Allison M20 gear failure

[CapetaniosG]

Hi to you all from the beatiful Rhodes Greece. And now to my problem.

After a five hour trip to a nearby island and while trying to dock back at my home port I noticed no response from my port engine. A glance at the gear oil preasure gage showed zero pressure. Shut the engine down and docked with one engine running (not easy with a 15 knot wind in a crowded harbor).

Once at the dock I checked for air leaks at the intake side of the oil pump. None found. Checked for oil leaks none found. Oil level is normal. Intake screen is clean. Oil in gear is 40 wt. clean and clear. Oil filter changed a month ago. Started the engine played with the adjusting screw on the Hydreco pump still no pressure. Put it in forward, neutral and reverse gears still no pressure and no turning of the shaft. That tells me the the Hydreco pump maybe shut. I'll pull it out and inspect it. Aside from the unthinkable ie. internal damage to the gear, have I missed anything? Checked previous postings in this forum, but found nothing to match my problem. Talked to Acadian Marine an they suspect the pump.

Do you guys have any thoughts as to what might be the problem.

Thank you for yor input.

CapetaniosG

 

[Genesis]

Likely the pump; start there....

An internal failure massive enough to result in ZERO pressure is unlikely.

 

[REBrueckner]

If the oil pressure suddenly went to zero...seems like there are limited causes: oil leak into bilge, sudden internal oil flow blockage, air leak on input side of pump, oil pump failure.

Sounds like you are on the right track.

The only other thing: You likely have to yank on pipe fittings and lightly yank on hoses to see if an air leak might have occured...say,maybe, if a pipe fitting developed a vibration crack....

 

[captddis]

There is a fiber disc that is the coupler between the gear and the pump. It either sheared[ it is supposed to shear if the pump seizes] or it is the pump.

 

[mike]

FWIW, one possible cause could be the check valve on the pump is plugged or dirty, allowing oil to by-pass the pump. I had this happen on our 53MY, the exact same symptom you described. To fix it we pulled the check valve, cleaned it, and was up and running. The check valve is located under a cap on the side of the pump opposite the intake port. Be careful not lose the orientation of any of the parts, and the check valve ball. The oil sump on the Allison’s collect a lot of particles from the gears, bearings and esp. the starter gear, all of which some times can make it past the pick-up screen and into the pump and check valve assy. This esp. true if you have been on a long run rough ride.

 

[CapetaniosG]

I thought I give you an update on the Allison M20 gear problem you were so kind to provide input and advice. I thank you all. Here is what I did and the results.

I installed a spare Hydreco gear oil pump that I removed from a DD 8V71N. The pump is identical to the one that failed.

Fired up the port engine and for about 45 seconds the gage showed zero pressure the same as with the old pump. Then, the needle started rising and at iddle speed it went up to about 80psi and stopped. Increased the RPM but the pressure remained at the 80psi level. I played around with the pressure adjusting screw of the pump, and was able to increase the pressure to as much as 180 psi at 800 to 900 rpm. Backed off the adjusting screw at this rpm and brought the pressure down to about 150. At iddle the pressure fell back to about 80psi. At this pressure and at iddle speed, the reverse gear did engaged but not the forward gear. Increased the rpm to about 800 and the gear pressure went up to about 140 psi and the forward gear did engaged.

At this point I reduced rpm down to about 650 to 670 the gear pressure dropped to about 90psi, put in forward gear and after about 5 seconds delay the forward gear did engage. So, I now have again forward and reverse gears.

However, I don't understand why at iddle (600rpm) the pressure drops down to about 80psi. Also, when I increase rpm to 1200 and up, the gear pressure keeps increasing to dengerously high level (180psi). It should be noted that the Allison manual on the Hydreco pupm was of no help as to how one can adjust the gear pressure. I'll search the Hydreco on Internet.


It should be also noted that the spare Hydreco pump was taken off from a DD8v71N and installed on a 8V71ti. Any thoughts will be appreciated as always.

One more thing is that the rpm sending unit that was on the spare pump gives me no rpm at the either helm so I'm guessing on the rpm (except at iddle) from the engine sound. How hard is it to romove the sending unit from the failed pump and put it on the spare pump? Thanks.

capetaniosG

 

[yachtsmanbill]

KALA !! If you need a walk through on this later today, I'll repost a bunch of pics about the tach drive, as for now I hafta go get the "pnuemonia hole" plugged up. Its about 30F here and pretty breezy to boot! ws

 

[Genesis]

Ugh - assuming the second pump is working properly (and there are no air leaks, etc) you've got fairly serious trouble here.

 

[Nonchalant1]

Sounds like the original pump had an internal seal failure. The seals are O-rings, bent into a 4 pointed star shape.

If the two pumps are identical as you said, just take the pressure regulator and pressure sender off the old pump and put them on the replacement pump. The replacement pump is giving you pressure so that could easily fix the regulating and pressure gauge problems.

If it doesn't, these pumps are extremely simple. There is not a problem with any fiber disk coupler between the drive gear and the pump because the replacement pump gives you pressure. So just take the pump apart and replace the O-rings, which are readily available. Then check for little pieces of stuff stuck in the pressure lines. Fanfare had that problem once and it was fixed by finding the small foreign object and removing it.

Doug Shuman

 

[Genesis]

Check the pressure and regulator on the replaced pump, or better, swap it to the other side and see if it works.

You need to eliminate the pump as being defective (finding a "different" one isn't sufficient as it still may not be any good!) As noted these pumps are very simple (they're positive displacement gear pumps with a relief valve mechanism to regulate pressure), but that valve mechanism can stick open due to debris and if it does it will not produce anywhere near proper pressure and that in turn will result in no (or very slow) engagement.

Here is how I'd proceed:

1. Swap the pump from the GOOD engine/gear to the BAD engine/gear. See if the problem disappears. If it does, you have TWO bad pumps - fix one of them and you're done.

2. If the gear STILL doesn't show proper pressure (with a MECHANICAL gauge, don't trust the electrical ones on the bridge) then you've got serious trouble inside the gear, likely seals associated with the engagement mechanism. If that's the case there's no alternative but to pull the gear and have it rebuilt. Acadian is the best option if the gear has to be gone through.

I'm suspicious of the gear because you've tried two pumps, but you need to know before tearing the gear out of the boat (which is a major pain in the ass) that the pump is GOOD. I would not dismount the gear until I KNEW that I was feeding it oil with a WORKING Hydrico pump.

For Allison gears I would NOT run over 150psi on the pump (measured with a MECHANICAL gauge) as there is the potential to blow seals in the gear. These gears normally run around ~135psi; some people will run 'em a bit higher but they generally will hold engagement all the way down to 90psi or so.

I really LIKE Allison gears but they have their detractors; the biggest issue are that they tend to pick up "gunk" in the oil as its a wet flywheel design and they're VERY heavy as a consequence of their case. On the other hand beef is not a bad thing when it comes to durability. They are one of the very few designs in existence that are rated for full-power shifts (don't try that at home on a routine basis kids, especially with the "over-driven" nature of mating these things to recreationally-rated engines that are cranked up, but if you need to for some reason in an emergency there's a good chance you will NOT grenade the gear doing it)