[many birds]

Shifts fine 1-2 and 2-3 when cold. Gear hunts 2-3 at about 25 mph and the speed for the gear hunting changes with adjustment of the vacuum modulator. Downshifts from 2-1 perfect on WOT. Downshifts from 3-2 also on WOT. However after driving about 20 minutes or so, the 2-3 shifts become very soft. After about 45 min of driving, it seems to not hold 3rd gear very well—slightest throttle and it seems to slip. The only thing I’ve done to this transmission is change the weights and springs in the governor in hopes improving the 2-3 gear hunting and get 5500 rpm WOT shifts. I’m still not getting WOT upshift to third. The fluid is in good shape, no filings or burnt smell. What could be causing this?

The car is ‘69 firebird, with a relatively stockish 400.

I’ve been having this problem with this transmission for as long as I’ve had the car running. Previous owner “rebuilt” or had the transmission rebuilt with a shift kit.

[Half-Inch Stud]

Replace the filter with a stainless/brass screen filter.

Or, add 2-quarts and idle/drive it on areas that you can afford to let it puke-out to the right level.

I'm south of HSV.

[Formulas]

Just dont apply engine power while its slipping because you will be possibly needlessly heating up the trans or a clutch pack and toasting it, IF it isnt already toast

they are not hard to rebuild ive done it 3 times get a book or use internet go slow git'er done

[Half-Inch Stud]

Found this Black Widow alive in the Tailcup area of a TH400 today. 1 had a a soft 3rd, the other no 3rd.

[many birds]

Originally Posted by Half-Inch Stud

Replace the filter with a stainless/brass screen filter.

Or, add 2-quarts and idle/drive it on areas that you can afford to let it puke-out to the right level.

I'm south of HSV.

PM sent

[OCMDGTO]

I had a similar issue, changing the vacuum port on the carb to modulator line solved it.

[many birds]

Originally Posted by OCMDGTO

I had a similar issue, changing the vacuum port on the carb to modulator line solved it.

Yep, checked that. I have it hooked up straight the the intake manifold.

[Half-Inch Stud]

Stop by this week so we can evaluate the. TH400 performance, and maybe do some tweaks. HIS

[Half-Inch Stud]

Well, after doing some simple vacuum line cleanup, and swapping Vac Mods (was off on spring tension) and mod valve (was binding in case) the 2-3 3-2 hunting became evident at 30-40 mph light cruise. Off the gas or mildly on the gas and that TH400 plays normal.

A very definite hunt, not a sloshy shift. the dsrn thing wants to hunt. That is a Shift-Regulator valve getting biased in a bi-static way, so governor pressure vs modulator pressure tug of war is going on.


My best hunch is the valvebody got messed-up with some sort of Shiffftkittt. So i offered to drop the VB and set it right or just flat replace it with a known-good spare.
And get the Checkballs right.


Yea it was a coupla Quarts low on fluid. typical. Buuut not the fix for the hunting.

[Cliff R]

Simply swapping out the VB may not cure the issues. There are a lot of different shift kits out there, and some "builders" use their own "home-brew" methods.

The separator plate and gaskets may not be stock pieces. They may have made mods to double feed direct and custom combinations of hole plate sizes and check balls locations used.

I've been into a LOT of TH400 transmissions, many had "shift kits" already in them. In any and all cases I basically removed all the "mods" and started from scratch. The simple way is to simply use the Transgo 1 & 2 kit and their separator plate, gaskets and hole size recommendations. If the trans is completely apart one can also double feed direct and modifiy the 1-2 shift valve and block a hole in the VB for full manual control of 2nd and 3rd.

In any case going behind someone else can be a "bundle of snakes" as modifications can go far beyond just the VB. Internally they may have left the center seal off the direct apply piston, a sealing ring off of the center support, and blocked feed holes in the case as well. You may also find some of the check balls not being used........FWIW.........

[77 TRASHCAN]

Originally Posted by Half-Inch Stud

Replace the filter with a stainless/brass screen filter.

Or, add 2-quarts and idle/drive it on areas that you can afford to let it puke-out to the right level.

I'm south of HSV.

The "true" screen filters (as far as I know) were used in Alliaon AT 545 transmissions. You may have to look at a picture chart of transmission filters. There's 2 AT 545 filters. Buy the one that looks exactly like a a TH400 kit (captain obvious), pitch the AT545 gasket, buy a high quality fiber TH400 gasket separately, Napa sells them.
If you just order up a TH400 filter kit, it will have a fiber filter element, that can be restrictive.
There is one other TH400 filter. It only fits the earlier Trans, not likely like yours.

[Half-Inch Stud]

@CLIFF, yea, good point that I may need to go through it. Shifts like a chirpy B&M kit.

@ 77 TrashCan, yez, the Allison screen filter used to be unique, now China has brass or stainless screen.