How can the transmission fluid in my truck leak into the crankcase?

 

I don't think it can, unless it's getting there through the radiator, and in that case you'd also have antifreeze in it. Even then, it'd be a stretch.

How do you know there's ATF in your crankcase?

 

Not able to speak from personal experience but, I've heard from other forums that fluid can pass through the ruptured (rubber) diaphram inside the transmission vacuum modulator.

It then passes through the vacuum hose leading to the engine's intake manifold, where, over time, it seeps past piston rings and into the crankcase. You should also be noticing the transmission fluid dropping in level as it's driven, assuming you're monitoring that as well, though there's no evidence of an external leak (puddles, etc).

It's easy to check by finding the vacuum line which leads down to the transmission case and disconnect it from the intake manifold fitting. Inspect inside the hose for transmission fluid. If it doesn't readily drip from the hose, use a toothpick or similar clean (wooden) stick to check inside the hose for the presence of fluid. If you find ANY, replace the vacuum modulator.

It's usually found on one of the sides of the transmission, down close to the oil pan bolt-flange. The modulator should be easy to purchase from most auto parts supply stores.

 

YOU ARE RIGHT Hooked-on-4WD!!!!
I have a 93' Ranger XLT V6 4.0L 4X4 with original 67K miles.
I had the same problem. Every 2 weeks I was adding a
quart of transmission fluid. To monitor the problem I put
a piece of cardboard under the trans but only a few drops.
At first I thought both the trans and engine were blown because
everytime I started the truck first in the morning I had a BIG white cloud of smoke. I went to the dealer and some tranmission
shops, all they said it was going to cost me about $2k.
So I decided to put on my backyard mechanic hat on.
I noticed on the vacuum tree all the rubber plugs were dry
rotting so I decided to replace them, When I removed the bottom vacuum plug about 2oz fluid came out. I removed all the vacuum lines and found the steel line from the transmission vacuum modulator was filled to the top with transmission fluid.
The modulator was $37 bucks from the dealer + 2 hours
of my labor and no more BIG WHITE SMOKE CLOUD!!!!!

 

Very true. My mechanic found the vacuum modulator's diaphragm ruptured and the fluid was being sucked up to the head and then draining into the crankcase.
All okay now. Thanks everyone for your help.
R.G.McDonald

 

I have to say I never seen any trans fluid in my crank.
How many miles does your truck have?
I have bean asking everyone about this problem but no answers.
It took me about a year, SLOW learning curve.

On another subject if your intested I went stainless steel from the headers back. I have photo's of my upgrades.

James