TH350 slip

[85/Silverado]

was running about 85 the other day and TH 350 B&M started to slip, so I let off and pulled over and checked the fluid and for some reason it was over full, strange since no one has touched it for a long long time. Anyway I turned around and went home. It seamed to be ok on the 5 mile drive back home. I took it out yesterday and after it heated up It didn't want to shift into 3rd. So I have no clue whats going on. I doubt it has 5000 miles on the new trans in the past 3 years.
Any Ideas?

[YoungPup1977]

How is reverse ? If you power brake it in reverse does it break loose any ? the reason I ask is the direct clutch is applied in reverse and 3rd gear so if you don't have 3rd, it could impact the engagement of reverse. you can check out a apply chart on the web to see for yourself what is applied in each gear on the T350.

[85/Silverado]

I'll try It today, but it seems to back up into the garage ok.

[85/Silverado]

It seems to start slipping just trying to take off after It warms up now. I think It my be the Torque Converter since the fluid seems to be over full and It was not before this started. I dropped it off at the installers garage this afternoon. Only has 7ooo miles on it but it's 2 n1/2 years old. So no Warrenty.

[YoungPup1977]

If the TC is bad it would impact reverse and forward gears.

Again does it slip in reverse ?

[85/Silverado]

No reverse works fine.

[YoungPup1977]

Originally Posted by 85/Silverado

No reverse works fine.

It’s not the torque converter if you have reverse. Most likely it’s in the forward clutch drum area Which would cause 3rd to slip as well. Since the forward clutch is applied in all 3 forward gears on a T350

Pic provided of the forward clutch drum with input shaft taken out of a core that I am rebuilding
Posted via Mobile Device

[85/Silverado]

The trans doesn,t want to shift unless I shift by had and force it to shift with the gas pedal, I have a 2500 stall converter so thats why It seems to slip what it starts out in 2nd instead of down shifting to first. I think the the shift valve Isn't working right, I have the TCI - Street Rodder tranny. So I have to get the correct part

[Bigdav160]

The fluid rising makes me think you have a pressure problem and the torque converter is draining back to the pan.

[cadillac_al]

The weird shifting makes me think vacuum/vacuum modulator.

[YoungPup1977]

thread started as driving 85 MPH and started slipping, reverse is fine, slipping on take off after its warmed up, dropped it off at installers garage, trans wont shift but only by hand (must of all of sudden quit slipping but has a 2500 stall and owned it for 2 1/2 years ?) "Starts in second gear, thinking its a shift valve so have to get correct part" …. what correct part ?

needs more ATF....

GOOD LUCK !!

[85/Silverado]

If I shift with the shift lever and let off the gas and hit it, it shifts like it's suppose to. Just ordered a new Vacuum Mod. from TCI and I'll see if that works. If that dose not work then I may just get a 700r4.

[YoungPup1977]

Originally Posted by 85/Silverado

If I shift with the shift lever and let off the gas and hit it, it shifts like it's suppose to. Just ordered a new Vacuum Mod. from TCI and I'll see if that works. If that dose not work then I may just get a 700r4.

It doesn't slip any more on take off like you indicated in post #4 ?

[85/Silverado]

It Is slipping because It was taking off in 2nd instead of 1st. so the 2500 torque converter was making the RPM's go higher to take off.

[YoungPup1977]

plus it quit slipping at 85 mph like outlined in your first post which now a modulator will fix a slipping at 85 mph issue I think I am starting to understand how it can be a modulator NOT


But If the trans was staying in second when you came to a stop and started out in second I would think it would be the governor. Maybe at 85 the slip you thought was actually not shifting from second to third....same scenario as starting out in second....

The governor gear looks like a apple core...….

If you pull it down into MANUAL low(1st) will it start out in low(1st) ?

[YoungPup1977]

Interesting to see this thread,

http://67-72chevytrucks.com/vboard/s....php?p=8414643

Seems like you have had a bit of a few issues pertaining to your new T350 as far as the modulator.

the above thread dated back dec 2018 indicates a no upshift condition now its not downshifting while you come to a stop....

Did this truck have a 700R4 or 350C from the factory ? Being its a 1985 year truck it could be either, the T350 you bought was not a T350C because your TCC connector is not hooked up...… assuming.

Did this have a 700r4 in it originally, and does this truck have cruise control ? from the pics I see, it looks like a 700r4 TV cable connected below the carb linkage on the drivers side of the nice holley carb..

One more thing, your vacuum advance rubber hose needs to be moved to manifold vacuum. Where it is hooked up is not a vacuum connection on the holley carb. see the pic posted in the thread copied in this post.

[85/Silverado]

Factory 305/ t350 thans. The Vacuum Hose was cut the last time. But It was checked this time. The truck did have Cruise new But when I bought It and after the motor swap we could never get It to work. I just ordered a new Vacuum Canister and will be installed also.
And have the vacuum lines redone.
Thanks