TH350c Slipping

[scenic760]

Hey all,

I was finally able to get everything put back together in my 1981 Suburban and my hope was to take it down to get a smog check and registered...however on my way back from getting gas (a painful $5.79 per gal btw) the trans was slipping something fierce. It's a fresh rebuild, I did the dual feed mod to it and a reman torque convertor.

It seems to work fine at initial startup and goes into reverse and all the drive gears but it starts slipping after running it just a little...like a 500 foot loop around a cul de sac. I measured the temp at the pan with a handheld laser thermometer at about 180deg F after about 2 loops. I have a feeling the first time I ran it it got hotter than that.

I have the cooler lines running to radiator and both upper and lower lines are hot so I'm assuming it's circulating correctly? A few things I checked already:

1. Modulator vacuum- seems to be pulling correctly
2. Modulator leaking diaphram- about 60sec or so after it starts there seems to be white smoke coming from the exhaust so I thought it might be sucking fluid but vac gauge test came up good, it goes away after I drive it
3. Brakes- made sure the brakes were not dragging
4. Fluid- I may have overshot the fill level by a pint or less after filling a bit after the first trip but still same behavior

So it seems to me like it's getting hot real quick? The fact it's going into all the gears is leading me to believe it's not a seal issue? However, I'm FAR from an expert...

Any ideas on where to focus my efforts would be GREATLY appreciated!

 

[Vbb199]

What fluid did you put in it ?

 

[scenic760]

What fluid did you put in it ?

Valvoline multi vehicle stuff...so I looked underneath and there appears to be some drops of fluid coming out from the bellhousing...Let me pull it off and see if I can tell where it's leaking from.

Its a lockup convertor, do you think it might have something to do with the o-ring on the end of the shaft? From everything I have been reading it sounds like a classic case of a TC issue...especially with it getting that hot that quick?

 

[scenic760]

I pulled off the lower bellhousing and it doesn't look like there is some massive leak..the TC is dry and it looks like it just leaking from the bolts a little bit..I have seen more than a few th350's leaking for more that function just fine..

One thing I did do when I installed it was to shim the TC to flywheel with washers because the gap was larger than what I saw spec'd somewhere (can't remember but I can find it if need be)...come to think of it do you think this might be causing some endplay issues and causing some (or a lot) of excess friction?

 

[Vbb199]

Valvoline multi vehicle stuff...so I looked underneath and there appears to be some drops of fluid coming out from the bellhousing...Let me pull it off and see if I can tell where it's leaking from.

Its a lockup convertor, do you think it might have something to do with the o-ring on the end of the shaft? From everything I have been reading it sounds like a classic case of a TC issue...especially with it getting that hot that quick?

I was asking about the fluid just to be sure it wasn't cheap stuff or the wrong fluid. That would be fine I would think.


The next thought after that is the tc and/or the o ring on the input shaft, assuming you're 100% sure the modulator is OK

 

[86c10twotone]

You should not put washers in there as shims .

 

[Matt69olds]

You should not put washers in there as shims .

Technically, you are correct. If the converter is built correctly, there is no need for shims.

Ideally, you want 1/8-3/16 clearance between the flexplate and converter with the converter seated completely into the transmission pump. 99% of the time, the clearance is correct. Occasionally you will find a converter with them overall build height is too short. This sometimes happens when the converter is cut open and rebuilt multiple times.

 

[Matt69olds]

I have tried the dual feed modification on the 350C in my dads 81 GMC, it never worked correctly. I could never get the converter to lock.

You may need to undo the dual feed modification. If you didn’t matching the direct clutch piston for a 5th clutch disc I would do that. It’s not as good as the dual feed, but better than nothing.

The 350C has been in the truck for over 20 years with the torque monster 455 Olds in it, it’s still going strong. Unless you have a really stout engine in your suburban, I doubt your going to slip the high gear clutches with 5 friction plates.

 

[scenic760]

I have tried the dual feed modification on the 350C in my dads 81 GMC, it never worked correctly. I could never get the converter to lock.

You may need to undo the dual feed modification. If you didn’t matching the direct clutch piston for a 5th clutch disc I would do that. It’s not as good as the dual feed, but better than nothing.

The 350C has been in the truck for over 20 years with the torque monster 455 Olds in it, it’s still going strong. Unless you have a really stout engine in your suburban, I doubt your going to slip the high gear clutches with 5 friction plates.

Hi Matt,

I actually did both, and installed a hardened sprag as well...I figured pulling the trans and reinstalling it was too much work not to do as much as I (practically) could with it on the bench..

If the convertor doesn't lock I can live with it...for a while anyways... but I would really like to have a driving vehicle I can register at this point..

I read another thread on here about someone rebuilding a th350 and had a thrust washer off by some miniscule amount jacking up his whole program...

My hypothesis is that the washers are not allowing the assembly to correctly expand when it warms up, causing it to heat up rapidly and causing the slippage...I probably shouldn't even call it a hypothesis because I believe that means an educated guess, of which mine is the wild ass variety vs educated..

 

[Vbb199]

If I can think on it, there's some notes I have jotted down in a notepad for the th350 and th350c regarding dual feeding (if applicable)

There's a specific seal I believe you are to leave out if dual feeding, or add a seal, it's wrote down, but I dunno off the top of my head, but recall there's no tc lock with or without it

 

[Vbb199]

I also saw you said the TC is a reman.

I have personally seen before, some of the cheaper no name TC's cause issues with lockup, or excessive heating from within the tc

Not saying you got a cheaper one, but where did you get it from?

 

[scenic760]

I also saw you said the TC is a reman.

I have personally seen before, some of the cheaper no name TC's cause issues with lockup, or excessive heating from within the tc

Not saying you got a cheaper one, but where did you get it from?

Going through stuff, it looks like I picked it up at AutoZone so I'm sure it's not a high end unit..

You think I got a bad one? I bought it about 6 months ago but haven't put it into use until now...is there any way to troubleshoot the tranny to isolate the problem to the TC?

 

[scenic760]

Ok...disclaimer- I haven't got around to a full blown road test yet, however I removed the spacer shims from the torque convertor and I took it for a spin and low and behold the temp at the pan was 114deg F...

Ironically when I took out the shims on the torque convertor, my starter started grinding so I checked the clearance on that and ended up needing to shim IT. BTW- starts 1000 times better now, almost instantly.

I started thinking about it though and I'm not sure why the starter alignment would change if I removed shims from the TC side of the flexplate? Obviously it was off before but it didn't grind and started somewhat normally...

And yet another question...I was watching the crank pulley at idle and it seems like it's wobbling a decent amount, is that normal? The engine was allegedly rebuilt and crankshaft/balancer/flexplate were balanced however I have ZERO confidence ANY it was done correctly.

Here is a link to video of the wobble:

PXL_20220324_195400167.mp4

drive.google.com

 

[scenic760]

I have tried the dual feed modification on the 350C in my dads 81 GMC, it never worked correctly. I could never get the converter to lock.

You may need to undo the dual feed modification. If you didn’t matching the direct clutch piston for a 5th clutch disc I would do that. It’s not as good as the dual feed, but better than nothing.

The 350C has been in the truck for over 20 years with the torque monster 455 Olds in it, it’s still going strong. Unless you have a really stout engine in your suburban, I doubt your going to slip the high gear clutches with 5 friction plates.

Hey Matt,

SirRobyn0 had a great idea to run this one by you as I had it posted in another thread I had going...

So after thinking about this a little more...I did "modify" 3 things when I put the trans back together (1) hardened outer race (2) 5 clutch piston (and I did indeed put in the 5 clutches, haha) and (3) dual feed by leaving off seal on direct, ring off pump and plug reverse feed.. I remember checking the thickness on the piston and clearance on the clutches when I was putting it together and all was good...

One thing I'm not sure about, thinking back, is the separator plate. Is it possible that someone had previously installed a shift kit and in conjunction with my dual feed mods is causing the slipping? Honestly, I would have had no idea what I was looking at if the separator plate was or was not original...

 

[Matt69olds]

Most shift kit plates are yellow anodized, TransGo plates are black. Stock plates are natural steel with no coating