th350 slipping off the line?

[gs82]

it seems my 76 350/th350 isn't handling the 35's i just put on it too well. it had a bit of a studder in 1st gear when taking off when i got it about a month ago, I had thought it was just the old carb needing a rebuild. Took it for the first big drive (150km) this morning since installing the lift and tires. Now when i come to a complete stop and go to take off again its shaking badly (especially when starting uphill) and has no torque through 1st gear. Once i get it up to highway speeds its shifting and running fine. Torque converter or?? I haven't even had a chance to see what the fluid looks like as i had to leave it at the airport and fly to work.

 

[mistaake]

it seems my 76 350/th350 isn't handling the 35's i just put on it too well. it had a bit of a studder in 1st gear when taking off when i got it about a month ago, I had thought it was just the old carb needing a rebuild. Took it for the first big drive (150km) this morning since installing the lift and tires. Now when i come to a complete stop and go to take off again its shaking badly (especially when starting uphill) and has no torque through 1st gear. Once i get it up to highway speeds its shifting and running fine. Torque converter or?? I haven't even had a chance to see what the fluid looks like as i had to leave it at the airport and fly to work.

Yeah... check the fluid. my th400 was doing exactly that (compltely stock, stock tires) and adding half a quart of fluid fixed it perfectly - maybe it's a bit low? a $3 solution!

 

[gs82]

So I've checked the fluid level and it looks good. Added a jug of Lucas stop slip. Ran and shifted good for the week I was home. But as soon as I drove back to the city to fly back up to work, it did the same thing once I drove the long hwy to Vancouver. Seems like it will only do it once the tranny is hot. Seems to lock up when I come to a complete stop. Shakes and studders when I go to take off again, really bad when I take off from an uphill stop. I'll try dropping the pan when I get home this time and change the fluid and filter. But does anyone have an idea as to why it would be doing this? Torque convertor out of adjustment.

 

[HotRodPC]

I'm kinda wondering if this isn't a torque converter issue rather than a trans clutch issue. I suggest the converter because you mention shuddering. Trans will usually just slip, converter can and will shudder. In a rare case, trans clutches can too. Like if you have a bad spot in the drum seal, as it spins, it grabs lets go, grabs lets go. If the 35's did anything in, it wouldn't be a drum seal though but it could have damaged the converter. What color is your fluid? Any signs of black clutch debris in the fluid? Or just ugly red from overheating?

 

[gs82]

The fluid looked fairly clean and didn't smell burnt at all. I'll have to wait until I fly home from work to drop the pan and see if there's any carnage floating around but it sure feels like a torque convertor issue. Just stumps me why it will only do it when it gets hot.

 

[HotRodPC]

Because that's what the fluid does when it gets hot. #1 sign of overheated trans is foamy fluid. Could even be a blockage in one your cooling lines. Either the supply line or the return line or within the radiator cooler tank.

 

[gs82]

Ok I'll try clearing them out and see what I can find. Thanks for the tips! I'll post some pics and update when I get home.

 

[gs82]

Ok so I changed the fluid and filter, replaced the old steel cooler lines and routed new hoses along the frame rail into my trans cooler. Blew out the cooler and it's flowing fine. Nothing in the pan as far as metal or clutch material. The old fluid and filter looked pretty good and didn't smell burnt. Truck did the same thing. Runs/shifts fine until it heats up and gets into stop and go traffic the. It feels like it's having trouble engaging 1st gear and slipping throughout 1st as I accelerate, especially uphill.
Any ideas?

 

[crazy4offroad]

Check the basics first. Torque converter bolts, bellhousing bolts, trans mount bolts, u-joints. Any of these loose, worn or broke will cause issues. Also what's the specs of your truck? How much lift, gear ratio, when was the last trans build, does it have a shift kit, etc.

 

[gs82]

Check the basics first. Torque converter bolts, bellhousing bolts, trans mount bolts, u-joints. Any of these loose, worn or broke will cause issues. Also what's the specs of your truck? How much lift, gear ratio, when was the last trans build, does it have a shift kit, etc.

Everything looks good as far as mount bolts and u joints. Haven't checked the torque convertor bolts yet.. I've only had it a couple months so I'm not entirely sure about the gear ratio. It's a 76 crew k25 with a 350/th350/np203. 14 bolt rear and 10bolt front (i think). Just did a 4" BDS lift. I'm sure it's probably time for a rebuild or a built tranny. Still not sure what I want to do for a motor yet. I would just like to keep it running for now and then start a proper motor and tranny build in the shop so I don't have to park it to do all my engine work etc. my first thought has been torque convertor from the start but I just keep thinking if the convertor was toast it would behave the same all the time not just when it gets hot.?

 

[crazy4offroad]

A converter would be a relatively easy thing to swap to check. I'm with HR if the clutches were bad it would do it cold not hot. Your description almost sounds like axle wrap except the part it only does it hot.

 

[HotRodPC]

Usually, when you have slippage after it's hot or warmed up, it's a seal issue. The fluid gets a little thinner then is able to sneak around the seals rather than compress the drum pistons. I've never been one for additives in engines and transmissions being it's my belief many are just witch potions that screw up the formulation of your current motor oil or trans fluid. BUT, GOOD, and I say GOOD, Auto Trans Conditioners have been known to have positive results in conditioning, softening and swelling seals so you can get more time out of them. For what it's worth, you might not have much to lose but a few bucks to try one of the better name brand seal conditioner treatments.

 

[gs82]

Sounds good I'll give that a shot before I dole out the cash for a B&M tork master. Like you said even if it's a band aid for the time being at least it'll be drivable while I build something

 

[gs82]

Hey HR do you think replacing the vacuum modulator would be a thing to try? Given that it shifts fine once I get out of first it seems like it's working fine but it is a cheap part. Just totally feels like the torque convertor is unlocking when I stop and then not re-engaging when I go to start again. But only once she's warm. Got me stumped.

 

[gs82]

Turns out it was the Forward clutch pack. Did a full rebuild, new torque convertor and stage two shift kit. Should be bullet proof now. Now just gotta do some locking hubs and start building a motor!