TH350/TH400:modulator block off plug mod’s

302049547Well, this is the first in a new series of posts in the name of performance upgrades you can do for your transmission. what you see in the picture here is an aftermarket aluminum block off plug you can get from most any performance autiomotive supply outlet for about 10 bucks. it not only makes for a much cleaner appearance on you tranny, but in demo derby application it keeps the modulator valve from ripping off on the floorpan and causing a major leak.  unfortunately as i have stated a few other times  throughout my blog, this creates a major pressure spike while in reverse causing severe internal damage to the tranny. usually the modulator valve will act as a blow off valve while in reverse, but with this rigid plug it doesn’t move and deadheads the psi. well, i am going to show you the simple mod’s to get around this.

396007828Basically, what we are going to do is render the boost valve of the pressure regulator useless.  on a th400 shown here, the pump is removed to expose all the oil passages. the pick is pointing to the oil supply port from the reverse circuit. this supplies oil to the boost valve of the psi regulator that is in the front pump of a th400. we need to block this oil. here i have tapped it and installed a 5/16″ set screw to block the oil. some builders simply jam a check ball in here with a punch. Aftermarket valve body kits will simply omit the supply hole in the valve body mid plate to accomplish the same thing.

400boost01Now, this is not a total seal to the boost valve when using a check ball or set screw. Even though it stops the signal pulse of the oil, some oil may seep through and slowly raise the boost valve off it’s seal and trap the oil in the circuit so even when it is suppose to dissengage the boost valve- you have a high psi situation causeing internal damage in all gears- not good.  To solve this issue we grind two small lands on the large face of the boost valve. this insures the boost will not come off it’s seat.

using this modification, your th400 system pressure will remain a constant 155psi or there abouts in all gears where a stock blue psi spring is used. your main regulator spring is the sole psi adjustment now and obviously can be inceased with a stiffer psi spring, which are available aftermarket. you can do these mod’s and not install a plug-leaving the modulator valve in place to go along for the ride- it will still maintain the constant 155psi. this is more than enough oil and psi to make a th400 do whatever you want it to in my opinion. if you have to run more psi to make it shift harder- you really ought to be looking at modify’n your valve body and/or mid plate rather than increasing system psi.

Now, these modulator plugs can be used on the th350 asth350 boost well, and yes they also need some internal mods as well, but ironically we are going a slightly different direction. pictured here is the psi regulator valve ocated in the valve body of a th350. Basically, the reverse/modulator boost valve needs to have a “slight” relief cut into it on the land the arrow is pointing to in the picture. what we are doing here is creating a small shallow groove on the large land of this valve to bleed some of the psi off the circuit while in reverse. now, this is somewhat of a trick as if the groove is too deep you will bleed way too much psi off in reverse and burn up the clutch packs: not enough and it won’t bleed enough and it will spike your psi and also cause damage. the goal is to create a slight bleed of psi in reverse to keep the psi realistic in reverse. don’t get the brilliant idea to just weld the valve in place through the sleeve cause then there is no movement and your pressures will do goofy shit in several different gear range.  i usually don’t mess with it on the th350’s as the short modulator valve sticks out the back of the tranny and really isn’t in the way anyways.

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