Th400 fail points

ok th400 fail points. after you get some kind of a clue what is in a tranny from above you can now recognize some of the common fail points.

 first is an easy one. if you guys know anything about hydraulics, accumulators are used to help cushion the effects of hydraulic shock. well, they are used in trannys too for this reason to give you that “cadillac” style. the 1-2 accumulator is aluminum and is not a big concern.

 however the 2-3 shift accumulator is not always aluminum. somewhere in the mid 70’s they started using plastic…and you guessed it, it cracks. when 316600931this happens it not only bleeds psi off the 3rd gear circuit, but the valve body backfeeds 3rd gear to help apply the direct clutch(we are going to call this the reverse clutch for sake of conversation)….yup you lose psi in reverse. this is often the first thing to go and create a weak reverse gear. and here’s where only the rear band holding the planetary in reverse is a problem. you lose psi anywhere in the circuit you lose everything….it has a lot to do with pascal’s law but you don’t need to concern yourself with it too much. since there is no mechanical sprag to fall back on reverse is often the first to take a sh!t in your tranny.

358343491the forward clutch is prone to burning out prematurely simply because it has a restricting orifice to cushion the shift into gear from park/neutral. when you slam shift it back and forth it never has time to apply fully and will slip & eventually destroy the clutch. reverse can go out for the same reason. on a used tranny it is really a crap shoot depending on how the tranny lived it’s life.

there are two different styles of clutch pistons aluminum and cheaper stamped steel. stamp steels started showing up in the mid 70’s aluminum is preferred as it is more resistant to warping from extreme use. however rebuild shops usually took them out and replaced them with stam316600827ped steel to make a buck. so there is no definite way to tell what your tranny has without tearing it apart. the forward clutch is usually just fine with a stamp steel piston but during a rebuild i susally replace the direct drum piston with an aluminum piston.

 the intermediate clutch can come apart, especially if a shift kit has been installed. the shift kit bypasses a lot of the cushioning effect of the accumulators and can crack the clutch backing plate(pictured right), or worse yet blow the groove’s right out of the case.  not  factor in a 1st/reverse only tranny as you don’t use any of this.

358343488reverse /direct drum quite often can shatter the backing plate that holds the clutch in. here’s an assy to the left as it looks upon removal when it goes to hell. this is a common failure in snow plow trucks and derby trannys. it’s more common than you think it can break like this which creates a weak gear..a stamp steel piston will crown and act like a wedge to make this happen quicker, but it will also happen with aluminum pistons358343493

here to the right is pictured just the direct backing plate. that let loose all at once and locked up the tranny the drum was also destroyed. on the leftis a good piece.

contrary to popular belief, a torque converter can explode internally and will do it more frequently than you think. a stock style or cheap aftermarket converters are not designed for the shock of forward/reverse shifting with you foot on the floor and not hitting the brake. if you drive like a jack ass you pay the price. more often than not it will give you the same symptoms as burned up clutches/seals. so if you have a tranny weak in forward and reverse you may have a bad converter. BUT if the converter went to hell chances are it sent sh!t through the pump and the rest of the tranny so you really need a new build reguardless.i have had real good results with guys using 6 bolt heavy duty gm truck converters. stall converters are a good alternative but then you will be creating A LOT of extra heat fast.

from what i have seen shift kits are a bad idea for demo derby. this is due to the fact that to create a hard 3rd gear shift most kits restrict if not block off completely the fluid flow to the reverse circuit comming out of the valve body.  some cheap made kits also boost psi to an ungodly level in reverse that encourages seal failure on hot trannys, especially with loose fitting stamp steel pistons. this is fine in a normal tranny , but there is already a boost valve internally on the th400 to aid in reverse band application. together with a higher line psi pressures well over 350psi  are not uncommon with the modulator unhooked.  in my book i like to see line pressure between 175 and 230psi. with shift kits, on a th400 you can make your own kit’s with a bit of research 😉 i think they are a waste of money on a th400.

also i recommend hooking up your modulator valve to vacuum on a stock transmission. this aids in spiking psi in the reverse circuit. as long as you shift low 1-reverse the psi will remain where you want it. my research thus far suggests the psi will remain in the 150-200 range and work fine. however if you decide to use regular drive range the psi drops down to less than 100psi. running without the modulator hooked up or using a mod plug should never be done without further internal mod’s to prevent psi spiking in reverse to dangerous levels.

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