rpm.

The rated minimum operating pressure is 270psi@1800rpm. This is the running pressure, not the observed pressure while shifting. The running pressure will usually be 300-320psi@1800rpm.

If the clutch packs are healthy, the observed pressure at the same rpm will be the same in reverse, neutral and forward. A moderate pressure drop while shifting is normal.

Assuming no external oil leaks on the transmission, the usual prime suspect for a Twin Disc 506 that is suffering from sluggish shift behavior is to check the pump suction strainer that is behind a cover located on the rear of the transmission at the bottom. The recommended service interval is to clean every 1000hrs, every oil change..

Many people, including mechanics, are not aware of the existence of the strainer so it is commonly neglected.

The other issue that sometimes occurs is that the relief/pressure regulating valve spool can get sticky and not compensate for low pressure as well as a regulator that is not sticky. This may result in slightly extended shift times.

You might also want to compare the transmission temperatures between Port and Starboard to verify that they are close to the same. Unequal temps may indicate a heat exchanger issue.

The Twin Disc design is very robust and should easily go 10,000hrs before needing an overhaul. The main operational concern should be if the transmission pressure is at least equal to or greater than 270psi@1800rpm and not noisy,


Steve said:

Ron made the comment that if the pressures are different in F / N / R, then that is an issue - I'm hoping that's what the reference was about.

        "The important question is if the pressure is the same in neutral, forward, and reverse.

         Impending clutch pack failure will usually show itself as a pressure drop in forward or

         reverse in comparison to the other positions."

Copy and pasted into a permanent file in my Chris Craft directory for reference. Thank you Ronald.

Ronald Zick said:

The usual failure mode in a Twin Disc transmission is for one of the two seal rings to fail on a clutch pack piston. This will cause a leakage flow that will be observable as a reduced pressure while the clutch pack is  fully actuated in comparison to the neutral running pressure or a healthy clutch pack, all observations at the same rpm.

The rated minimum operating pressure is 270psi@1800rpm. This is the running pressure, not the observed pressure while shifting. The running pressure will usually be 300-320psi@1800rpm.

If the clutch packs are healthy, the observed pressure at the same rpm will be the same in reverse, neutral and forward. A moderate pressure drop while shifting is normal.

Assuming no external oil leaks on the transmission, the usual prime suspect for a Twin Disc 506 that is suffering from sluggish shift behavior is to check the pump suction strainer that is behind a cover located on the rear of the transmission at the bottom. The recommended service interval is to clean every 1000hrs, every oil change..

Many people, including mechanics, are not aware of the existence of the strainer so it is commonly neglected.

The other issue that sometimes occurs is that the relief/pressure regulating valve spool can get sticky and not compensate for low pressure as well as a regulator that is not sticky. This may result in slightly extended shift times.

You might also want to compare the transmission temperatures between Port and Starboard to verify that they are close to the same. Unequal temps may indicate a heat exchanger issue.

The Twin Disc design is very robust and should easily go 10,000hrs before needing an overhaul. The main operational concern should be if the transmission pressure is at least equal to or greater than 270psi@1800rpm and not noisy,


Steve said:

Ron made the comment that if the pressures are different in F / N / R, then that is an issue - I'm hoping that's what the reference was about.

        "The important question is if the pressure is the same in neutral, forward, and reverse.

         Impending clutch pack failure will usually show itself as a pressure drop in forward or

         reverse in comparison to the other positions."

Thanks Ron, I really appreciate it. It doesn’t seem to be noticeably lower in pressure than before when running - it still sits around 270 where it always did, the other one has always been slightly higher, +/- 275psi, but both in the normal range as far as I know. The troubling behavior is that starboard now takes longer to come up to pressure and the gauge will drop noticeably by 100psi+ when shifting. There is no pressure difference between forward/reverse/neutral once the pressure comes back up, it’s just that it falls on its face while it’s shifting. Port maintains pressure when shifting, there is no large drop. I will get somebody to check that screen and the valves. 

Hey also - given what it’s doing is there anything I should check that’s more likely than the other things ? Also until it gets fixed will it hurt to run it this way?

Ok I went down there to start messing with it and the transmission was almost bone dry. So I guess that explains the pressure fluctuation. I can’t see where it’s leaking from - this is gonna be fun. Sigh. I filled it back up and will have to get somebody to go with me and watch it when running to see where it’s coming out.

Ok well got to the bottom of it, the transmission oil cooler that’s located in the cooling circuit by the heat exchanger was leaking so it was pumping all the oil out of the transmission. Am replacing both of them since I would assume the port side is not far behind in condition. It’s always something. Doesn’t look like I got any water in the transmission thankfully, I am guessing that’s because the oil pressure far exceeds the water pressure when it’s running.

My port transmission has started to act 'differently'. Shifting feels fine, no noise, same engine/trans as Chris. Normally I  see 325PSI in F/NR and a drop to 250 PSI when I shift. The port now often runs at 325PSI, but sometimes drops to 300 for a period of time (measured in minutes) and then back to 325. Shifting is now dropping to 150PSI. I checked the oil level with the engine running (but cold) and it was just at the low mark. I don't know when the oil was last changed, but it's very clean on the stick and the engines were rebuilt a lot less than 1000 hours ago (I would guess at no more than 300-400 hours.)

Could this just be the low oil level (but still on the stick, just at the low mark) and all I need is a little top up? Or is there something else I should be looking at in the off season?

Steve,

Biggest thing to always be looking for is if there is a difference in pressure between forward and reverse at the same rpm. Having a substantial difference would indicate an O-ring problem.

In your case, I would top the transmission off and see if that addresses the problem. With a cold transmission, low oil level and the SAE30 oil, it is not hard to get a certain amount of air entrainment in the oil which will cause some pressure fluctuations, especially in neutral.

I would be investigating where your oil went. Chris's problem was a rusted oil line to the HXC on the front of the engine. These types of leaks usually start out very slow and get progressively worse over time.

I would be looking for external issues first before getting too concerned with the transmission internals.

Top off and see if that fixes it, then look for the small oil leak.

Thanks Ron. Is SAE40 okay to use as well? It's what I have for the engines. I haven't looked at the manual yet - am I right to think that filling is just done via the dipstick location?

Ronald Zick said:

Steve,

Biggest thing to always be looking for is if there is a difference in pressure between forward and reverse at the same rpm. Having a substantial difference would indicate an O-ring problem.

In your case, I would top the transmission off and see if that addresses the problem. With a cold transmission, low oil level and the SAE30 oil, it is not hard to get a certain amount of air entrainment in the oil which will cause some pressure fluctuations, especially in neutral.

I would be investigating where your oil went. Chris's problem was a rusted oil line to the HXC on the front of the engine. These types of leaks usually start out very slow and get progressively worse over time.

I would be looking for external issues first before getting too concerned with the transmission internals.

Top off and see if that fixes it, then look for the small oil leak.

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