[73LeMans]

I'll start by saying, I've done nothing to diagnosis this TH400 and I don't expect to diagnose it here without a lot more information and actually diving into it. Its still in the car exactly how I left it after Octobers track outing. Haven't even dropped the pan to see whats up. I'll likely bring it to a pro shop as Ive done for all other transmission needs, but curious if it can determined where the focus should be...and then maybe I try it myself.


TH400
10"Continental converter
3800 stall
Fully Manual Reverse Valvebody w/Tbrake
TCI Max Shift trans fluid
3775lbs race weight
I'll get crap for this I'm sure....Burnouts are and have been for 25 years, 1st into 2nd, and usually (but not always) 3rd and roll out.

For the last 3 passes, while under full throttle, there was no 2nd gear for runs 1 and 3. Its as if I had shifted into neutral. The engine hit the rev limiter set by the MSD so I shift into 3rd. For the 2nd run in between, everything was fine.

- 2nd gear worked consistently during all 3 4500 rpm burnouts without issue
- 2nd gear always works normally driving down return road and parking lot
- Fluid doesn't smell burnt and its full, checked hot, running and in gear
- TCI 34 element sprag is 25+ years old by now. Trans has been refreshed one time since, in 2010

If its the sprag, which a lot of indicators point to, why would the run in middle be OK? And if it is the sprag, doesn't it just break and then become a consistent problem?



video of car hitting rev limiter because, why not

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https://www.youtube.com/watch?v=zjgpGic2_LY
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[Half-Inch Stud]

Intermediate Sprag located on the Direct HUB fails by rolling over center and becomes a tickle bearing. No TQ transfer while in 2nd.

Move on to 3rd means Direct Pack locks its frictions so FWD TQ can transfer to hollow Sun Shaft. FWD Pack locked with Sun Main Shaft + DIR Pack locked with hollow shaft means the Planetaries are locked as a rotating chunk.

So, the converter eats-up the "neutral drop into 3rd while you risk rev liming "for the win", as you did.

You have an uncomplicated Sprag upgrade to do. Question persists about using the 8-element Spring Roller Hub instead of the 34 element Sprag upgrade for low ET runs; how low of an ET can the Spring Roller handle?
I've always went with the 34 element Sprag. Spring Roller may be fine if others would bring theirsuccess stories.

[73LeMans]

Thanks HIS. I've always appreciated your thoughtful replies. Unless I hear resounding success stories on the 8 element hub, I'll keep it simple (in my mind) and make a choice between the following direct replacement pieces for the fix. My trans guy who passed in 2012 said the next time its out, the direct drum needs to be upgraded for safety reasons, so I'll include that in my list of "to do's", especially since I wind this new engine up pretty good. That leaves me with the following choices from BTE.

- TH400 Aluminum Direct Drum with Pro Mod Sprag - overkill yes~! but I like the idea of reduced rotating mass
- TH400 Pro Mod Drum with 36 element - an upgrade for sure
- BTE TH400 Direct Drum w/ 34 Element Sprag

While choice 3 is the least expensive way out, I believe in making things better than I found them. Looks like Ive got some thinking to do.

[cgeise]

BTW I would stay clear of BTE stuff like the plague

[242177P]

Broken case lugs can also lead to no second gear. Snap ring can't do it's thing. If they're laying at the bottom of the pan, another case may be in order.

[73LeMans]

Originally Posted by cgeise

BTW I would stay clear of BTE stuff like the plague

Say no more! JW, ATI, FTI, Sonnax...I'll go that route

Originally Posted by 242177P

Broken case lugs can also lead to no second gear. Snap ring can't do it's thing. If they're laying at the bottom of the pan, another case may be in order.

I've been talking about a Reid case or at least an UltraBell for safety for about a year now. Or should I say Ive been jinxing myself for about a year? Lets all cross our fingers its NOT the case. $$

[Bill Eveland]

I never do burnouts thru all the gears, I have a reverse manual valve body and do them in 3rd gear only to avoid damage to the sprag

[Ken MacNicol]

I have been doing burnouts through all 3 gears for over 30 years, and have never broke a sprag in a th400.

[chuckies76ta]

TCS has a nice Billet Aluminum direct drum. Holds a 34 element sprag. Comes with Billet piston. http://www.tcsproducts.com/products/...ect-high-drum/
First to second in the burnout box is considered a no no. Ultra bell housings only cover the rules in NHRA for bell housing. The case is suppose to have protection also. There's were the after market cases comes in with SFI ratings. The Cast direct drum is the one to worry about from what I've read. It can explode if it gets into the higher RPM's I guess. No neutral at higher rpm at end of track is what my instruction say with manual valve body.

[73LeMans]

My ATSG THM400 manual was supposed to arrive on the 6th but the shipping date got changed to the 28th. I question how useful it'll be since I have the 1973 GM Service Manual, but it cant hurt to have too much information on such things. I ordered the book from Oregon Performance Transmission instead. It'll be here Tuesday.

In lieu of preparing myself with useful knowledge I decided to shift gears (see what I did there?) and tear into this thing anyway. What better way to start any endeavor?


Pressure plate for direct drum clutch pack



Direct drum frictions/steels




Direct drum (looks like a vinyl record)



Intermediate pressure plate


According to my caliper measurements, the thickness of this plate ranges from .2715 to .2740, so by all accounts, its not truly flat, although you can't really tell laying a straight edge on it or putting it back on the drum.

Other things of note -

• There are 4 intermediate clutches. All frictions and steels look stellar, as seen below.
  
  
  
  
• The last steel however, the backing plate I believe it is, up against the center support, has impressions on it. You can see that here. I'm pretty sure this is normal.
  
  
  
  
• The sprag makes no noise, spins smoothly clockwise, and doesn't move a single bit in the opposite direction. Has about .013" of play up and down, but the gap between it and the retainer is not consistent. Some areas increase to .015". I was very much hoping the sprag was my culprit, but it doesn't seem so here.
  
• The reverse manual valve body looks great as does its separator plate. No elongated hole in the plate and its flat as flat can be.
  
• The case lugs are all intact (thank goodness!)

I'm not sure what caused the excessive heat to the direct drum clutches or the wear between the direct drum and intermediate pressure plate, but I'll listen to any and all opinions on it. Maybe a low oil situation? I run a trans cooler - always have. Maybe a previous converter ballooned? I can't imagine it would take a ton of thrust to cause such a thing, so I might not even notice that. Too much line pressure combined with mostly street miles?

I don't believe clearances were too tight from the get go - its been in the car for far too long for that to be the issue. I'm tempted to replace the drum/clutches and put it all back together, but to my untrained eye, none of this strikes me as the smoking gun to my "no 2nd under duress" situation, so that would seem dumb. These pieces certainly aren't helping me, but I don't see any as the root cause.

There are also a few other sub assemblies left to look at, namely the forward drum. It looks great externally, but I haven't opened it up yet.

I don't see a need to take out the center support ring and assembly. Would that be a mistake?

[mysticmissle]

I have parts and seals in RI if you need anything quickly. Is your pump restricted or does it have the factory sized passage. I recommend that mod if it is not.
what sprag is in there? i think it can roll and still have enough 'grip' to seem to function on a bench but under real pressure it just flips.
is your center support carrier still preloaded? snap ring for direct drum appear to be normal and flat?
Im no expert, but have been inside or upgraded a few. Look forward to hearing the diagnosis from far more knowledgeable trans guys.

[Half-Inch Stud]

@73LeMans my guess is the DIR frictions suffered from poor programming, maybe low fluid level.

Check Sun Gear Shaft to Center Support for slip-fit. Any play spells doom to DIR pack holding PSI.

[73LeMans]

Originally Posted by mysticmissle

I have parts and seals in RI if you need anything quickly.

Very much appreciated. The parts list is growing, so the odds of me reaching out are also growing, but I havent called anyone yet. As a side note, I tried the TCS site recommended by Chuckie, but I cant put anything in my cart for some reason. Tried a few different browsers too.

Originally Posted by mysticmissle

Is your pump restricted or does it have the factory sized passage. I recommend that mod if it is not.

You're referring to the converter charge pressure orifice mod, right? I haven't opened the pump, but seems like a perfect opportunity to check. If we're talking the same thing, I'll make it happen if it isn't done already.

Originally Posted by mysticmissle

what sprag is in there? i think it can roll and still have enough 'grip' to seem to function on a bench but under real pressure it just flips.

This is Interesting. I thought it would freewheel or at least make noise. Its a TCI 34 element. Its been in there quite a while, and its relatively cheap insurance, so I'll replace it, along with the drum just because I'm here.

Originally Posted by mysticmissle

is your center support carrier still preloaded? snap ring for direct drum appear to be normal and flat?

Yes and yes. Nothing seems out of whack, but then again I've only watched videos and read what should be true. Snap ring came out as expected and lays flat on the bench.

Originally Posted by Half-Inch Stud

@73LeMans my guess is the DIR frictions suffered from poor programming, maybe low fluid level.

Check Sun Gear Shaft to Center Support for slip-fit. Any play spells doom to DIR pack holding PSI.

I will check for this. I stuck my hand down in there and nothing seemed to give any, but I'll pay closer attention and report back.

[Half-Inch Stud]

The long Babbit bushing in the Center-Support must absolutely be checked for slip-tight fit.

[Dragncar]

Originally Posted by Half-Inch Stud

The long Babbit bushing in the Center-Support must absolutely be checked for slip-tight fit.

In a lot of rebuild kits that center support bushing is offshore made while the rest of them are Clevite or FM bearings. I never used any of those center support bearings in the 3 T400s I have rebuilt.
I had to go to a local trans shop and have him dig through old stock and find me center support bushings that said "Clevite" on them.
He told me it was very important and not to use any bearings that had no markings on them.

[73LeMans]

Just updating the thread with what has transpired.

First, I'm blaming circular burnouts back in 2014 for all the scoring witnessed in the trans now. The engine overheated and with no movement across the trans cooler, I imagine the trans did as well.

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https://www.youtube.com/watch?v=EZgVmKz5tX4
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I will never do that again.

@HIS - I pulled the center support and assembly. Happy to report most all looks great there (question below) and the center support babbit is a perfect slip fit to the sun gear shaft. Thank goodness too. I don't have a good puller for that bushing.

@mysticmissle - Pump mod has been done. Didn't know about it until you mentioned it, so thanks for steering me that way. Still needs cleanup, but here's the completed task.



Other things of note.


Sprag looses teeth easily - These fell out before I even took it off the drum.


The forward clutch hub was also a witness to some heat. The associated steels show slight hot spots and some gouging.







The direct clutch hub also has some wear



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Questions -
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1.) The reaction carrier drum bushing - does it look worse than it should?



2.) The low speed roller clutch assembly looses rollers pretty easily after taking it out. That normal? Seems normal given the contraption they're in, and they look good and go back in fine, but I have nothing to compare to.


Next step - getting all the right parts gathered.

[Half-Inch Stud]

Overall, that TH400 saw high-heat and probably burnt frictions from lost pressure due to low fluid level.
Lo-roller roller springs lost their tension from heat, lightly stretch them each and reuse. They can break from over stretching.
Dir hub (Int) sprag and it cage are suspect: the dogbones are rarely (like never for my experience) able to fall out. Can test 1-way action and visually inspect remaining tilt for reuse.

New steels (or 320 grit scrub followed by 0000 steel wool scrub) to replace scored steels. I didnt study the friction images.

Re-check: sungear shaft clearance with Center-support babbit bushing. Imperceptible clearance/movement or is it bad with 0.005-0.015" slop?

What was valvebody? GM?

[73LeMans]

Sungear shaft to bushing clearance is slightly tighter than .004". Replace?

Valve body is a fully manual reverse pattern unit using a GM core. (8626577A) Manufacturer? Not sure. My trans guy installed it in 2002. The receipt doesn't mention brand, but it did cost me $468 at the time, so I'm hoping it was a good one. My understanding is a lot of companies used the GM core to do their thing, so I'm not surprised to see a stock looking VB.



Good advice on low roller. I'll likely go new on steels, at least for direct drum anyway. I'll clean up fwd steels. The intermediates look great, except for their associated pressure plate. That'll be replaced. New frictions all around.

Sprag is definitely getting replaced along with direct drum for a stronger, lighter assembly. May also replace fwd drum for a lighter, stronger billet piece.

[chuckies76ta]

Did the valve body use a gasket on both sides? Looks like maybe a Hughes Reverse valve body. Can you flip it over and post a pic of gasket.

[73LeMans]

If I said there were no gaskets, would that surprise you? Surprises me, but I don't recall taking one out, much less throwing one away. But then again that was months ago. This plate, which may double as a gasket, sat between the VB and the case.






Forgot to mention, this VB has a transbrake.