1989 Chevy Silverado 4x4 constantly low on tranny fluid
 

Can not find a leak ,filled and let sit for a week. Had a big sheet of plastic under truck to see where a leak could be. Not even a drop was on it. Checked the whole under neath of the truck after every time I drove it and it is dry too. So not sure where it is going,it looses 4 quarts a week to 10 days. Anybody with ideas?

have you checked out the radiator to see if theres any fluid in there?
that being said i would image 4 qts would displace some other fluids in the rad

It's not under any pressure when not running so the leak may not show up under those conditions.

Check your cooling system for contamination from transmission fluid.

Also, while the truck is running, trace the transmission cooler lines back to the radiator and look for signs of leaks along the way.

I just did and it is fine. One thing I have noticed and is a issue too though,is my oil. I put 5 quarts in and when I drain it out in a month it has around 8 to 9 quarts in it. Which is about what is low in tranny fluid. So I was thinking there was a connection with it. But couldn't figure out how.

yup, it can mix in the radiator if they both cool through it. i would fix this issue asap as oil in the trans would damage it pretty fast. ive heard of people running trans fluid in the engine to quite a noisy lifter but to me thats an old tale that shouldnt be trusted

I did and cooling system looks fine and tranny cooler lines are fine too. And whole underneath of truck is dry . Usually with a leak it usually blows all over the underneath of the truck too in my experience atleast. So it has me puzzled.

It doesn't look like there are,what I see is both tranny lines go into radiator, then there is what looks like a tranny cooler that the lines from the A.C. unit go into. And then there is what appears to be a pretty small tranny cooler on the other side of that with 2 lines running to what I'm going to call a gooseneck that the oil filer screws into.

Is this vehicle new to you or did it just start happening when it was previously fine?

Unless someone had previously plumbed something wrong, I don't see how transmission fluid can be getting into the oil without getting into the coolant.

The radiator has an oil cooler inside the radiator on one side and a transmission cooler inside the radiator on the other side. The two don't interconnect. If one or the other leaks, they will mix with the coolant.

i dont see how someone would plumb up the two things when oil cooler is on one side while the trans cooler is on the other. but thats not saying someone didnt really mess with anything. and i do see your point about the two never being able to mix in the rad but thats the only thing i could think of that would happen by "accident" but then im not sure how all three even plumb up in the rad. that all being said i know some trans have a vacuum modulator and that can suck up trans fluid but his being an 89 it should have a 4l60/700r4 and those dont have one

It is new to me,it appeared fine when I bought it. But a couple days later noticed this stuff going on. Like you said I don't know how the 2 would mix. Tranny lines are going into radiator. And 2 lines coming off like a goose neck that comes out of motor and the oil filter screws into. They run to a smaller separate cooler in front of the radiator.