Th350 shifting up and down repeatedly on hills.

GBodyVillain

Not-quite-so-new-guy
Jan 4, 2021
28
16
3
I've got a th350 sitting behind a 4.3L v6 (how I received the car) and the th350 only has 10k miles on it after getting it rebuilt. Whenever I start driving up a hill with the pedal on just when you feel the resistance on the kickdown cable, the trans just starts shifting down then back up over and over again until I let off the throttle or push the pedal to fully engage the kick down. I've disconnected the kickdown cable and if I hold the throttle in just the right place about where I had it before I disconnected the cable, It still does it. Still I went after adjusting my kickdown cable just to make sure. I'm at a bit of a loss as to what's going on. I've got an adjustable modulator and wasn't sure if that would really affect anything and I played ith adjusting it a bit but couldn't get the trans to stop shifting up and down. Any of you have any ideas what I might be dealing with?
 

stew86MCSS396

Greasemonkey
Aug 1, 2022
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(808)
My '85 Caballero came with a TH-200C, just curious if you had substantiating evidence that yours is a TH-350???
94111-e0516f7d734aa150811ce9e7bfab29ee.jpg
 

GBodyVillain

Not-quite-so-new-guy
Jan 4, 2021
28
16
3
Yeah, it's a turbo 350 with lockup. Pan shape matches as do some other identifying factors. Had it out of the car twice and had people task me why a 350 was in the car, as well as did the shop that rebuilt it for me.
 

ELCAM

Royal Smart Person
Jun 19, 2021
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What is the vacuum modulator connected to under the hood?
Manifold or a carburetor port?
 
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GBodyVillain

Not-quite-so-new-guy
Jan 4, 2021
28
16
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It's hooked to the manifold port, but there is a tee going to the vacuum canister on the driver side front corner of the engine bay, then the other side of the tee goes to another tee that splits into the vacuum modulator and a purple vacuum line going into the cabin. The hose goes all the way to the modulator, there is no steel line although I'd like to make one up. Just gotta figure out how to go about it.
 

69hurstolds

Geezer
Supporting Member
Jan 2, 2006
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You have a TH350C. Which is technically the same as a TH350 but with a lockup converter. Basically the front pump and input shaft lengths are different to the best of my knowledge. I have heard, but cannot verify, that the TH350C may have plastic washers inside and some very minor differences in the clutch pack, but not sure what those are. Essentially, they're nearly identical on the inside.

Not enough information on your car to determine whether it's factory or not and what has or has not been done to it.

What makes it lockup if it didn't come in the car originally? I don't know. Never had one. It's obviously got an MPH point that the converter locks up. Is the converter kicking in and out or is it something else?

Perhaps the line pressures aren't correct somehow in your transmission due to leaks in the valve body/gaskets or somewhere. Sometimes steel check balls beat the living crap out of their seats to where they don't seal exactly right and you lose pressures there. So while it might do fine putting around, when you put it under load it doesn't act right. Could be a governor issue as well. Pull the vacuum line off your modulator, draw a vacuum on it using a mighty vac or something you can check with a gage, and see if it holds vacuum. If it shifts well otherwise, it may not be a problem. Try the easier, non-invasive stuff first.

It's extremely difficult to diagnose via a web posting.
 
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GBodyVillain

Not-quite-so-new-guy
Jan 4, 2021
28
16
3
I think stock the car is suppose to come with a 200 or 400, can't quite remember but I know the 350 isnt stock. The car has a vss and I'm assuming the computer doesn't care what transmission it is if, if you have the solenoid wired up it just tells it to lockup. The lockup does work properly and kicks in at 45mph and I can hook a light up to it to see when its coming on and it isn't acting up on a hill. I'll try checking out the modulator and governor. I know it's hard to diagnose stuf fon the internet and it's probably headed for the shop but I appreciate the info on stuff I can check before it goes there.
 

ELCAM

Royal Smart Person
Jun 19, 2021
1,085
1,200
113
Hook up a vacuum gauge and run the hose out the back of the hood and tape the gauge to the windshield so you can see it when driving the car. Try the hill climb and see if events in the transmission match vacuum events.
 
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78Delta88

Royal Smart Person
Supporting Member
May 23, 2022
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SW Arizona
Older post, so just curious if you got it figured out.

From reading, first thing I would check is fluid level. Also if this was a swap, you might have wrong dip stick or wrong markings on it.

The TH350 comes with two typical filter styles. The mesh type feeds from the top and the felt style feeds from the bottom.

The act of climbing the hill will move all the fluid to the back of the trans and not enough in the pan where it needs to be picked up for the pump. If the trans is low on fluid the filter will come uncovered and cavitate meaning ... (sucking air).

Low fluid causes all kinds of issues.

Beyond that, it could be an issue with valve body or clutch pack or band wearing out.
 
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GBodyVillain

Not-quite-so-new-guy
Jan 4, 2021
28
16
3
I haven't really had the time to get into it. I did drop the pan because it was leaking and it is holding the correct amount of fluid, I've replaced the dipstick and measured the replacement to make sure it is correct for the transmission as I've gotten an incorrect one before but for another trans. So it reads correctly and as for the filter I'm running the felt style filter. I bought a mesh type as its what I used to run but it doesn't mate up properly to the mounting surface and I can't even start the screws to get it bolted up. When I get a free day which should be this sunday I'll pull the pan off and see If I can find anything. I've done a service I think 2 maybe 3 times on this trans as it was rebuilt when we got the first round of stim checks and it doesn't have many miles on it. Each time I dropped the pan I've only ever found a normal amount of metal inthe pan. Pretty minimal, looks like a tiny line or pile of silver antisieze, a few flakes here and there and the fluid was still pretty clear, wasn't burned or anything. It's gotten a little bit worse where it will do it on a slight incline but not on flat ground. In any case, I'll check all the things you've mentioned this weekend, dipstick, fluid level, filter. Beyond that if there is an issue it's beyond my capabilities at the moment. I'll order a couple filters because I might as well and try to make sure I get the correct ones. Either way, I'll try to remember to update with what I find.
 

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