Allison gear oil pressure

[Bob Bradley]

My port gear pressure has been spiking randomly the last couple times out. It starts out just over 200psi, and slowly works its way up to 400+. If I throttle back, it drops somewhat, but still is around 375. Taking it in and out of gear a couple times will sometimes bring it back to the 200 - 225 range, but it eventually climbs back up to 400 again. The oil is clean and full, and it was changed about a season ago.

In addition, my stbd gear only runs around 125psi. Since I am new to this boat, and these gears, I don't know how concerned I should be about this. I'd appreciate any knowledgable input. Thanks.
Bob

[Traveler 45C]

Hi,

I don’t know if you’ve got the same gears but, I’ve got Allison M20 gears and the operating oil pressure should be 110-130psi.

Greg

[Orion]

Check out page 3 of this sight under Transmission Gage it will tell you how to adjust the pressure.
Orion

[Genesis]

125psi is about right.

The other side has a stuck pressure relief valve in the vickers pump (the pump is on the rear of the ENGINE, not on the gear).

I don't know how much of a PITA it is to fix this - but I bet the easiest way is to remove the pump from the engine. That's not terribly difficult - a few bolts and the two hoses and it will come right off, as its gear driven.

[Bob Bradley]

Thanks for the replies. Do you have any suggestions on how to determine the best approach to repair? I'm OK with mechanics, but have 0 experience on this application. Also, what is the damage potential from running with high pressure?

[mike]

This sounds like a sticking check valve. It may or may not be the problem, however I would remove the check valve and clean it in fuel oil. I had the valve stick in the open position after a hard run, with the opposite effect, no oil pressure at start up. To remove the valve is simple, there is a cover on the pump body, remove it and back out the check valve. Just pay close attention as to the positioning of the ball, orfice/seat and spring. There is a good cross-section picture of the assembly in the DD engine service manual (if you don't have one I will try to scan and post it). Do not back out the pressure releaf set screw as you may not have to re-set the pressure if it's a stuck valve. I later also found very small particles in the intake pickup line pre-filter screen. These tranies are simple and robust so I'd bet it's a simple problem. Good luck. mike

[Bob Bradley]

Thanks, Mike. If you could scan the page and either email it to me (rbradley@hubbell.com), or fax it to 203-799-4223 to my attention, that would be fabulous.
Bob

[captddis]

First I would confirm pressure with a mechanical gauge. It is common for failing senders to act like this. I think 400 psi would blow the filter gasket out. Ditto the other comments, but if it is a MH gear on a H.P. engine the press is approx 150-160 psi.

[Bob Bradley]

Can you advise me where to obtain, and more importantly, where to install a manual guage?

[Genesis]

The sender is right on top of the gear, on the top of the selector manifold.

Look where the cable goes to the shift lever. You'll see a gauge sender sticking out. Unscrew it, replace with mechanical gauge. The sender is a 1/8" NPT, most manual gauges are 1/4" NPT, so you may need an adapter.

Gauges are available at most better hardware stores; get one with a 400 or 500 psi range to be 'safe'.

I suspect the gauge is wrong, because 400psi would likely make a huge mess at the filter and/or blow the selector valve seals.

[Bob Bradley]

I'll pick up a mechanical guage and try to tee it off if there is room at the sender, otherwise just install it temporarily. If the reading does indeed prove false, I can also swap the connections on the bridge to see if it's the sender or the guage that is faulty.

[saltshaker36]

Bob,
If you have any questions, need parts or repair of the Hydreco pump, call Acadia Marine (1-888-HYDGEAR)
I found them to be the best source for Allison parts and repairs and their prices are less than 1/2 of Detroit or JT prices. I had to have a pump rebuilt and they were the only one who would do it. I think it was around $150 as opposed to $400 for a new one.

Jack