TH350 intermittent leak

[WFarm]

I have an intermittent leak from my transmission that puzzles me. It’s a TH350 behind a 454 and driving a NP203 transfer case in my 75 Blazer. Transfer case is stock therefore full-time 4WD. Everything drives fine with no issues. The only thing I’ve done to the transmission was replace the filter and what fluid I lost in doing that. That was done last summer. I installed a Lube Locker pan gasket and there is no evidence of any leakage at the pan.

Months will go by and not a drop will be on the floor where it’s parked, then one day there will be a small puddle (couple of tablespoons?) under the transmission. It’s coming from where the kick down cable attaches. I figure a new seal may be needed but what puzzles me is why it only leaks occasionally.

The last leak occurred after I returned from the Cruise In last Saturday. Temps in the 80s, drive time maybe 20 minutes each way. Have driven quite a bit longer distances and not had leakage so not sure what’s up.

 

[Sad Sack]

Common -
Dipstick, shift seal, modulator o-ring, TV cable seal and possibly the speedometer drive gear. But if it were me I'd try a leak detector die.

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Remember reading something about convertor drain back situation but still researching that topic.

 

[WFarm]

The source of the leak is clear, but not sure why it only leaks occasionally or if this is one of those things you just have to live with for this transmission.

 

[Sad Sack]

Surfin in my spare time (throwing darts) and found this article:

converter drainback

Torque converter drain back occurs with many types of transmissions, but is especially prevalent with Chrysler trannys.

As soon as you start your vehicle the trans pump will begin to fill, or "charge" the torque converter with fluid. But because of the design of the manual valve, [in the valve body], on most Chrysler transmissions, converter charge was inadequate.

In a perfect world, you would park your vehicle, turn off the engine, the converter will be fully charged, get up the next morning, or 5 days later, start your vehicle, then drive right off.

But in the real world, if your pump bushing is worn, or you have wear in pump stator bushings, converter splines, valve body issues, or a host of other issues, the fluid will drain back into the sump [pan].

Then when hop into your vehicle, start it up and expect to drive right off, you may not take off so well.

Let the vehicle run for a couple of minutes, the converter will be "charged" with fluid, and the cycle starts all over again.

If you had this condition, you could qualify this concern by removing the trans dip stick, before the vehicle is started [after sitting overnight], then have someone start the vehicle for you and immediately check the fluid level.

If you have converter drain back, the fluid will register very high, or overfull, on the stick. Until the vehicle runs for a while, then it will register correctly.

 

[Edelbrock]

I have seen transmissions push fluid out of the filler tube. It might be worth cleaning the outside of the filer tube, and then rubber band a small amount of paper towel around it. Then check it the next time you have fluid on the ground.

 

[WFarm]

Thanks, but I know exactly where it’s leaking from as I described in my original post. What I don’t know is why it happens only occasionally.

 

[Edelbrock]

You seals are probably dried out and hard. When the temperature changes the seal cant form to the parts that way it used to and a leak happens. Once the parts cool down, the tiny gaps close and stop the leak. This is not the standard "stop leak". This is a specialty product. It does not swell seals or cause any problems. Its simply replenishing the missing chemicals that have evaporated out of the rubber, making the rubber soft and supple like it was when it was new. Works great.

 

[bucket]

I'm curious if it has something to do with internal case pressure. Do you have a locking type dipstick, or just the regular old one with a wobbly steel cap?

With that said however, I'd just go ahead and replace the seal on the cable.

 

[bucket]

You seals are probably dried out and hard. When the temperature changes the seal cant form to the parts that way it used to and a leak happens. Once the parts cool down, the tiny gaps close and stop the leak. This is not the standard "stop leak". This is a specialty product. It does not swell seals or cause any problems. Its simply replenishing the missing chemicals that have evaporated out of the rubber, making the rubber soft and supple like it was when it was new. Works great.

What stores sell that stuff? I've never used it, but I'll give it a whirl.

 

[WFarm]

I have the locking type dipstick.

 

[edgephoto]

If your vent is restricted it might cause odd leaks.

 

[bucket]

I have the locking type dipstick.

I'm wondering if you drive around with the dipstick not fully installed into the tube (just loose, basically) if it will keep the kick down cable from leaking.

It would just be a test out of curiosity. The seal itself would still be the culprit, of course.

 

[Edelbrock]

What stores sell that stuff? I've never used it, but I'll give it a whirl.

Many auto parts stores do, but the most convenient seller is Amazon. Maybe $18 per bottle.

During the pandemic, the production of this product stopped due to supply chain issues.

I saw listings for $200 a bottle, and they were getting it. Its good stuff.