Leak Driving me absolutely crazy

[Tdrcreyn4]

I've got a leak coming somewhere from my TH400 that I can't seem to find and stop. It will sit in the garage with no leaks. Take it out and run it and the next thing you know I've got tranny fluid all over the bottom of the truck. It does not appear to be leaking at the front or rear seals, pan, speedo gear, etc. It's driving me crazy. Any suggestions???

[Grumpy old man]

Clean the area as best you can and then dust it with baby powder and run it , the baby powder will reveal even a slow seaping leak and easy washes off . Also works great on valve cover and timing chain / distributor area leaks that are really hard to pinpoint .

[Jrainman]

fill tube grommet or O ring , early 400 have O-Ring later 400s have a grommet more like the 700 r 4 have.

[n33k0]

I don't have a TH400, but maybe it isn't too different from the TH350s. I had a hard time tracking down a leak that would soak the underside of my truck with fluid while driving, turned out to be the TCS switch that was leaking (only when under pressure while driving). I pulled the switch out and put a brass plug in the hole and the leak stopped. Like I said, maybe this issue doesn't affect TH400s, but it doesn't hurt to check.

http://67-72chevytrucks.com/vboard/s...d.php?t=676288

[Mike C]

Common leak spots are from dipstick o-ring or from bent in end of yoke. These things can be frustrating to find. May want to run a wire in the vent if you can get to it to make sure it's functioning.

[kbill45@gmail.com]

Originally Posted by Tdrcreyn4

I've got a leak coming somewhere from my TH400 that I can't seem to find and stop. It will sit in the garage with no leaks. Take it out and run it and the next thing you know I've got tranny fluid all over the bottom of the truck. It does not appear to be leaking at the front or rear seals, pan, speedo gear, etc. It's driving me crazy. Any suggestions???

Same issue here with 69 sbc and t400

Replaced pan and gasket
Replaced front and rear seal
New o ring on dipstick tube
New o ring on modulator

Cleaned transmission thoroughly - ran engine for 10 minutes - rolled up paper towels and tucked under and behind the torque converter and discovered this was where the leak was coming from.

Will now remove the trans again and check front. At least I know where the leak is coming from.

[Wrenchbender Ret]

Ive had a lot of them leak around the shifting shaft seal. It looks like it is coming from the pan gasket.

[davepl]

Shift shaft seal.
Shift shaft seal.
Shift shaft seal.

Says it three times fast because it's fun. But then do check it, like Wrenchbender said, that's where I had a leak that took a long time to find.

[Tdrcreyn4]

Thanks for all the help......gives me something to sort out this weekend!! Appreciate you all

[regan wilson]

You might want to look at the vent tube on the upper pass side of the transmission. I run a overflow on mine because 400's seam to find there own happy level. Once mine spits everything out that it doesnt want its good to go.

[mMad Dawg]

I had the same thing happen with a 69 Chev. I finally fixed it by changing the speedometer driven gear(the plastic one) and put a new o-ring in the housing.The gear was worn from contact with the o-ring.It was hard to notice but it was worn just enough to leak. You might want to make sure that the vent isn't plugged.

[Freddo]

I feel your pain. I have chased leaks on my turbo 400 for quite a while & still haven't completely resolved them. I've read here on the forum that the torque converters sometimes drain if they are parked more than a day & the fluid level rises inside the transmission & the fluid will find its way out of any spot prone to leakage even if you have no leaks while it's running. I've never got a solid answer on why they drain back or how to fix it other than a 'rebuild'. My front seals & rear seals appear to be ok with no leakage. I replaced speedo gear & gasket, (it leaked), replaced shifting shaft seals . (Actually had it done), blew out the vent, replaced modulator o-ring, & fill tube o-ring. Replaced pan gasket twice & had it done professionally the third time. It still leaks a little, but only when parked a couple of days. Hence the drain back from the torque converter.(?) many suggestions on hammering the bolt holes out a little because they get pushed in from over tightening. We tried hammering them back & used a straight edge to ensure a flat surface for the gasket, but there is still a little leakage through the pan after parking for a while. If you find its still leaking through the pan, maybe it's a little warped? That's going to be my next attempt to stop the leaks, I'm going to get a new pan that is the best quality I can find for a reasonable price. Good luck