This is a strange one, can't figure it out so I am posting this to hopefully get some ideas. I have a 79 Vette that I have been toying with over the last year, rebuilt the trailing arms, front end, headlights, all new ujoints, rebuild calipers. Finally took it out for a drive to get a paint quote, went a good 100+ miles that day and had some grinding issues with the rear end. Came home, rechanged the fluid in the rear end (already did it once) but used GM fluids this time and did the figure 8's in the parking lot and the noise went away. Thought I was set. It was running a little rough so I adjusted the carb and retimed it, went to take it for a test drive and got about a block away when the car came out of gear (automatic). Tried reverse, low gears, wouldn't move. I turned the car off and restarted it and it went back into gear and got it home. Soon as I got it home it did it again. Checked the fluids and it was about a quart low, dumped a quart in but it did the same thing again. The next day I saw fluid under the car and saw that the cooling lines were leaking so I changed them. Checked fluid again, started it up, went into gear, backed up, went forward and then put it in park for second, Went to shift it back into gear and nothing. Rev the car up, put it in any gear, nothing. Turn the car off, turn it back on and it will go into gear but the same things happens... very baffling.

Now I know everyone will say it low on fluids, but it is not. And when I had the car apart I replaced all the seals in the transmission and changed the filter and has all new fluid. And as I said, I have drove several short trips in it and a good 100 mile trip in without any issues with the transmission, this was just all of a sudden. There is a vacuum line that goes form the carb to the back of the transmission, which seems weird to me and I question that. I attempted to move that vacuum to another place but it did nothing. Is it a pump? Torque converter? Or something simple (I hope) that I am not seeing. I had hoped to get this painted and taking it to a car show I was going on August 15 but I think I am going to miss that now, which sucks.

Once again, sorry for the lengthy post, just wanted to say everything I have done. I have searched but can't find anyone with this particular problem. Hope someone out there is a transmission guy. This is the TH350.

 

The vacuum line from Intake to tranny is for the shift points. That brass looking canister is called a transmission modulator. On occasion they will fill with ATF, which gets sucked up into the intake via the rubber hose / steel line. That ATF will clean your cylinders but also cause rough running.
I wonder if that's what was actually leaking and not the cooling line fittings? Those modulators can be purchased that are adjustable also. This would change the RPM at which the tranny shifts 1-2 and 2-3 either higher or lower.

The fact that the tranny was recently rebuilt tells me something was left out or the valvebody is loose or the front pump is wonky. IDK
Before you think about pulling the tranny, you might want to drop the pan and have a look around for something amiss.

Also, were do the cooling lines go to? Still go to the Rad? Any mods done recently? New cooler?

 

He put seals in the trans. I don't think a rebuild. Front pump is going. Needs trans work. Don't do it. Replace it with some form of overdrive. 200r fits same place the 350 is now. 700r tougher but 3 or 3.5 inches longer. Or hang pedals and start rowing...

 

It wasn't rebuilt. As I said, ran fine and it just happened all of a sudden, not a gradual thing. I backed it out of the garage today and went from neutral to reverse, neutral to drive, did it about 6 times before it quit working. Shut it off and start it back up and it works again. I didn't think the modulator kept it from going into gear, would be great if it was the problem because that is an easy fix. But, it was the cooling lines that were leaking, I was wondering if air could have got into the lines and caused an issue? I have heard of people disconnecting the cooling line and letting the car run to pump out the fluid, wondered if that might pump out any air?

 

WAG
Maybe trans was low on fluid as you First backed it out / Before your 100-mile ride / During your 100-mile ride ? ...
... Maybe you drove w/ low fluid & smoked band / frictions ?

Hopefully, it's Not too serious ... But if it is ... agree w/ OverDrive replacement trans.

 

Any air in the system would be dissipated on the return of fluid to the pan. Its not like a brake line. Its self-burping so to speak.

Your cars tranny runs just long enough to empty the trans pan, then stops. Wait a few minutes for the fluid to refill the pan and its fine again.

H-m-m-m-m-m-. I vote a blockage somewhere. I don't believe its the front pump. But you are running out of fluid & pressure soon. What's the history of the pan filter?
Is there a kink in the return cooling line from the radiator? The kink could be in the rubber hose section near the Rad.

As you know, you can't build pressure if there is no fluid in the pan.

 

I changed the modulator on the transmission, and ran the vacuum hose from the manifold instead of the carb. I took one off one of the transmission cooling lines and turned the engine over to see if was pumping and it dumped a quart or more all over the floor (had it set up to go into a bottle but that didn't work). Put everything back together, put more fluid in it and drove it down the road. It quit about two blocks from my house, just quit moving. Stopped the engine and restarted it and it went back into gear. I drove faster this time and went about a 1/2 mile and turned around to come back and made it about half way when it went out of gear as I was doing around 50 mph. I put it neutral and was able to coast back to the house. There, it would not go into any gear even after shutting it off. After it sat for while it was working again. Very baffling. The blockage idea might be correct, but it is a new filter, new fluid, the cooling lines are a mix of steel and hoses. I was going to buy two replacement steel lines but really didn't want to shell out another $100 on something I didn't think I needed. The mystery continues.

 

are you checking the fluid level on dipstick, WHILE CAR IS Level, HOT AND RUNNING AND IN PARK? or while engine is off?

 

I doubt the steel lines are the issue here. Not much that can go wrong so I see no need to replace.

If the pan is running dry, that would explain the short distances before things begin to happen. But where is all that fluid going? Or, why isn't the fluid returning to the pan, might be a better question. IDK

Torque Convertor issue? Over full? No return of fluid? IDK

 

Ask around & find competent honest trans shop ... might be a low-profile one-man show.

Ask if he can diagnose without pulling trans ? and how much ?

It'll be worth Your time & $ to know from a real pro ... and to pay him accordingly.

Lotsa auto trans choices: pay to have your 350 rebuilt, buy rebuilt replacement 350, buy a junkyard 350, retro-fit a 200r4 or 700r4.

With a reliable diagnosis & quote in hand, you'll be equipped to make best choice for you.