Allison Transmissions, where is the oil going?

[zigzag930]

My Detroits (471's) have Allison transmissions. The Allison transmissions have, ever since I've owned the boat, consumed oil.

I'd say they require my adding about 3-4 ounces every 18 hours or so of running time. No a big deal, but I should would like to know where it's going...

The oil is clear on the dipstick, so I would imagine, I'd see clear oil dripping or pooling somewhere, but there isn't any oil under the transmission or engine. Well actually, there is always a little dark oil in the pads under the engines but these are DD's so I expect that.

I was thinking maybe the oil in the transmission was somehow making it's way past a seal and filling the engine, but the oil level in the Detroits does not seem increase and there is never any smoke from the exhaust. Maybe I'm just not noticing the increase as it's so small in overall capacity that I can't detect it?

Sort of reminds me of that movie "Captain Ron" where he says "Diesels love their oil, where does it go? Nobody knows..."

Sure would like to know if anyone has had a similar issue or figured this out.

 

[rsmith]

Oz? Don’t fill Detroits to the full line they just burn the top quart off.

 

[zigzag930]

The transmissions are consuming oil, not the DD's.....

 

[rayswift]

We had the same issue but maybe more extreme.The transmission would lose oil and the engine would gain.The solution was to replace the engine rear main seal. Apparently the Walker aircepts created enough negative air pressure to pull oilback into the engine. The transmission oil always stayed clear and the difference in volume between the engine and transmission made it manageable until the seal was replaced. Prior to replacing the seal I did replace the transmission pump seals but that was not the problem. 8v-71s

 

[JD5652]

I've never heard of oil migrating from the tranny to the engine if the main seal leaks. I'm no expert on the pressures on either size of that seal.

My starboard engine main seal leaks a very small amount, but only enough to change the color of the oil on long runs. I haven't detected a significant rise in the tranny oil level since noticing the darkening of the oil.

 

[rwaldrop13]

We had this issue with Pau Hana. The Detroit tech I was friends with said it's a crossover leak between the oil cooler and the transmission oil cooler. Only way to stop it he said was to replace the intercooler assembly, or pull the assembly and have it leaktested, then seal off the bad capillary tubes.

 

[zigzag930]

Rob, I'm not familiar with the intercooler. Would oil between the engine and transmission mix there without water getting into them?

 

[bigbill]

maybe the transmission cooler is leaking some oil into the raw water side and going overboard out of the exhaust.. trans oil pressure is higher than the water pressure so you would not see water in the oil.

 

[zigzag930]

Now that is a great possibility. Will need to check that out.

 

[zigzag930]

The Allison transmission is one area of the propulsion system that I am not real familiar with. I've started reading the service manual, etc to understand the external components, pumps, filters, oil and water plumbing, etc.

From what I see, there is a transmission oil cooler on the front of the engine (DD 471), right below the main engine heat exchanger tank. There are two oil line connected there, but no water lines, so I am taking that to mean that the oil cooler somehow uses the coolant from the engine block (anti-freeze) and not raw water.

If there was a leak in the cooler core, I would expect to find oil in my coolant, right?

Has anyone had one of these fail?

I suppose there might be a way to remove the oil lines, plumb a couple of air fittings to the cooler housing where the oil lines attached, and pressurize to test for leaks? If so, this could be done without disassembling the cooler.

If not the oil cooler, then maybe the rear engine seal...

 

[zigzag930]

After some more thinking about this... I am refining my thoughts on what might be going on.

1) If the rear main seal of the engine was leaking oil into the transmission, the oil in the transmission would not be clear (it is) and would be darker like the engine oil. So I don't think the rear seal is the problem, unless the transmission oil is being forced into the engine. Don't see that as likely, but perhaps.

2) If the transmission oil cooler was leaking, I'd think I'd see water in my transmission or some milky oil. Now thinking the transmission cooler less likely culprit.

3) Transmission oil pump??? Maybe a seal on the back side of the Hydreco transmission oil pump is worn. Might that allow oil into the engine? I'm not sure about that, but maybe worth investigating. I did recently notice that there is a slight leak on the front side of the pump. Called a company (Acadia Marine) that repairs them and was told that is a common issue with these pumps. I'll call them again tomorrow and ask about the engine side seal maybe causing this problem.

 

[SKYCHENEY]

Trans oil pressure is much higher than the water in the heat exchager so oil would leak out and not water in.

 

[rwaldrop13]

Rob, I'm not familiar with the intercooler. Would oil between the engine and transmission mix there without water getting into them?

I was told the capillary tubes are joined/fastened together and the joints can create bleedovers.

 

[zigzag930]

After speaking with another trusted Forum member last night and then to Brian at Arcadia Marine, I'm 99% certain the issue is the Hydreco transmission pump.

So now, I just need to decide if I want to rebuild them myself or send them out. Looks pretty easy, so I may go that route.

Either way, I'll update the forum with the results once I know for sure