Transmission oil foaming

[llldks]

On my starboard Allison M20 transmission, the oil is foaming. The oil level is correct, the transmission holds 135 psi in forward at 1200-1800 rpm, and shifts fine. I have had the return line from the transmission to the pump replaced without improvement. A mechanic suggests rebuilding the pump as the next step. Any other suggestions?

Larry
Seadation
1972 53MY

[Barry Lunn]

I was told an issue such as this is caused by low fluid level or contamination. if you remove some and put it in a clear glass container does it look normal with no "fish-eyes"?

[dsharp]

It sounds like it is getting air in the system from somewhere on the suction side. Is it bubbles and and discolored like water in it or air bubbles. I was thinking those gears ran close to 180 to 200 lbs. Of pressure but, it has been a few years since I have worked on one

[Timeout]

Did you check the level with the engine running? On mine, the oil cooler and lines will drain down into the transmission when not running and appear overfilled.

[GJH]

Here are the words from the manual:

a. Importance of Proper Oil Level

(I) Maintaining the proper oil level is very important. The transmission oil is used to apply clutches and lubricate and cool the components. Transmission performance will be affected when' the oil aerates. The primary causes of aeration are low oil in the sump or too much oil in the sump.

(2) A low oil level can cause the in- put pump to cavitate, causing irregular oper- ation. The aeration also changes the viscosity and color of the oil to a thin milky liquid.

(3) Too much oil in the sump can introduce oil into the gearing and clutches. This can cause aeration which can overheat the transmission or cause irregular operation.

[davidwigler]

As suggested in another thread on this site, check the oil level while the engine is running at idle and warm, and fill to the full line. Then scribe a new full line on the dipsticks so you can check the oil with the engine off. It's much easier.

On my 8V71 M20s, the new full line is 1.25" higher than the original full line. If you have the same engines that should give you some idea whether you are over or underfilled. Foamy oil may affect the reading, so wait for the air bubbles to separate.

[SKYCHENEY]

Check the suction side, including all hose fittings and the seal at the pick-up screen. Did you clean that screen lately? Did you replace the gasket?

[randy]

I beleive the manual states that you raise the full line 1/8in for each degree of angle to the engine /gear installation. The pump seals do need to be replaced after 30yrs probably. It's on my list to do.
Randy

[Bugsy]

Did you happen to change the oil lately? I started using the same oil as for the engines (as EVERYONE) suggested and the foaming began. I changed one side back to multi-grade (15W40) and the foaming stopped. I thereafter changed the oil on the other transmission and have had no further issues.

I first suspected all of the other causes discussed above so spent a LOT of time eliminating factors that in my case were irrelevant.

[llldks]

I use 40w in both transmissions with recent appearance of foam in only one transmission. Have replaced pump suction line already. Will clean screen, and if problem exists, will consider pump rebuild. Have a good local mechanic but have also seen Arcadian referenced in prior threads. Any feelings on whether it's necessary to send it off?

Larry