Coolant leaking from the bottom of the transmission.

That’s the most likely cause. Hopefully you haven’t ran it too long and wiped out the bearings.

 

Do you know if your transmission is coolant or air cooled?
Just some ideas I would try.
Follow any hoses from your transmission to see if they connect to your cooling system.
Did you remove the inspection plug on the side of your transmission to verify coolant inside. There should be a drain plug lower in the transmission you could check for coolant there.
Did you look above the bell housing for leakage?

 

Use the magnifying glass to search your problem, top right of page.
Many recommend deleting the coolant and install an air-cooled system.
First you should drain your transmission. Your own diagnosis is probably correct.
Good luck

 

If its not coming out the housing vent and the oil in the transmission is not milky, its not going to be a cooler failure. I'd look and see if you can see coolant leaking from above the transmission or back of the engine.

 

I'm have the same problem, I'm wondering what you found out on your truck. Was it the cooler or something much worse. Like blown head casket.

 

I used to think the same. But synthetic gear lube doesn't normally become Choco Milk. It will stay separated when stopped and just a fluid buffet while running. Even harder on bearings and races.

Eaton has no front seal. So when it's over full, it comes out the input shaft retainer. Normally from ingesting coolant. The "Lightning Series" also had internal coolant passages. That was a wonderful design! Not sue what they have. Looks like a FRO.


But your right @AModelCat they need to psi check the coolant system and look for leaks above. Could be other things. Heater core, hoses, engine, head gasket or trans cooler.

But if it's internal the bearings are shot.


Edit, I read it again and see 13sp.

 

When my transmission cooler went out, it pushed the coolant in the transmission. I was losing coolant and transmission fluid mixed through the breather on top of the transmission.