M20 Tranny sluggish shifting

[MikeP]

THe other day, upon initial startup, the starboard tranny was decidedly slow going into gear. To the point where I got a bit concerned since I put both trannys in reverse to back out of the slip but only the port side provided any thrust. After about 5-6 seconds - maybe more, the starboard engaged.

From that point on it was perfectly normal going in/out of gear and forward/reverse.

I've been noticing over the past year that it has been more and more delayed on initial startup so I'm thinking it's time to investigate. 40Wt oil level is correct, pressure is ok - actually quite high - around 165 - but it's been there since we bought the boat and the PO's logs show it was that way since the mid 1990's. I have done nothing whatsoever to these trannys at all other than change oil a couple of years ago. Oil is not dark/discolored. I have not fooled with the pressure setting, opened the strainer, or anything mechanical related to the tranny.

Based on considerably experience with car automatic trannies, it acts like insufficient pressure but the manual gauges says otherwise.

Any thoughts?

 

[JLR]

Mike - assuming these are hynautics, have you tried bleeding the control heads and the cylinder that throws the gear down in the engine room. The usual description is "spongy" rather than sluggish but a good bleed might take care of both. If you need details on the procedure let me know--one thing to remember is to disconnect the linkage before you bleed.

 

[yachtsmanbill]

Your due for a screen AND a filter job. 165 PSI is actually a little high, and she outa slam into gear right now.
If its not cable actuated, pull the actuator off and shift it by hand. The coupling is right there to witness the engagement. ws

 

[MikeP]

Thanks guys - It's cable actuated so no hynautics involved. It has a reasonably new (remote) filter. I'll take a look at the strainer; my wife's here now so my cleaning Jihad is done and I can go back to work!

 

[MikeP]

Just pulled the strainer - it was clear/clean.

But one thing I noticed is that the manual tranny pressure gauge in the ER is connected to the output of the pump, not to the selector area where the oem pressure sensor is located. I made up/installed new manual gauge ER panels/new gauges/new hoses in '04 but I used the same fittings that the PO had used for tranny pressure.

The tranny pressure on my gauges (from the pump output) doesn't vary based on forward/reverse/RPM as the book specifies so now I'm wondering if the choice by the PO of that port for monitoring pressure isn't essentially useless. MAYBE the actual pressure at the selector is low - I have no idea. The only way to be sure is to make up a t fitting so that the oem sender AND the manual gauge will be reading from the same point.

 

[yachtsmanbill]

Now get out the Allison book and have a look... Theres a forward and a reverse clutch disc with the selection disc in between. That disc rides on a square O ring and is sealed between the disc and the ALLISON ONLY DD flywheel. Thats what holds the clutch pressure when engaging F/R gears.
When I had mine out for Ohaul, the guy put air on the disc and it struggled to escape the flywheel. HARD SEAL was the diagnosis. Ya know where this is going dont ya?? You might get away with adding an oz. or two of brake fluid to help soften that seal. Did you make sure that strainer flange was not leaking? Vacuum that is...
Mine is still slow going into fwd, like about 3-5 seconds but R is now. I just deal with it... ws

 

[MikeP]

Thanks Bill - I will ensure that there are no vac leaks at the strainer when I put it back together. I don't think there was a leak there but who knows, the gasket looked OK.

Appreciate the info re the o-ring seal. I have the Allison tranny manual and I've been doing a bunch of reading this afternoon!

Still thinking that the choice of the gauge port is wrong.

 

[yachtsmanbill]

You really do need to read it at the shifter valve. Mine has a hose right at the top. Easy to T in there. Who knows... maybe bubba installed an orifice at the pump; stranger things have happened! ws

 

[MikeP]

I bought the fittings necessary to T the oem sender and the gauge hose so I'll get at least one side done today.

But I have another question...In addition to the oem sender mounted at the selector housing, there is another sensor (actually looks more like a switch) mounted to the top of the strainer fitting. What is that for? I don't see any picture of it or reference to it in the Allison tranny manual.

 

[Genesis]

Probably a temperature switch. Hatt installed a high drive oil temp alarm warning from the factory on many of their boats.

 

[MikeP]

I ran the starboard eng/tranny with the pressure gauge now reading from the selector position - as does the oem hatt pressure gauge. Big difference!

When cold on startup, the pressure at the pump outlet, where the gauge USED to read from (and the port eng still does) pressure was 170PSI. At the selector (where the gauge is supposed to be), in neutral it reads 108 PSI. Interesting that it had been set up incorrectly since 1992, when the PO installed the tranny gauges.

Another one of those little lessons in never assuming ANYTHING was done correctly. I didn't pay the slightest attention to where the gauges were plumbed when I installed new ones. I just made up new hoses and connected them were they had been connected...

 

[SKYCHENEY]

That switch you are looking at is the drive oil temperature switch for the 12pt Hatteras monitor.

 

[yachtsmanbill]

Hmmmm.... ws

any input across the hall????

 

[MikeP]

Re-plumbed the gauge on the port tranny. Now, the readings make sense...

Instead of 170/170 when cold, the port reads 120 and the starboard 108. SInce the starboard is the sluggish shifter, the lower reading makes sense. I realize they are not supposed to be checked/adjusted at idle and I will perform the actual adjustment at the correct RPM but the fact that the sluggish shifter has a lower pressure setting seems to indicate that a pressure adjustment will take care of it.

I guess I could just adjust the idle pressure of the Star to match the port and see what happens to the shift quality even though that's not the "book solution."

 

[SKYCHENEY]

Could also be a worn clutch.

 

[MikeP]

I found that the pump adjustment would only increase pressure to 118 at idle on that tranny. The pump adjustment increases pressure but once it hits a certain point, the pressure then heads downward. It's as if the adjustment mechanism is not a spring/plunger but has a cam that hit's a high spot and then, as you continue turning, the plunger goes back down the other side.

Again, I didn't try to set the pressure at the specified RPM, just found it interesting that the pump does not appear to steadily increase pressure to some unlimited amount. Also, I had read that it took very small movement of the pressure-adjusting screw to make substantial differences in pressure. This was also not the case - at least at idle. Turning it in 3/4 of a turn increased it by 10 PSI. Additional turning beyond that point dropped the pressure.

 

[yachtsmanbill]

The pump's tired. I just did mine last week and 5 turns raised the pressure from a walloping 60 to 115lbs. That is the max on the pump output. Looks like DEPCO will be getting a package around December! ws

 

[MikeP]

I haven't seen (or looked for) pump repair parts but I assume these pumps can be rebuilt like any other pump. I assume it's just a positive displacement pump like a common oil pump...though don't know that.

 

[yachtsmanbill]

Mines a HYDRECO ws

 

[captddis]

I run my MH's @155lbs. I know those pumps when healthy should hit 200lbs.
Acadian transmission in LA has exchange pumps at reasonable prices or you can get kits to rebuild them. They are aluminum housing pumps and if debris gets run through them it trashes the pump.