th350 question

[stevef]

Hi. I tried my hand at rebuilding my th350, but failed. First and reverse works, but it doesn't shift out of first. I tried the easy things that can be done with the trans in the car, but no success. So, I pulled the transmission back out of the car, and it's on the fixture. I noticed in Cliff's th350 book, page 103, that there's an air pressure test for the clutch packs shown, where you put air to 3 holes in bottom of the case. I have removed the valve body to expose the bottom of the case and the 3 holes shown in Cliff's photo on page 103. I'd like to air check those, but the holes are in an awkward location to get a  good seal with the air nozzle to get good pressure. I have an air gun similar to the one shown in the book, with a fairly small diameter angled tube and a tapered nozzle on the end if it. Any tips on how to get good air pressure in those holes, and how much air pressure should I use? The shift lever position doesn't matter does it?
I will be taking it apart and can air check the drums when they're out of the case, but I thought it'd be interesting to see what they're doing now, still being inside the case, if I can do that air check.
Thanks for any advice!

[77cruiser]

Can the intermediate sprag be put in the wrong way? I've only done 1 350 but it worked fine.

[Cliff Ruggles]

If it has no up shifts that is typically caused by no vacuum to the modulator or a problem with the governor.  A stuck modulator valve would be something to look at, but that is not a common problem with those unless it got bent somehow.

It's not easy to pressure test a TH350 thru the case but can be done with a little patience. I use small shop rag to help seal off the air in the passages around the end of the air nozzle being used. 

The selector position will have no effect.

Did you have the valve body apart for any reason?

I'd also verify check ball locations and separator plate gaskets in the right place and didn't get swapped out or shifted, etc........Cliff

[stevef]

Hi Cliff. Thanks for the tip on using a shop rag to help seal around the holes. What air pressure is good to use?

I did take the valve body apart to clean it. I was careful to lay the parts out in order and put them back in order. But, I'll double-check it.
 
I have good vacuum, and the modulator valve and governor work good. I was very careful with installing the balls and gaskets, so I don't think that's it.

I'm thinking I have a leaking piston seal. That's why I'm interested in the air pressure test. So I'm going to re-do those, and double-check everything else too.

Thanks for the quick reply!

[Mudsport96]

Was it just no upshift at all or was it acting like it shifted into neutral?
I would think if it was a piston seal, the trans would try to shift and leave 1st and not apply second.
If the governor was sticking it would never leave 1st or have to rev very high to build enough pressure to force a shift

[stevef]

It was just like driving around in 1st gear. Now that you mention it, it might have gone to neutral a couple of times, when driving around the block, but I was able to get back in gear to get home. It's been a couple months since I drove it, so it's hard to remember exactly how it was acting. I just know that it never shifted up.

I don't think it's the governor, since I checked the valve inside it, and it's moving freely, not sticking.

Thanks for replying.

[stevef]

Okay, I found a pretty good way to air test those 3 holes. I took a piece of small diameter rubber fuel hose and cut a small notch in one end. I slid the other end of the hose over the end of the air nozzle tube. I inserted the notched end of the hose into each cavity aiming the notch at the hole in the cavity. When holding the hose tight into the cavity, it seals up pretty good and blows air into the hole. So, I did that for the 3 holes, using 25-30 lbs air pressure.

Referring to the pictures on page 103, from left to right: 
Applying air pressure to the hole in the left picture, I can see the intermediate clutch moving as the book says. It seems to work strong without significant air leaking.
Applying air to the middle hole, I hear the clutch applying, but there is the sound of air leaking. Is that the forward clutch pack?
And putting air to the right side hole, I hear the clutch applying. It's a duller thud when applying than the 2nd one. It has air leaking too, but not as loud as the 2nd one. Is that the direct clutch?

So, what would you conclude from this test?

[stevef]

I'll answer my own question. If I'm not supposed to hear much air leakage, I think there's too much, even though the clutches do engage. I've got a new seal kit, so I'll have to see if I can get those clutch packs as air tight as possible this time. So, this was a helpful test that seems to be confirming my suspicion of what the problem is, before taking everything out.

[Cliff Ruggles]

Leakage should be very minimal during an air pressure test.  I've never tried low pressure air, we use 125-140psi when testing them here. 

The metal hooked end sealing rings seal pretty tight but you can here very minor leaking with each test. 

We air test every unit we build here during the rebuilding process.  A very easy way to test the fwd and direct are to invert the pump is a large shop vise with soft jaws and air test thru the supply holes in the pump.  This is described in my TH350 book, Chapter 5 page 90......Cliff

[stevef]

Thanks again for responding Cliff.

The reason I used the lower air pressure was I figured if air leakage was the problem, then the clutches wouldn't apply with lower air pressure and I'd know that's the problem. But, I hear leaking, and even with low pressure they're applying anyway, which I didn't expect if the leaks were too bad. 

I thought if I use high pressure, even if it leaks badly, the high pressure would overcome the leakage and make the clutches apply anyway, which would defeat the purpose of the test. But if you do it that way, I guess that's not the case.

During the rebuilding, I did do the forward and direct air test on the bench. I got a little leaking at that time, but the clutches applied so I thought they were okay. I was using higher air pressure then, and afterwards, when the transmission wouldn't shift, I was thinking maybe I used too much air pressure, which was overcoming the leaking and making the clutches apply in spite of the leaking, making it a false test.

Now, I have a better idea of how much air leaking is normal. I suppose even if the clutches apply with a bad air leak during a test, the apply could be too weak to work when the transmission is in actual operation.

So, I'll re-seal them and test them again. And I'll double-check the other things that you listed that could go wrong too. Thanks a lot for the help!

[Cliff Ruggles]

In all the years I've been doing transmissions I had one TH350 that didn't work after putting it together.  Ended up being a snap ring that I didn't get seated well getting kicked off the drum and the spring cage/apply piston followed it.

I've seen a few TH350's come in here not working due to clutches not applying even though they were assembled correctly. The culprit was a failed bushing (prior to the rebuild) on the direct drum allowing it to contact the support and wear the sealing ring grooves so hard that they held the rings from expanding.  The inside of the drum was also worn and grooved pretty bad in each case and the builder didn't catch it.

I've had almost perfect success with TH400's, the one unit that didn't work pushed the snap ring off the PR valve, and was a very easy fix when the pan was dropped for inspection.......Cliff

[stevef]

Interesting to hear about the things that go wrong, so those are more things to look for. I installed all new bushings, so don't think I have a problem there. I have a couple of parts transmissions to choose the best looking parts from, so I didn't use any parts that showed wear, not that there were many that showed wear.

One other thing that I remember was that I bought a new sprag, and while getting ready to install it, a couple of rollers fell out. But I put them back in the sprag and it fit tight and seemed to function fine.

I have a bunch of 70's project cars, and most use th350's, so I'd like to get good at rebuilding them.