Twin Disc 506(?) delayed oil pressure and shifting

[Peter Brownell]

I have a Twin Disc that I believe to be a MG-506(-1?), original equipment on a Cat 3208T circa 1982-83. OEM trolling valve was added by previous owner

It worked just fine. and then there was a (possibly related) starting fluid high rev event. 

Now the transmission takes about 15 seconds to build pressure and shift the first time you engage it in either direction. If it’s been in gear and you go neutral and back into either gear, it will come down to as little as 4 seconds, depending on how long you pause in neutral.

Initially, it was also turning slowly backward (reverse) when the shifter was in neutral.

I’ve cleaned the oil strainer and pumped out the old oil. It was clean, no burned smell. There was a bit of crud in the strainer, but it didn’t seem particularly constricted.

I put in new SAE 30 (1 gallon Rotella T1 and a quart of Napa conventional). It reads between low and full at idle (but I haven’t run it long enough to fully warm up). Previously it shifted fine in this range. FWIW, it is about at the full mark when not running. 

It stopped turning slowly when it was supposed to be in neutral, but still the same delays in building pressure and shifting.

I am guessing a problem with the pump, since it shifts normally just fine once the pressure eventually comes up.

Any troubleshooting next step suggestions?

I know I should better identify the model, but Cat decided to mount the oil cooler for the transmission right on top covering the plate(s). It looks like this one (except in Cat yellow paint): https://downeastboatforum.com/threads/twin-disc-mg-506-for-caterpillar-3208.28575/

Oil fill is on the upper port side. Dipstick is conveniently located on the lower aft, underneath the starter. Oil strainer is behind an oval plate with 3 bolts on the lower port side.

The engine serial number begins with 01Z0 and it’s rated at 300HP

Topic

[Peter Brownell]

Thanks for the suggestion.

I’ve tested shifting on the unit by moving the selector valve arm fully to the detents, and still the same result. I need to do the same with the trolling valve arm, even though the helm control is fully disengaged.

I had a phone conversation with Twin Disc corporate, and was told that based on the symptoms and the model/vintage, it was most likely the clutch piston seals starting to go, and I should contact one of their service vendors about parts and repair. They said the older 506’s were more prone to this problem.

After getting off the phone with them, my skeptical mind started wondering if that really made sense, since there are separate clutch packs for Forward and Reverse, and the delay is the same in either direction, and the problem started at the exact same time for both forward and reverse. Seems unlikely that both sets of piston seals would start to go at the exact same time, especially since I use (and shift into/out of) forward a lot more than reverse.

I am thinking I might take the strainer back out and try sticking a piece of HDPE tubing down the suction tube between the sump and the strainer, to make sure it is not constricted.

I might call the nearest TD authorized service vendor to see what they would propose to do, and if they have a marina near their location I could bring it to. It’s about 100 miles between their location and my slip, but there are marinas within 12 miles of their location. If they work on it, I’d rather avoid paying a lot in travel time (although don’t want to pay more in transient fees at a closer marina, either). I also have the name of a local marine mechanic I might call about an estimate, too.

[Perry]

3308

We had a similar issue with same tranny/engine combo.     We were having 5 sec delays on flybridge with PT  shifting forward.     Lower helm did not.     Upper shifter adjustments rectified the issue

[Author]

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