DIY TH400 shift kit

Chris S

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Okay, this will be a picture-heavy thread, consider yourself warned.  I have found that there is a severe lack of pictures on this, so I dove in with camera in hand.  If you see something that is incorrect or incomplete, please chime in, but only if you can explain the CORRECT way in a comprehensive manor - pictures prefered.

Remember there are many different ways to modify an automatic transmission to get the same effect, I am only attempting to document one of those ways.

Here goes....

Pull the cover.  Remove the filter, the valve body is under it. 
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Remove the valve body.  If you are very carefull, you may be able to reuse the gaskets if they are undamaged.  There are 2 gaskets with a thin plate in between them, we will be drilling 3 holes larger in this plate.  The gaskets are labeled with a "C" and a "VB" to tell them apart.  The "C" goes next to the case, and the "VB" goes next to the valve body.  The detent solenoid will have to be unbolted as well to remove the gasket.
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Now you can see the check balls, there will be up to 6 of them.  Don't panic if yours has less, some had less, and someone may have been in there before you.
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Note where the 3 arrows point.  These holes will be opened up to a maximum of 3/16", any larger serves no purpose as this is the limit of flow anyways.  (The hole size can be smaller, but I'm after a firm shift so I went to the maximum.)
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These 3 balls can be removed, as their purpose has been deleted by the drilling of the plate.  You may have more or less balls, but make sure to leave the frontmost ball & the rearmost ball.
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This piston is the 2-3 accumulator valve, its spring can be removed to firm up the 2-3 shift even further.
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Replace the piston without the spring. 


This mod is to allow us to hold 1st gear manually without the factory upshift at a specified level.  Remove the pin with a pair of needle-nose pliers.
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With a magnet, remove the piston.  Keep things in order & CLEAN. 
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See the black mark on the ringland on the right?  Grind a "flat" area on this to allow holding 1st gear manually.
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"Flat" ground.
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Reassemble everything & try it out.  Things should feel a little different than the factory setting, although even the factory used these settings on select performance models, so don't worry that you have done something radical to your transmission, these modifications will actually help the transmission last longer, as the soft slow shifting (and subsequent wearing of the bands) is now greatly reduced.

Enjoy!
 

Novillac

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printed, stapled and put into the folder, I paid someone to do this for me with trans #1, now with trans #2 I will DIY it myself.

PS, I also had the long tail shaft replaced with the shorty at the same.

John
 

Andy V

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Chris, thanks for the great post.  On removing the 2-3 accumulator spring....I hadn't heard that one before.  Is this recommended for a primarily street driven car....or does it make it pretty hard shifting at all speeds for the 2-3 shift?

thanks,
Andy
 

Chris S

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The 2-3 accumulator is essentially a "delay" that further stretches out the timing of the 2-3 shift. 

(The balls and the holes in the plates are the 1st "delay") 

Remember driving your first actual new car?  How silky smooth everything was?  These built in delays were what created that "feeling".

Later as we get used to the car, that same feeling starts to feel "mushy" to us weirdo performance freaks, and we start wishing for a more solid shift feel.

The good news is that the "silky-mushy" feel is a compromise that was designed into the transmission.  The compromise was "comfort" in exchange for "longetivity". 

Fortunately that can be reversed easily.  The factory used plates with the larger holes and even they eliminated the balls & accumulators on some models; SS & such.

To more directly answer your question, the trans will shift much more "firmly", but I would describe it as hard or harshly only while at WOT, lesser throttle settings are more acceptable.

According to Ron Sessions, many rebuilders install a shift kit as SOP during rebuilds, helps assure a longer lasting rebuild & a happier customer.    (I would be suspect of a rebuilt trans that shifted the same, but happy with one that obviously shifted "stronger"; its all in the perception.) 

Thanks everyone. 
 

Chris S

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Nathan, can that be done without getting deep into the trans?  Doesn't that require drilling the direct drum, or something similar?
 

shiftless

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The way I did it was to remove the center sealing ring from the center support and block off a feed hole in the case. If the trans is standing on its tailshaft with valve body surface facing you, the big hole just to the right of the center support bolt is the one to block off. I just used a cup plug with a light swipe of grey RTV.
 

MOCRUZN

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Will the manual 1st gear mod eliminate the automatic gear shifting in Drive ?

Would someone please locate the hole shiftless installs the cap plug into in a picture,  maybe one thats posted already ?

Thanks  ..
 

Dave Parks

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I have been reading Sessions book ( about 47 times)it explains this on page 210 about plugging the reverse oil passage.      Dave
 

coppercaddy

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I would recommend that everyone of us should have the Ron Sessions turbo 400 book. I have 2, one in the garage and one in the house, both with a ton of highlighting in them.
 

dave brode

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MOCRUZN said:
Will the manual 1st gear mod eliminate the automatic gear shifting in Drive ?


Thanks  ..

No, it just disables a safety feature in most TH400s = they will make the 1-2 shift even with the lever in "D", at a certain rpm, which varies and is setup per vehicle.

Dave
 

Chris S

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shiftless said:
The way I did it was to remove the center sealing ring from the center support and block off a feed hole in the case. If the trans is standing on its tailshaft with valve body surface facing you, the big hole just to the right of the center support bolt is the one to block off. I just used a cup plug with a light swipe of grey RTV.

Here's a picture of the hole to block off.  Sessions states that this MUST be done when the center sealing ring is removed.

To me, the picture does NOT make it clear exactly which hole to plug.  Maybe its obvious when the trans is in front of you, but for the sake of clarity, can someone mark the picture so we know for sure?

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Does the center sealing ring have to be removed in conjunction with plugging this hole?  Or can the plug be done independently?  (most people won't want to disassemble the trans to remove the ring)
 

dave brode

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Does the center sealing ring have to be removed in conjunction with plugging this hole?  Or can the plug be done independently?  (most people won't want to disassemble the trans to remove the ring)

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One of two things MUST be done along with the plug. Either remove the center lip seal from the drum, OR the sealing ring from the the center support, as shiftless explained.

The hole to plug is the upper of the three in the pic. The middle hole is where the bolt goes.

Dave
 
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