Twin disc transmission

Anyone familiar with twin disc transmissions? Both transmissions within hours of each other started to loose pressure when warmed up (20 mins). Whereas they both would run approx. 300 psi, now they will drop to 250 at cruise, 8 kts., and under200 at idle. I've cleaned the screens which were clean, changed trans oil, checked cooling system integrity. Ideas?

No I'm not an expert.
Which gears are they?

Have you carefully checked the guage wiring, ALL of it?

Are there any other signs such as slippage or revs picking up with boat speed loss.
Have you checked the gear operating temperature? In most cases should not be over ~ 180F, better between 120 - 150F but T.D can confirm.

Which oil are you using? Should be a 30 or a 40 wgt HD engine oil, NOT a multigrade. The wrong oil can be trouble.

How are you checking the pressure? If you are simply using the electric guage then try a good quality mechanical guage at the same place and see what it says. You will need one with a range of at least 500 lbs - 1,000lbs but don't go past that too much or you will lose too much accuracy.
These gears often reach up to 320 or more psi thus the minimum 500 lb rating so you don't monkey the guage.

Use a HYDRAULIC STEEL TEE OR OTHER FTGS, NOT BRASS.

There should be other ports for checking the clutches specifically as they are operated. For that you will need the manual from T.D. to get the locations.

Double check that you do indeed have a potential problem before going too much farther. It could be the pressure guage going away.

Twin disc trans

c electric: thanks for the reply. In order of your comments:
Have not checked wiring, but will. Do so, but pressures show the same on both stations.
No slippage noted.
Did not check with temp gun, but did not feel too hot.
Don't kkmow what oil was in it, but used 30w oil of proper classification
I've got a pressure gauge and will make up a tee ti check pressures at the hoses
After I get a tee, I plan to run the boat to pint of loosing pressure, then check with gauges. That takes about 20-25 mins. It's odd that this began on the port engine and then 2-3 hours later the stab. Engine also.

dach side said:

... this began on the port engine and then 2-3 hours later the stab. Engine also.

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Therein lies a clue.

I have a service manual for the MG-502. The trouble shooting section lists the following as potential causes for low pressure: Partially clogged strainer, stuck regulator valve in selector assembly, broken seal rings on clutch shaft, damaged or worn pump, improper selector valve linkage adjustment. For checking pressure it says to hook a 0-500 psi gage to the 3/18-18 NPTF selector valve inlet from the heat exchanger with a tee. At an input rpm of 1800, pressure should be between 310 and 340. At an input rpm of 600, it should be between 290 and 335.

Temperature range is 140 to 180 degrees F measured at the same port as for the oil pressure check. Max temp at the heat exchanger inlet is listed as 225 F.

The only problem I've had with the transmissions in our boat is when I was messing around near the cable run and disturbed the "free" positioning of one of the cables. That caused issues with shifting (and presumeably impacted the pressures, as it in effect messed up the selector valve linkage adjustment). I knew immediately what I had done, so did not actually check the pressures. If you did any maintenance in the vicinity of the cables, you might look there first.

All of this obviously assumes you have a 502. I'd certainly agree that the nearly concurrent appearance of the problem in both transmissions is a great clue.

It is a twin disc 502 trans., I have not done any work near the cables, but will check the cables.

I agree it's a great clue, but a clue meaning what????

dach side said:

...this began on the port engine and then 2-3 hours later the stab. Engine also.

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Clue ... what do all three units have in common with regard to the pressure gauges?

Are the indicators electric?

How about just answering the question vice playing a guessing game? The OP asked for assistance, not a test.....

Rick b: I checked pressures at the sending unit port, 280/290 port, 300 approx. stbd, both @ 600 rpm's cold. Tomorrow I will put in a tee and check both from heat exchanger. Re: your thoughts on common denominator of this issue, there is a hydraulic/electric pressure sensor and electric gauges. I cannot think of anything that would suddenly afflict both transmissions. Any more clues?

voltage

Pau Hana said:

How about just answering the question vice playing a guessing game? The OP asked for assistance, not a test.....

Click to expand...

Doesn't always work to spoonfeed someone an answer all the time. It's great to have someone in the know, step through the methods that are helpful in diagnosing a problem.

theran5317 said:

Doesn't always work to spoonfeed someone an answer all the time. It's great to have someone in the know, step through the methods that are helpful in diagnosing a problem.

Click to expand...

Then provide the answer AND the troubleshooting diagnostic tree so the problem can be solved quicker and with less back and forth. When I learning about how to build high performance turbo engines in Buick Grand Nationals, I had plenty of questions- and the answers came with the logic behind the answer,

That was much better than trying something based on a hint, getting greasy, and having to clean up to go back to the computer to ask for more information. Hell- even a tech manual will walk you thru a diag tree based on a sympton, not give clues to the answer.

dach side said:

... there is a hydraulic/electric pressure sensor and electric gauges.

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What do they have in common? Voltage? Yes, but the voltage or current transmitted by a transducer or sensor varies (which is what we interpret as the "reading" so what is left that is (or is supposed to be) a constant value?

Where does the voltage or current go after providing the information we want?

A bad ground connection will cause instruments to fluctuate. I usually check for that first.

Most senders ground through the threads on the sender base where it screws into whatever it is measuring. The wiring to it from the guage also must be good including any crimp terminal ends.

The guage also needs a voltage supply and a ground of its own to make a circuit. If these connections are poor it will affect the operating voltage and thus the voltage to the sensor which can throw readings off.

Thanks. I'll send my wife under the dash to check all connections! If a gauge on the bridge has a bad or poor connection, would that also skew the gauge at the lower helm?

Also, assuming it may be related to the electrical side, why would it read 300 psi when cold, then go to 225-270 psi after 1/2 hour of 8 knot cruising? My first thought was dirty h/ex, but the passages were clean. Could there be some restriction on the oil passage side of the h/ex ?

Try calling Peter at Nopper Marine in Fairhaven, MA. Nopper is the local Twin Disc repair shop. He is a real nice guy and may have an idea if the problem is related to the TD instead of some extraneous issue. Just as an aside, would you not expect the oil pressure to go down somewhat as the oil heats up from cold? I'll check my TD (MG5050) next time out.

I would start with the strainer , and a check the proper opil is installed.

Proper oil on our TD MG is 30SAE or 40SAE (warmer climate) - not multigrade!
Th strainer is just like wire mesh.

Chris's, I did call norras, and he said it sounds like an internal leak. I just checked pressure cold on stbd. Trans, 300 psi; on the port engine it's the same. I'm letting it run to warm up, then I'll take it out for a ride and monitor the gauge. FF, screens were cleaned, fluid replaced on both transmissions 3 days ago.

dach side said:

... screens were cleaned, fluid replaced on both transmissions 3 days ago.

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"3 days ago"

Was this immediately before the unexplained pressure drop that did not exist before?

Presumably there is no leakage out of the TD? You did not mention loss of fluid. If it is an "internal"leak, it must be something common to both TDs. If you are losing fluid it might be going out of the transmission oil heat exchanger. It is hard to believe that you could suddenly develop the same problem in 2 separate TDs. I would focus on the plumbing shared by both transmissions. I hope it is not a TD problem. They (ours) need to be removed for service and are heavy/difficult to remove , then expensive to repair.

The screens were cleaned and fluid changed in response to the drop in pressure. Chrisjs, I would agree with you re: pressure dropping when warm, but this has not happened in the previous 2 years and isn't the pressure regulating mechanism supposed to compensate for that? As another aside, I've had the port engine running for 50 minutes and the mech. Gauge showers a constant 30 psi, whereas the lower helg reading started at 345 then dropped to 250; the upper gauge had started at 300 psi, but had dropped to 250. I'm hoping to get out and run the boat to compare readings. Also, temp at hx outflow is 118 degrees.

Rickb, the screens and fluid were done after pressure dropped. Christi's, there is no loss of fluid but Peter at norras thought there might be a leak internally from one of the pistons.

Did you use the mechanical gauge yet?

If not...I'd definitely not panic till I saw the results from it....

Paneled: I had run the port engine 50 minutes with mechanical gauge, 300 psi steady. I put another gauge on the starboard side so I can monitor them underway, both showed 290-310 cold. I'll post a reply tomorrow after a reasonable Ron to kickimuit river.

Don't keep fretting untill you have done the mechanical guage test. But do it. If that shows a problem then you may have a problem.

Electric guages are great but they can also cause all kinds of trouble when they goof up. They make installation easy as the wiring is easy to run to panels in various places. Mechanicals can be difficult to rig to panels and fly bridges.
The trouble is it gets you running around like a chicken with it's head cut off when they goof up. I've been down this road as have many, many others for pressures and temperatures.

Once you really know one way or the other then you can assess the situation PROPERLY and if the guages then figure what specifically.

Keep the electrics connected so you can compare the mechanicals to the electrics for interest sake. These goofy guage, pressure and temp. problems often resolve out to something in the guage, wiring or senders, not the equipment.

Have you topped off the oil in the transmissions recently, prior to the problems appearing? I note you said you weren't sure what the original oil was but you added 30w.