TH350 Troubles / Questions

[YourBuddy'sTruck]

Having trouble with my TH350.
1970 C20, 350cc, Th350.
Here's the story.....
Started leaking out of the pan over the Winter.
Drained fluid, pulled pan, new seal, then refill with new fluid.
Ran fine, everything okay.
I don't use it much in the winter...literally 100 miles or so the whole winter.
As we've warmed up, I'm driving it more and now the transmission seems to be slipping.
A little at first, now it's quite a bit of RPMs till it finally shifts and gets up to speed.
Once up to speed, seems fine.

The fluid I put in is Dexron-VI / Mercon LV.
(I don't even know what these words mean, but that's the one most of you here suggested in threads)

Did I do something wrong? Or is this thing in need of new friction rings and a rebuild?
Any help / advice would be appreciated as I know just about nothing on Transmissions.
Thanks in advance.

Also, last year I replaced the Actuator (vacuum shift) Valve with your help, as well as the Speedometer Cable and Gear.

[Stocker]

If my TH350 gets low on fluid, say one pint or more, it will sometimes slip when first starting to move the truck. RPM increases momentarily, then it hooks up and you're off.

I don't know if that's similar to your situation, but it's easy to check fluid level. Make sure the trans is fully warmed up, truck on a level surface, idling in Neutral or Park (I don't think it makes a difference with these trucks).

[AussieinNC]

Sounds like either:

1/ Low fluid level: Start truck, park brake on, trans in Park...place trans in D, then N, then R, than back to Park. Engine running, check fluid level on dipstick. Cold it must be above lower level mark, Hot it can be at upper level mark.

2/ Vacuum leak at modulator pipe: Check the rubber tube at top and bottom of the vacuum modulator pipe to ensure there are no splits or cracks.

3/ Hard clutch seals: If the trans has never been overhauled, the clutch pack seals tend to go hard. The clutch seals need to seal correctly for a shift to occur. If the issue "goes away" once the trans has warmed up, and the oil level is correct, I would be looking at either having this trans overhauled or exchanged for a rebuilt unit.

You mentioned the vacuum canister...more correctly known as vacuum modulator,...
Was it replaced due to a leaking trans fluid into the engine ?
Did you replace the filter in the trans when you had the pan off?

Some of these modulators, have an adjustable feature to fine tune shift points. However, most of the described issues seem to point to one of the three items I listed above.

Hope this helps...

[YourBuddy'sTruck]

I did not replace the filter. Thought I was just fixing a leak?
I'll recheck the fluid after running it some tomorrow.
Also look for vacuum leaks.

Yes, it was pulling fluid into the engine and smoking out the passenger exhaust.
but come to think of it, that was two years ago.

I actually plan on overhauling the whole engine and transmission, just keep putting it off because of life and best laid plans and all that.
I'm not afraid to tackle any job, just limited on time to devote.
That said, I'll hope it is one of the easy solutions!
Thanks fellas!

[YourBuddy'sTruck]

Still slipping.
I topped off the fluid. (less than a pint)
Shifted while holding brake and rechecked fluid level. Good to go.
Went for a test drive and still slipping.
Pulled into an empty parking lot and came to a full stop and put it in reverse.

Strange? It totally performs in Reverse?

Put it in Drive and slipping. Reverse again and it performs fine?
What would this make you think is going on? I'm really confused now.

[AussieinNC]

Originally Posted by YourBuddy'sTruck

Still slipping.
I topped off the fluid. (less than a pint)
Shifted while holding brake and rechecked fluid level. Good to go.
Went for a test drive and still slipping.
Pulled into an empty parking lot and came to a full stop and put it in reverse.

Strange? It totally performs in Reverse?

Put it in Drive and slipping. Reverse again and it performs fine?
What would this make you think is going on? I'm really confused now.

The forward clutch pack is toast...time for an overhaul....

Time maybe to consider an upgrade to maybe an overdrive trans...

[Bigdav160]

I think the two most commonly mis-used words in the automotive lexicon are "slipping" and "hesitation".

What they mean to a technician and what they mean to a vehicle driver are usually two different things.

[YourBuddy'sTruck]

Originally Posted by Bigdav160

I think the two most commonly mis-used words in the automotive lexicon are "slipping" and "hesitation".

What they mean to a technician and what they mean to a vehicle driver are usually two different things.

This is some obvious slipping.
Lots of RPMs and no go.

Regardless...looks like I'll be reading every thread here on TH350 rebuilding!
Thanks for the replies and sending me in the right direction.

[54blackhornet]

My 71 GMC with a 350 sat for twenty years. When I first tried to drive it I was surprised to find absolutely no fluid showing on the transmission dip stick. I filled it up with Dextron. It moved but slipped when you first gave it gas. I had the filter replaced and refilled it with Ford type fluid. Now it operates perfectly.

[67swb72klb]

Used turbo 350 are pretty cheap these days you may find a great deal on a fresh one..

[68bbc/20]

I need to put a new gasket and filter on my 350 turbo also, will the pan clear the crossmember in the back and drop down or do I need to unbolt the crossmember and mount to move it out of the way?

[54blackhornet]

I didn’t have to.

[Daron65]

I may be asking this incorrectly, but what should be the shift points be?

[Stocker]

Originally Posted by Daron65

I may be asking this incorrectly, but what should be the shift points be?

Depends on how light or heavy your right foot is, among other things. My K20 is virtually stock and with the loud pedal pinned, it shifts at 4500 RPM.

[Bigdav160]

A little at first, now it's quite a bit of RPMs till it finally shifts and gets up to speed.
Once up to speed, seems fine.

This sounds like a late shifting problem. I know Aussie is a good tech (I was Chevrolet Master myself) but you wouldn't get up to speed with a forward clutch slipping. In third you are using direct and forward clutches. Direct are used in reverse also so a solid reverse doesn't indicate a problem with that clutch.

[YourBuddy'sTruck]

Thanks for all the replies fellas.
I'm pulling everything.
Started a new thread to ask about pulling the Transmission and such.
This one is a c20 with the 2 Drive Shaft Set up.
So I pulled the Shafts and all.
This, so I can get new Universals while at it because...whey not?
Learning this stuff is mostly fun with some swearing, but when finished, I'll know it was done right.

Also, to answer one of you, the C20 with 2 Drive Shafts has a massive Cross Member that is right in the way to remove the rear drain pan bolts.
Looks like the same Engineer at GM designed that AND the amount of clearance for holding the bolt to remove said Cross Member. That fella was EVIL!