Allison M20 issue

[skrmetta90]

my boat has been idle for a few years- nothing but the occasional crank and run. We have finally moved home and now it's time to start working on the old girl. First issue I have is no forward capability in the port Gear, however I have reverse. This is being caused by low oil pressure per the gauge- I checked the oil level and the gear is full of oil when running. I'm thinking it's the oil pump- I'm heading down to the boat in a few to start troubleshooting. I've searched some old threads and most of those issues revolved around low oil- which is an issue I don't have. Any thoughts or experience with this specific issue?

Thanks in advance!

[davidwigler]

Clean the oil strainer screen.

[saltshaker]

If that doesn't work, try adjusting the pump pressure. Might need to rebuild the pump but I'd try changing the pressure first to see if she comes around. They're pretty simple or you can get a rebuilt one from Acadian Marine

[skrmetta90]

Thanks guys, cleaned the screen, changed the filter and the oil and forward is now working. I only have a little under 100 psi on the pressure gauge- I'll try to adjust the pressure but from the way it looks I'll probably need to rebuild the pump. Again, thanks for the help!

[Nonchalant1]

Thanks guys, cleaned the screen, changed the filter and the oil and forward is now working. I only have a little under 100 psi on the pressure gauge- I'll try to adjust the pressure but from the way it looks I'll probably need to rebuild the pump. Again, thanks for the help!

Frequently the problem is an air leak in the intake side or a clogged screen at the three bolt flange. Near the gear is a 3 bolt flange that leads to the larger hose to the pump. Take the flange off and check for a clogged screen.
Then check for an air leak by seeing of the elbow at the pump is tight and all the hose fittings are tight. Air in the system reduces the pressure.

If it is the pump, they 're easy to rebuild because there are just 2 big O rings. One is like an O and the other is bent into the groove so it looks like a 4 leaf clover pattern. The pump just splits into halves to reveal the O rings.

[Steve Mannshardt]

If a leak on the auction side, air expect the pump to be noisy. Like marbles being crushed or a whine at higher rpm. That will be cavitation on the pump and you will see aerated oil on the dip stick.

[skrmetta90]

I feel like I'm in Ground Hog Day..... Getting the boat ready for the season again and the port gear gives no response when kicked into forward; still functions as normal in reverse. I took the selector mechanism off of the gear and checked to make sure the shifting components were ok, all was good there. Put it back on and still a no-go. Local mechanics are saying that the clutch plate is worn and it probably needs to be replaced. Before I commit to removing the gear, is there anything else I should check? Screens clear, I obviously have pressure from the pump as it's engaging in reverse- What about the dump valve? I've read that could cause this issue but I don't know what or where that is. As always, any help is greatly appreciated!

[skrmetta90]

Oh, one other thing, the shaft spins in the forward direction when the gear is in Neutral. I can hold it from turning with my hand for about 10 seconds, but after that enough pressure builds up and I can't hold it anymore.

[kjcampos]

have you looked at your shift actuator may be bad ....my trany was stuck where could not put it in neutral and they were the problem ....there are 2 per engine ...i replaced both on my starboard engine...now trany works like new...

[saltshaker]

Oh, one other thing, the shaft spins in the forward direction when the gear is in Neutral. I can hold it from turning with my hand for about 10 seconds, but after that enough pressure builds up and I can't hold it anymore.

Sounds like low pressure. Did you ever replace or rebuild the pump? have you tried adjusting pressure? Very common to have reverse but no forward when the pressure is low. I doubt it's the clutch.