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Discussion Starter #1
i want to put a shift kit in my offroad truck that will have a 6bt in it soon. the truck is a 1985 chevy 4x4 1 1/4 ton 6inch lift and 37's th400 . is it a good idea or a bad idea? i dont know i have heard mixed results.
thanks for your help
 

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A shift kit doesn't change anything. They fix factory shortcomings to keep the transmission working right and last longer. They typically improve the reliablity of the pressure regulating valve and include modifactions to prevent seal and case blowouts.

Shift reprogramming kits change the shift speed and harshness. They increase line pressure and limit or eliminate accumulator piston travel.

The faster an automatic shifts the less the frictions wear in theory. In reality, most slushboxes are pretty sad excuses for modern engineering that will more than likely have some sort of component failure or malfunction that causes the clutches to burn up before they die of slipping in shifting.
 

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Discussion Starter #3
what to do

so what is the best way to do this? or where do i get something to reprogram the shifts? I know what you mean about the accumulator pistons as i removed the springs on my crown vic to give it harsher shifts but i dont want them that harsh because i will be shattering u joints left and right thanks for your help.
 

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Discussion Starter #4
i know the 400 is mechanical so nothing i can do will help the shifts? this doesnt seem right? i know shift kits provide more fluid in most cases to get more firm shifts but i hear they are not always the best this is all i was asking. thanks nick.
 

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i know the 400 is mechanical so nothing i can do will help the shifts? this doesnt seem right? i know shift kits provide more fluid in most cases to get more firm shifts but i hear they are not always the best this is all i was asking. thanks nick.
They can be adjusted ! you want a nice tight shifts but not slam shifts.
 

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I had a TH400 in my truck for a short time. It does not work well with a 6BT if you want to run over 60mph. I have 35's and 4:10 gearing and it just does not work. I ended up going to a 4L80E. The TH400 also requires vacuum modulation to shift properly. The modulator from the bread trucks is no good on a 6BT. I thought I could make it work, but soon gave up and forked over the money for the new tranny.
 

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Discussion Starter #7
so there is no way to make it work properly? because it wont be able to shift properly. right now with my 6.2 in it it feels sluggish could that be because the 6.2 doesnt generate enough vaccumm thanks for your help.
 

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Discussion Starter #8
cable operated modulator

anyone heard of a cable operated modulator for the shifting problems that turbos10 was talking about? i imagine that it would work because it states it can be used on diesels. i imagine that once the motor gets a certain load on it will allow it to shift properly? thanks for your help.
 

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Discussion Starter #9
sorry what i meant was not the one that comes with the p30 van but one from patc? it sells for like 120
 

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Discussion Starter #10
vacuum pump

would it be possible to run a line from the vacuum pump to the brakes and the vaccumm modulator on the trans in order to gain proper shifts? thanks for everything nick.
 

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so there is no way to make it work properly? because it wont be able to shift properly. right now with my 6.2 in it it feels sluggish could that be because the 6.2 doesnt generate enough vaccumm thanks for your help.
Nope, diesels won't generate enough vacuum to operate a vacuum modulator on a transmission.

anyone heard of a cable operated modulator for the shifting problems that turbos10 was talking about? i imagine that it would work because it states it can be used on diesels. i imagine that once the motor gets a certain load on it will allow it to shift properly? thanks for your help.
sorry what i meant was not the one that comes with the p30 van but one from patc? it sells for like 120
That's the ticket for diesel + TH400.

The "modulator" on the P30s is just a fixed point type, basically a screw threaded down to set the shift point / firmness. It shifts at the same RPM and with the same firmness EVERY time, no matter how hard you're hammering the pedal.

Cable modulator will make the trans respond to the engine throttle input.

would it be possible to run a line from the vacuum pump to the brakes and the vaccumm modulator on the trans in order to gain proper shifts? thanks for everything nick.
It doesn't work that way. A vacuum pump is producing about the same vacuum all the time. The vacuum a NA gasser generates is inversely proportional to engine load. High load = WOT = low vacuum. Light load = mostly closed throttle = high vacuum.
 

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The cable modulator is probably the best way to go if you have to run a TH400. I had one but never installed it. Sold all my stuff to another member so I dont have it any more. I am running a vacuum pump with vacuum regulator on my 4L80, but I would much rather a cable setup. If my pump or valve shells out I could shell the transmission pretty easy due to over pressure :(
 

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Discussion Starter #13
manual valve body

can you run an entirely manual valve body to eliminate all of this hassle or do you still need to have the modulator?
 

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can you run an entirely manual valve body to eliminate all of this hassle or do you still need to have the modulator?
You still need something to modulate line pressure....
 

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If you eliminate the modulator you're fixing line pressure. This can work OK, but there is always a downside. If the vehicle is street driven and longevity is a concern you cannot just crank the line pressure because it will force the torque converter into the engine and wipe the crank thrust. If the engine is pretty low output you can probably get the tranny to hold with a middle of the road setting, but more power and you'll toast the tranny.
 

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Discussion Starter #16
thanks for the replies guys i think im gonna get the modulator from patc and hopefully all will be well.
 
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