Twin disc transmission

Aqua belle: I had not topped off the oil, but changed the oil in both transmissions, (after cleaning screens)' because of the lower pressure readings. In regards to the oil that was in the transmissions, I meant I did not know what the p/o had used. I replaced with 30w hd oil, cc/cf as required Bo td.

If the 2 transmissions share an oil cooling system, then I can now see how an internal seal/pump leak in one unit would affect the pressure for both transmissions. If that is the case, then you would somehow need to identify which is the bad unit. Perhaps Peter has an idea. As previous, hope you do not have to deal with the effort and expense of removing/repairing. If you do, Nopper is the best local shop to RI.

If by chance the tranny's share the oil system (I doubt it very much)....CHANGE THAT IMMEDIATELY!!!!!....

They do not share oil or cooling systems

Then I do not understand how a seal/pump leak in one TD would affect the pressure in the other unit. So, presumably you have 2 pressure gauges that are both showing the same issue?
It is starting to feel more like an electrical problem.
It has to be extremely unlikely that both units would independently show the same issue at the same time.

Chrisjs: I agree that it's very unlikely that both transmissions develope a similar issue at the same time. Yesterday, I plumbed mechanical pressure gauges into both transmissions. I'll monitor them today, but I think I'll find the mech gauges will show proper pressures. I'll be back.

Thus RickB somewhat playfully making some of us think through a common denominator such as an electrical issue.

I guess I'll be chasing the wiring soon but I need to confirm pressures under various conditions first, especially since my wife refuses to climb around under the dash areas.

I read somewhere, probably BoatDiesel, that the biggest killer of Twindiscs was shifting while still having the revs up. I could see that happening to both boxes at the same time. Running hard and then you see a crab pot, slam her into reverse.

Baggiolini: no crab pot issues. If I ran into that problem (no pun intended) I would run them over first. But today, cruised at 8.5 kts for 2 hours, checked mech. Gauges every 1/2 hour: mech gauges consistently 300-320 psi, while dash gauges went from 290 down to 200 psi. Temps were consistently 135 at the case & 124 on the cold side of the hx. I'm feeling a whole lot better about this issue now.

Great! The trouble with internet troubleshooting is getting off on tangents. We all have had issues of our own at some time and pass them along...the trick is not all issues are what we had.

The guys who have been around a lot of issues know better...and that doesn't just mean guys that have been boating for 70 years or have owned 15 boats. You need the experiences of guys who have worked on hundreds if not thousands of boats. They will try to put your brakes on and make you focus on symptoms rather than jump to conclusions.

Wise mechanics know that it's usually something simple...look there first...as many pointed out that 2 trannys with the same symptoms rarely go bad at the same time but the voltage (and/or total electrical system) feeding those electric gauges is common so starting there has some promise.

Instead of unreliable electric gauges , perhaps a camera looking at a set of mechanical gauges would be less hassle?

Swapped pressure senders and the lower pressure is now on the starboard side. Confirmed proper operating pressures with mech. Gauges. Per Peter from Norras, I will replace both senders.