The Stovebolt Site Forums The Shop Area Driveline *350 Turbo Leaking*

I took my 49 in a few months ago to have the leak fixed at the shiftier shaft. While the transmission shop checked things out they changed the speedometer cable seal and a couple other seals while they were at it. Well....the shiftier shaft seal is leaking again and at the speedometer cable seal. It never leaked at the speedometer cable before so I guess my transmission is worse now than it was when I took it in. I tried to snug up the speedometer cable to the transmission but did no good. I was afraid to tighten it to much. It looks like a long knurled nut. Should that just be snugged up or can I tighten it more?

The speedometer cable housing uses a o-ring to seal to the case,https://cdn.shopify.com/s/files/1/0049/5697/9273/products/Turbo_400_Heavy_Duty_Performance_Parts_1048x700.jpg part 078. Same for the shifter seal

Well there is a thing that happens to auto trannys when they sit alot. I dont know the official term, but ive always called it, "converter drain back". The torque converter drains back into the tranny slowly, but never the less, overwhelming the seals, and hence the leaks. Dont know if this is whats happening or not, but should be confirmed. The tranny builder should have a idea if thats the case. A new torque converter is usually the fix. Why this happens, i dont know? Good luck and keep us posted.

Yup....that is what is happening. I drive or start my truck at least once a week. I'm not sure how long it takes but after it sets for a few hours or maybe over night it starts to leak as all the fluid drains down. Wasn't aware that a torque converter could cause a leak at one of the seals.

There's another source for that leak that's hard to find, but fairly easy to fix. The driveshaft slip yoke that plugs into the transmission tail shaft has a cap on the back of the splined area with a tiny pinhole in it to prevent air pressure buildup as the yoke slides in and out- - - -a classic example of engineering overkill. Once the converter drains and the fluid level gets above the centerline of the output shaft, that vent hole becomes a drain hole. It will drip constantly and the fluid looks like it's coming from the speedometer cable housing. Pull the driveshaft, dismantle the U joint and epoxy or braze the hole shut. It doesn't fix the drain back problem (that's a leaky one-way check valve in the valve body) but it does eliminate a known leak point to the outside.
Jerry

Tightening the speedometer cable nut has no effect on the speedometer adapter seal effectiveness, so I would not crank it down.

Also, at the risk of sounding like Captain Obvious, I would double check the fluid level to ensure the trans is not overfilled.

Its been like 40+ years since I touched a THM350, but I seem to recall two seals on the speedometer gear adapter. An O-ring that seals the adapter to the extension housing, and another one that seals the speedometer driven gear to the adapter -which helps keep ATF out of the speedometer cable.

Call me a skeptic, but it just seems suspicious to me that the speedo adapter did not leak until the seal was replaced by the trans shop. Is it possible that during installation, the shop either nicked the O ring, or a mating surface ?

Question to all the trans gurus out there: I have heard that converter drainback is a fairly common issue on older THMs. However, it is unclear to me if this is mainly just a problem that shows itself when the seals are not 100%? Some reports claim that when various seals were replaced, the leak was corrected. Maybe it just depends what degree of drainback occurs? (i.e. perfect seals can tolerate a lot of drainback, marginal seals some, and worn seals none ? )

Seems like by design, the seals would have to be able to hold back a fair amount of fluid. What if the vehicle were parked on an incline ?

With regards to the selector shaft, could there be a burr, nick, or pitting on the shaft that could be tearing up the new seal? Or could it too have been nicked during installation?


Best Regards...

Drain back should not be a problem on any automatic trans, just for the reason suggested, parking on a hill. If the seals are leaking, they are bad, fluid level should not play any part in it. The fluid is all over the place when the vehicle is in motion. Now if it's running out the top vent tube, you might have a level problem, as the fluid gets aerated from all the moving parts.
You should be able to fill the transmission to the top without leakage. It won't like it and should never be done, but the seals should hold!

That vent hole in the slip yoke isn't an exercise in theoretical semantics- - - - -I've fixed several chronic leakers by plugging it up. It's several inches lower than the case vent near the front of the transmission.
Jerry

Well guys....it's been a while and after a close inspection the leak is at the shifter shaft and the speedometer cable. I have two pans under the truck to catch the fluid. Odd thing is, the fluid lever never has never gone down. These leaks are not excessive but over time there is a good amount. I always get the engine up to operation temperature then check the level. Sometimes it's hard to tell on the dipstick so I always pull the dipstick out and clean it off, on a paper towel I mark the bottom of the dipstick and the high level. When I pull the dipstick out I put it on the paper towel lining up my marks and slide the dipstick which transfers the fluid to the paper towel. The fluid level is always at the high mark. How can this be??? I'm getting ready to retire for the 3rd time at the end of the month so I'm taking my 49 to a different transmission shop and give them a shot at it.

Those two leaky areas are not difficult to fix. There is a seal which rides on the plastic speedometer gear shaft. Make sure the plastic shaft isn't worn. You can find seal kits on ebay with or without the speedometer gear.
TH350s have a lot of fluid in them, and puddles on the floor always look like more fluid than what is actually there.
The torque converter holds a lot of fluid which migrates into the pan over time, giving you an over-full reading on the dipstick.
You need to drive the truck until the transmission reach full operating temperature, and the fluid gets distributed inside the transmission where it belongs before checking the fluid level while the engine is running.

[quote=olezippi]
The fluid level is always at the high mark. How can this be??? [/quote

Sounds like it is overfilled. Engine running, shift to drive & reverse pausing for a while in each gear. place in park. Fluid level should be at lower end of cold mark. Drive rig & get trans up to temp & recheck. If you can hold the dipstick in your hand it the level shouldn't be in the hot range. Hot level is burn your hands hot.

Ok....got it up to temperature, put it in drive and reverse for a few seconds and back in park. Not quite on full this time. The two lids that I catch the fluid in has been cleaned one other time in the past few month. According to the dipstick I may only be a few ounces down. Kinda looks like I've leaked out more than that.