Can anyone help a guy solve a TH350 operational mystery?

[Tim Gordon]

My rig is a 1973 C20 heavy duty, with nine-leaf springs, a 14 bolt axle that probably houses a 4.56 (never actually checked!), 8.75x16.5's, and used to have a forged crank, 4 bolt main 350 which worked great when pulled after the iron around the starter bolt bores cracked and broke away. I swapped in another 350/350 from a 1976 Caprice, but kept the Quadrajet and intake. I did put the Caprice's HEI distributor in, though. Current mill is at about 176k miles with a new cam and lifters about 23,000 miles back, compression ranging from 100 to 125 psi and valves that act somewhat burned or eroded just like a 176,000 mile motor that's never had a valve job! A bit of noise coming from near cylinder #8 lately, but starts and runs very well.

Recently, the transmission would not engage in any forward gear. Both seals leaked badly, a result of the truck mostly sitting and not being driven very much due to high fuel prices and very low fuel mileage (8-10 typical, original motor was a consistent 13 mpg). Figuring that too many trips out with fluid that was too low for good health but just enough to make the truck go had taken their toll, I swapped the original TH350 back in. It's a 150,000 mile unit, but had been sitting since 1997 or so. Fluid looked good, little to no moisture observed. All seals, o-rings, and fluid was replaced, and a new filter and gasket were installed. Adjustable (orange stripe) modulator held applied vacuum well, and was just treated to new rubber hose. The '76 converter was used, as it hadn't been sitting for 15 years and any sediment that may have been held in suspension in the fluid wouldn't have settled out and 'packed in' anywhere.

After about 75 miles of use after re-installation, I pulled onto a nearby highway and accelerated up to 55, but the upshifting just didn't seem to be happening like it should. I never felt 1-2, but 2-3 happened maybe at 50 mph or a bit faster, and was very noticeable. RPM's were high, but they always are with this truck at highway speeds! A mile later, I slowed down and turned off onto gravel, and pulled over to the shoulder to let a 'terragater' pass. Suddenly, the truck acted as if the transmission had just lost a bunch of fluid and the converter just wasn't doing its job anymore. No leakage around transmission has been observed. Fluid is very clean. About ten quarts went in and reads over full on the dipstick; I'd drained the converter of old dark brown fluid as thoroughly as I could and put at least one quart of the new stuff in before mounting it and installing the trans. Gee, I thought the pump would push much of what I'd poured down the dipstick tube into the converter, but I do wonder just how much ever got there! No signs of foaming or aeration, though.

I made it about 150 yards from the first sign of trouble until I stopped and checked the fluid level. About that time, my neighbor happened by on his tractor and we stopped to visit a bit. Truck didn't seem to want to go, so he went to grab his truck and a tow chain. I started the truck after a few minutes to give it a try and see if anything different would happen. It went into reverse just fine, then forward. I got about 200 yards, then it happened again. Shut down, wait a few minutes, start up, goes into reverse fine, goes into forward, well, 'okay', but never shifts out of first, go a couple of hundred yards, then pretty much that's it. Neighbor arrives with truck and chain, tows me about 500 yards back home. I add a quart and a half just to see what happens, start it up, reverse is fine, seems better on a very short road trip (200 yards), but then acts balky in the driveway.

I REALLY need my old truck for hauling so I can finish building a house in town I'm working on. Summer is slipping away fast, I need to be sawing and hammering, not turning wrenches again! I'd love to put a 700R4 in it, but the speedo, throttle valve cable, and converter lockup adaptions are more than I've got time for at this late date. I'm about ready to just buy a 5x8 trailer I can pull behind any of my other vehicles and let my old Chevy wait until next year...but does anyone have any ideas on this or any experience with this kind of strange behavior? It was great for four days and 75 miles, then this! Any input would be greatly appreciated as I've never personally seen anything like this before.

 

[HotRodPC]

Wow, and they think my posts are long??? OK let me go take a piss, refill my Pepsi, and I'll be back in a few.

BTW, Welcome to GMSB !!!! BRB.

 

[HotRodPC]

OK, reading all that and trying to make sense of it all. I think you've got a combination of problems here. Sticking valves in the valve body, and dried up hardened seals in the clutch drums. This is all due to the long time of sitting and age of the transmission. The clutches are probably not in to good of shape either. They can become crusty and brittle. SO, with the mileage and age on these transmissions they will need rebuilt. I'd keep the original truck transmission if it were me, OR install a Th400 which is what GM went to in all the 3/4 tons in like 77 or 78 all the way up til 91 then the 4L80-E came about.

So give up on even trying to repair them. They need rebuilt. You do have a chance of at least making it thru the summer though. It's pretty much a 1 shot deal, and it'll either work or it won't. Find a good Trans Conditioner additive. I'm not one for additives very much at all but for a transmission that has sat for a very long time a good conditioner like maybe Lucas and other big brands, just read the bottles of what they do, you might get those dried up hardened seals to soften up and swell a little bit and it be enough to make the trans work again. They can also clean up some of he gumming up of the valves in the valve body and free sticking valves. Fluid level is important, a little over won't hurt, but to much over it can hurt. Also make sure don't have a clog in the trans cooler lines or radiator trans cooler tank.

More is also not better, it will either work or it won't, so try 1 bottle, if it makes a great improvement but doesn't quite make it perfect, you might be able to try 1 more bottle but past that, no need to add more or you're just wasting your money. If you can get those seals conditioned and the valves moving freely again, it might just last you thru the rest of the summer and part way into fall, but IF it does work, it's just a temporary fix, but the transmission does need a rebuild.

Now if your problems seem related in both transmissions, as in the donor trans out of the Caprice and the truck trans back in it, since you kept the same donor converter, you might have a converter problem and no additive is going to fix that. Unless it's a lock up converter with a clutch in it, sitting around and age really don't hurt converters much. That I know of, there's no rubber seals in them to harden or go bad.

I'd avoid the 700r4 in a C20 if it was you. I might try it myself, but I also build transmissions, so if I break it, it won't cost me $1000 - $1500 for another one.

 

[chengny]

Not sure if the OP is saying

1. That the transmission has no forward drive at all

2. That it is not consistently upshifting out of 1st.

If it is able to move the truck but only stays in 1st (or shifts late/erratically, skips second and goes straight to 3rd and bangs when shifting) look at the governor.

It is very possible that something caused a valve to stick, but maybe the govenor is malfuctioning.

The govenor is an easy check. Pop off the bulbus metal cover on the right rear side of the transmission and slip the govenor out.

Check to see that the gear is not chewed up and that the spool valve inside moves freely. Otherwise, it will be necessary to pull the valve body and inspect the valves for free movement.

The modulator is a possibility, but it should eventually shift although late and hard. Check the rubber connectors at both the manifold and the modulator nipple. Look for cracks/splits.

Long shots for sure - but worth a try.

 

[Tim Gordon]

Thanks!

Thank you, gentlemen.

The situation is very puzzling, which is why I took care to be as specific as possible and leave out no details.

To clarify, the very leaky '76 Caprice trans that was in the truck for the last 15 years 'up and died' one day when it would engage no forward gears, just reverse. If enough fluid had leaked out that it wasn't going to go very well, usually it was reverse that wouldn't work on a cold start (probably would after the truck warmed up) but Drive would engage. I added a quart or two, but pretty much knew I'd be swapping a transmission.

I had the original transmission on hand, which worked just fine when I removed it with the original engine back in the late 90's. It wasn't my first choice since it'd been sitting, but we got so much rain up here this year the salvage yard was a total quagmire until late July. By that time, I'd figured I'd just run the unit I had in storage. The fluid looked good, so I figured it was worth a shot. I replaced fluid and filter, front and rear seal, and all o-rings.

It worked very well for four days and 75 miles. What seemed to have 'done it' was punching it a bit hard and 'hauling' for a mile. After that, the transmission would act like it lost a quart or more of fluid. However, if I shut the truck off and wait two or three minutes, I could start it up and it would act just fine. I could get it to go a few hundred yards, then this would repeat. Letting it sit a few minutes would remedy it again. THIS is what totally puzzles me.

The fluid is very high on the dipstick, but shows no signs of foaming. What I'm thinking about doing next is using the drain plug I installed in the pan to empty the clean looking fluid, examine it closely, then pour five quarts of it back in, provided it still looks good, filling as if I'd just replaced a filter.

I would imagine that as much fluid that's going to end up in the converter has long since gotten there!

I'll keep you guys posted on what I find. I wasn't able to find anyone else out there who complained of something like this happening to them, so the situation appears to be somewhat unique.

 

[HotRodPC]

So what about when you let it cool completely down like for an hour or 2. How does it do then? It really could be bad internal seals. Not sure how long this sitting for a few minutes is, but it could be sitting long enough to cool the fluid a bit and it's a bit thicker. Thicker fluid is able to help compensate for the hardened seals. Fluid gets warm or hot and starts to thin, then it can easily get past the hardened seals and not compress the clutch drums. A good seal conditioner may soften those seals and make them swell enough to bring the trans back to life for awhile.

 

[Tim Gordon]

That's a good question!

Just an hour or so yesterday afternoon and then late morning and early afternoon today are all the time I've had to evaluate this. My 'cool down' periods hardly have been, like on the order of two or three minutes. Certainly nothing for an hour or more!

I pulled the plug, drained the fluid, and examined it very carefully. It is a consistent burgundy color and smells just like new Dexron.

While the fluid was out (not in the catch pan, but sitting in a 2 gallon jug that'd I'd recently emptied, with a lid on it), I pulled governors and compared. They are definitely calibrated differently! The difference in the weights and springs is easy to spot. The Nylon gears were mostly OK, but the Caprice governor had a bit of roughness on the leading edges of a few of its teeth, the truck unit looked great. Both miked out at .7999 on the shaft segment closest to the weights when I checked with my Starrett 231. The bore diameter was a lot tighter in the truck, though. It was like .806 and something, the Caprice case had a bore that was enough larger to raise an eyebrow. I don't remember, but it's not that important as that unit isn't being used.

With the governor reinstalled, I added five pints. Verifying with the dipstick, it read quite high. I then started the truck and warmed it enough that it'd kick out of high idle. I have very limited reverse right now, just like I'd have if I was a quart or more low. Forward works, but tends to be very jerky. The pump is whining, so I don't know if it's just sucking air or if it's on its way out. Touching my stethoscope probe to the case near the pump didn't render encouraging results, there is a definite grating, grinding sound that I don't like to hear in rotating parts going on there!

I don't have a pressure gauge to plug into the ports. That's always been a 'someday' item, and someday hasn't come yet. I suspect that if I pull off a cooler line I'll find that the output isn't very good. I strongly suspect that the pump is on its way out.

You'd mentioned not putting a 700R4 in a C20. If I did a lot of heavy towing with it, I'd be with you on that, to at least a degree. The one I'd likely install is from an '88 GMC Sierra. I can have it for $200, but no one knows if it's a good unit or whether it's the reason the vehicle was junked. I may just buy the whole vehicle for the numerous small parts I can swap over. This old truck needs just about everything! Half the time I'm empty (on the way to pick up a load), and when I'm loaded, it's usually not more than 1000 pounds or so.

More than likely, my old truck, which has served me very well all these years, is going to sit this year out and I'll have to wrench on it next year. I should have time to tinker and do things I wouldn't even think about messing with right now.