I checked the fluid level in my TH350 transmission, in park, hot with the car running; on the low side. I shut it off, check the dipstick 5 minutes later and it's halfway up the fill tube. The good news is even when it drains back it isn't leaking out the dipstick, shifter, or speedometer ports. But I still imagine this can't be very good for the transmission long term. What's the correct fix? Is it as simple as replacing the input/converter seal? (Well, as simple as dropping the whole transmission again can be.)

 

Disregard any reading with the engine off as it will always read high on the dipstick. Trans should be checked/filled warmed and with engine running in park or neutral only.

 

Yet another reason to READ the Chassis Service Manual...

 

I am fully aware of that, and if you'll notice that's how I checked it and added it. But I've never encountered one that has drained back that much so quickly. Usually that is indicative of a problem, no?

 

This is a constructive comment

 

About as constructive as worrying about the trans fluid level when you are checking it in a way that you're not supposed to...

To recap:

Trans shifts fine (apparently, as we haven't heard about problems).

Fluid is slightly low when checked using the correct method.

What's the problem? Of course the fluid drains back when shut off - the converter is filled when the engine is running and drains out when it is stopped. This is why you're supposed to check it with the engine running.

 

You should consider yourself lucky it's not making a puddle on the floor. I have one that's leaking now.

 

Shifting is a little funny on the 2-3 shift; it likes to hang, but it may not be related.

What I am concerned about is you have to wait awhile for the converter to refill once you start it, otherwise it hesitates. In addition to that if you check it right after starting there's some bubbles/signs of aeration, which is not good for the transmission. Some degree of drainback is to be expected, but this is way above normal.

That seems to be the way people notice drainback issues. I haven't seen much on how to fix it with TH350s; other transmissions tend to have some sort of check valve that has to be replaced.

 

Well I took it out today, with the fluid topped off hot in park, running. It drove ok there, but on the way back wouldn't kick into 3rd. When I got back and popped the hood, the engine bay was sprayed with transmission fluid from the dipstick tube. Fluid isn't burnt. Not 100% sure on the root cause here, but between not shifting, drainback and puking fluid, I think bottom line here is the transmission is borked.

 

Verdict: Plugged transmission vent. It was plugged solid with mud. Not sure if due to daubers or something else; the perils of used equipment. I removed the crossmember, and tilted the engine back to get at it. At the same time I noticed one of the hoses to the modulator came loose. I clamped that back on. Between the two, it now shifts fine and no longer pushes fluid up the tube.