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Old 06-30-2013, 08:08 PM   #1
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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?
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Old 07-02-2013, 02:07 AM   #2
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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.
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Old 07-02-2013, 08:50 AM   #3
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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.
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Old 07-02-2013, 09:05 AM   #4
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... this began on the port engine and then 2-3 hours later the stab. Engine also.
Therein lies a clue.
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Old 07-02-2013, 10:28 AM   #5
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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.
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Old 07-02-2013, 12:56 PM   #6
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It is a twin disc 502 trans., I have not done any work near the cables, but will check the cables.
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Old 07-02-2013, 02:14 PM   #7
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I agree it's a great clue, but a clue meaning what????
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Old 07-02-2013, 03:57 PM   #8
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Quote:
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...this began on the port engine and then 2-3 hours later the stab. Engine also.
Clue ... what do all three units have in common with regard to the pressure gauges?

Are the indicators electric?
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Old 07-02-2013, 04:50 PM   #9
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How about just answering the question vice playing a guessing game? The OP asked for assistance, not a test.....
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Old 07-02-2013, 05:47 PM   #10
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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?
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Old 07-02-2013, 06:27 PM   #11
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voltage
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Old 07-02-2013, 06:41 PM   #12
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How about just answering the question vice playing a guessing game? The OP asked for assistance, not a test.....
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.
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Old 07-02-2013, 07:31 PM   #13
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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.
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.
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Old 07-02-2013, 09:27 PM   #14
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Quote:
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... there is a hydraulic/electric pressure sensor and electric gauges.
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?
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Old 07-02-2013, 10:47 PM   #15
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A bad ground connection will cause instruments to fluctuate. I usually check for that first.
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Old 07-03-2013, 12:42 AM   #16
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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.
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Old 07-03-2013, 07:40 AM   #17
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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?
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Old 07-03-2013, 08:18 AM   #18
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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 ?
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Old 07-03-2013, 08:45 AM   #19
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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.
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Old 07-03-2013, 09:26 AM   #20
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I would start with the strainer , and a check the proper opil is installed.
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