Seems both of my TH400s are leaking from the tailshaft area. Up until now neither has had a problem and one was recently rebuilt. Going into the lovely PNW rainy season they were fine. This spring I find both have sprung leaks. I have refilled them but they continue to leak. No issues with either trans until now. Has anyone had TH400 leakers out of the blue before and what should I look at to remedy the situation? Neither were abused prior to the drip, drip, drip. Any and all thought are appreciated.
I like to install the split in the babbit tail housing bushing to the top side even though there's not much fluid that lives back there when parked level. I've battled these tail housing leaks on a TH400 a couple of years ago, & used a thin coat of Indian head shellac ptfe Teflon thread sealer on the bolt threads, a quality National seal & let it all sit a few days before adding fluid which usually isn't necessary if working with it level or nose down. Good luck sir. Flux
Had the same problem on my ot car, there is an o-ring on the output shaft. Most transmission shops should be able to fix you up with one. Pull your driveshaft, then either carefully slip it over output shaft, push in with thin blunt object until you feel it slip into its groove. Or you can pull tailshaft housing so you can put it on and see what you are doing. Lube output shaft with some bearing grease, protects oring from the sharp splines.
By the way, no abuse needed according to my transmission guy., they are an o-ring and sometimes get cut and fail. I did mine without removing tailshaft almost 3 years ago and not a drop since, had quite the mess before and was the same as you, couldn't figure why just out of the blue I had trans fluid all over the floor. Good luck, hope this helps.
there are several things that could be leaking. The speedometer gear is a prime suspect. The yoke/O ring thing is another possibility, there are different yokes available, some require the O ring, some don't.
Sometimes they leak from the ujoint side of the yoke. Yokes had a hole in them. Don't know why . but weld are sealer fixed the leak.
More than likely it's the weep hole in the back of the yoke. But also worth checking is where the rear seal rides on the yoke, the yoke should be smooth with no grooves worn into it (this goes for the rear bushing area too). I would also make sure the tail housing is not cracked or distorted (out of round) where the rear seal is located as this'll allow fluid around the seal. Also check tail-housing bolts , the ones on the governor cover and the pan.
Make sure you have a new bushing in the tail shaft housing. If not and the yoke has too much play in it, it will take out the rear seal in short order.
Years ago I had an OT p'up with T-400. If it sat unused for too long it leaked, even after replacing every seal and O-ring, etc. that went to the exterior of trans. Turns out that a T-400 can develop an internal leak from the torque converter to the trans case that allows fluid to leak from torque converter into trans. main body, which then is overfull and fluid manages to seep out all over the place .from all sorts of places. Never leaked if driven regularly.
clean the transmission and pour in a bottle of oil dye and use a black light bulb or ultraviolet light to properly diagnose the leak. it is an industry standard used to find leaks without guessing for engines, cooling systems, rearends, transmissions, power steering, etc. your leak might be traveling all the way from the torque converter seal. we once fixed a transmission leak by unplugging the transmission vent. the pressure would build up and blow out the seals. differentials do it all the time out the pinion seal when the vent is clogged. just my two cents.
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