Allison Transmission Problem

[mgernes]

Last night when I was taking the boat over to our gas dock, I noticed that the port transmission did not engage in forward, but would engage in reverse. After leaving the dock I backed out into the channel to play around with the port engine to see if I could get it to engage.

The good news is that it engaged and I was able to successfully run the boat, and returned to the slip safely. As a precaution I asked my Marina Service group to look at the transmission.

Now the bad news.

They found that the idle pressure was at 90, but would drop down to 40 when engaged. They also found that the transmission fluid was a little milky and they were not sure if it was antifreeze or water. There was also a leak at the shaft coupling.

Later today they are going to service the transmission by changing the fluids so that they can learn if there is any metal shavings in the fluid. That should tell me a lot.

There are only a couple of DD(I have 871TI's) in our harbor and my service group had a few questions that hopefully someone can answer.

Is the transmission pressure pump mounted externally or internally inside the transmission? Also, is the transmission cooler that is mounted at the heat exchanger cooled by raw water or anti-freeze?

Are there other things that I should be exploring?

Thank you for your feedback,

Marcus-Beach House

 

[Genesis]

The oil pump is external and bolted to one of the accessory drive points on the back of the engine. Follow the hoses; there are two possibilities. Most of these setups have the gear cooler raw-water cooled, although there IS a second core in most of the factory Detroit heat exchangers intended for gear cooling use. The problem is that it's marginal in size so most marinizers used a separate, external unit that is in the raw water circuit.

 

[Pascal]

Engaging in reverse but not in forward, or with a long delay, is a classic symptoms of low oil in the tranny. Check that first, don't forget it needs to be checked engine idling. If you are at the full mark engine off, you are low. Engine off, the proper level on my dipstick is a couple of inches above the full mark.

I would check that before tearing anything apart.

Another thing is an air leak at the screen which is held up by 3 bolts. If the gasket is damaged, air will get in and cause low pressure.

If these guys aren't familiar with Allisons make sure they don't push a "rebuilt" for something that simple.

 

[sandspur1966]

Ditto what Pascal said.

If I had a technician on board who didn't know, just by looking at the gear, that there were oil hoses running to and from a PTO pump, I would be concerned with his ability to diagnose and make any repairs.

I would get the oil tested as well, which will tell you what is causing the cloudiness.

 

[davidwigler]

The problem could be a simple cable adjustment. Shortly after I bought Windsong the port transmission stopped going into forward from the flybridge helm. I discovered it worked fine from the pilothouse and learned how to adjust the cables. Problem fixed. Try engaging the transmission in forward from the other station.

 

[davidwigler]

Also, after you have refilled the oil to the full line while HOT and in neutral, shut the engine off, wait 10 minutes and scribe a new mark with a file on the dipstick so in the future you can check the oil before startup.

 

[mgernes]

As always, this forum provides such a wealth of information, even on short notice.

Thank you for your thoughts.

Marcus-Beach House

 

[Nonchalant1]

I had that problem and it was air getting into the pump from the intake line. Check for loose fittings from the gear to the pump mounted on the back of the blower drive. There is a 3 bolt flange coming off the gear on the large hose, which is the pump intake. That can have leaks, as well as the hose fittings and the pipe threaded elbow on the intake side of the pump itself. In my case, tightening that elbow fixed it forever.

 

[davidwigler]

Also I forgot to mention if the oil level gets low it can whip air into the oil and look kind of milky. Let it sit for a few days and see if the oil clears up. Hopefully that's all it was. Keep us posted.

 

[captainwjm]

I agree w/ David and Pascal; similiar thing happened to me, was low oil.

 

[bobk]

I too have had shifting problems on an M-15. In my case, the oil looked milky to a mechanic who did not know Allisons. The problem was bad internal seals. Entrained air caused the milky appearance.

My advice based on a bad experience is to never let a mechanic with no Allison experience any where near one. And the guy at your marina is surely inexperienced. The Allison can be easily rebuilt on board with the correct tools at half the price of taking it off. Been there and done that.

IF the output shaft seal is leaking, be sure to check the shaft for pitting. A new seal will not correct that.... it would need a jiffy sleeve.

Bobk

 

[mgernes]

Yesterday my Service Group changed the transmission oil, and fortunately did not find any transmission shavings in their screen.

We idled the engine for some time, engaging the transmissions in both forward and reverse without any problems. I went out for the evening on the boat, then this morning checked the fluid and it was still clear.

I think it may have been air getting in through the intake line which caused the milky appearance. This is something that I will definitely keep an eye on over the next few weeks.

As a general question, how often do you recommend changing the transmission fluid and filter?

This issue has also led me to look at some additional maintenance questions. I already change all the engine and generator oils each year, and racor fuel filters. What about the Walker Air Sep Air Filter(big black coned shaped), how often does that need to be changed? Then this conversation with my Service Group led to the discussion about how often and which hoses on the engines should be changed regularly?

Since I am up in Minnesota, where I am not running more than 150 hours a year(which is actually a lot on the St Croix river), I want to be cautious, but I do not want to go overboard.

Any feedback would be appreciated?

Marcus-Beach House

 

[Genesis]

Hoses, change 5-10 years (depending on service), inspect annually or on each oil change. Make sure you catch the little jump hoses behind the oil pressure block (they are often missed!); a busted one will wreck your day.

Gear oil, change w/filter annually or ~250 hours if you run more than that a year (most recreational folks don't), check before each run (level).

Airsep, change or clean element when dirty. If you have restriction indicators on the Airseps (you should) those will show (with engines under load) if there is excessive restriction in the element.

Front elements should NOT be black; the oil used on them is red in color. If they're black they're probably dirty and might be sooty -- if there is soot you have an exhaust leak and need to find and fix that pronto.

 

[mgernes]

As always, your feedback is precise and helpful.

Thank you,

Marcus-Beach House

 

[Jasper]

Pascal and Dave have said just about all for what you have found so, I would add that your problem may be solved for a short run, however, I would take the boat out for a long run to allow the oil to get warmer and keep an eye on the gear oil pressure.
I have just gone through an entire Gear Box rebuild on the Port side (Allison MH20).
Before this, the forward gear would operate normally until I had a 30 minute run at various speeds, then the gear failed until the oil cooled.

bobk has made a very good point in not allowing any mechanic without Allison knowledge work on them. Also regarding rebuilding the GB on board, which I also did.

Was there any aeration of the GB oil directly after your run?
What sort of Oil Pressure pump are you using? I assume it is a Hydreco, then get that checked. My last 3 years experience (on DD's and Allisons, but 20 years boating) maybe limited in comparison to many on the Forum, but I hope it helps.

 

[Pascal]

Btw if your oil level is getting low under, because of a leak for instance, the first symptoms will be the gear slipping, suddenly picking up 100 rpm or so. It s more pronounced at higher rpm, so you can limp home at a gradually reduce rpm setting.

Then you get to a dock, and switch to neutral, only to find out you can't get it back in forward...