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Old 08-12-2019, 10:39 AM   #1
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Is my transmission slipping?

Took a trip this weekend and due to time constraints was running about 3000 RPM on Yanmar 4LH-STP and Hurth 63-IV-2.5 transmissions.

About 2 hours into the return leg I felt the boat pull to port while at the same time losing a few knots. I thought I hit something but it all seemed normal so I powered back up. After another 5 minutes it happened again. Engines were at normal temps.

I took a temp measurement on the trans housings, stbd was 130 and port about 150. Trans fluid level looked ok and no discoloration or burnt smell.

Is it possible the port trans is slipping due to being too hot? The manual says 176 F at 2500 RPM so I don't think so but maybe I'm missing something.

Ran at 2000 RPM the rest of the way home, about an hour. and both trans cases were at 110F when we got to the slip.

Any thoughts?
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Old 08-12-2019, 10:49 AM   #2
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Take your digital tach (of course you have one aboard. don't you?) and check the shaft RPM against each other once you are sure the engines are synched up. And check the cooler on that warmer tranny.
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Old 08-12-2019, 12:05 PM   #3
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You may get a slightly different rpm due to ratio differences in the transmissions, but anything more than 10% is probably slippage.

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Old 08-12-2019, 02:21 PM   #4
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Go for a run and duplicate conditions. Watch tachs carefully. If tranny slips, tach will jump and boat will veer (to port if port tranny). Don't watch tachs so carefully that you hit something.

Don't use synchronizer for this test as it can mask symptoms. Hand sync throttles. Looking for about 100-200rpm jump on the tach.

Nothing wrong with 150F case temp. They never run the same temp as one gear has more bits in play to get counter rotating props.
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Old 08-13-2019, 10:20 AM   #5
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All good advice, I will keep an eye on the tachs. After reading the manual I realized I was checking the trans fluid wrong. There are two kinds of dipsticks, one threaded and one not. You are not supposed to screw in the threaded kind, just leave it to rest on the opening. I might be low, since I had been screwing the dipstick in all the way which would give me a higher level reading. I'll check that this weekend.

If it is slipping, what damage is being done if I continue to use the boat?
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Old 08-13-2019, 04:17 PM   #6
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Geez, Gary, I dunno, not being a tranny man, but could some pieces or grit from the slipping parts gum up some other really expensive parts like gearing?
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Old 08-14-2019, 01:11 PM   #7
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Quote:
Originally Posted by garmstro55 View Post

If it is slipping, what damage is being done if I continue to use the boat?

If it is slipping, it will trash the clutch plates (inside the transmission) in very short order. Of course if it is slipping, it may be because the plates are already worn out.
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