Vacuum Modulator Questions

[81_Chevy]

i figured id start a new thread on this subject since its diff from my old thread.
heres the first part of the story:
http://forum.73-87chevytrucks.com/smforum/index.php/topic,25636.0.html

i just replaced my Vaccum modulator to fix a shifting problem and the old one had a bad diaphram,

so i went to adjust it and it didnt do anything. i was told from my buddy out west that if you turn the screw on the nipple of the modulator all the way out that you would have very fast shifting and it would be very soft, and if its all the way in it would be long shifts and it would be very hard.

so i turned the screw out all the way, nothing still shifting really high and still shifting hard.

turned the screw in halfway where the old mod's screw was, shifting got lighter but still shifted at high rpms.

turned the screw all the way in, still shifted at high rpms and shifted very hard.

ive check, and there is vacuum at the modulator, about 15-20 at idle. ive read somewhere on another forum that the rod behind the modulator in the transmission should be able to move freely back and fourth opening and closing the valve, well mine doesnt move at all. also when i replaced the mod this morning zero trans fluid came out, even after driving it to work (10 miles) and back, i removed it and no trans fluid. but after i removed it again about 3 hours ago and  stuck my needle nose pliers in there to pull the pin out i hit it and then a driping stream came out, maybe a couple oz's of fluid then it stoped.

would i be correct in assuming that my valve behind the modulator is stuck? this is the only conclusion that i can think of.

any help/ advice is much appreciated

-Nic

[werewolfx13]

I'm not that familiar with the TH400, but on a TH350, the modulator valve has a spring behind it. As a part of the transgo reprogramming kit, you can remove that spring to firm up shifts. When I was taking apart the transmission for my 76, the spring behind the modulator valve was broken in 3 pieces, and the valve piston was stuck..I was able to pull it out with needle nose pliers pretty easily, and once the spring shards removed, the valve didn't stick anymore. Only a small amount of fluid will come out.

[81_Chevy]

ok, well i got the pin to come out but i didnt see any kind of spring behind it. didnt see any broken peices either, my dad says he just gunna make an appointment at our trans shop and have them fix it, so were done messing with this.

https://www.automaticchoice.com/Catalogue/th400.pdf i also came across this too

[Engineer]

I don't think you will find a spring behind the modulator piston. However, it should move freely.

If you are still having late, harsh upshifts and the modulator passes the test, the other object that affect upshifts/downshifts is the govenor. if the govenor has an internal leak not allowing it to build pressure like it should it will give late upshifts........ That is assuming that the plastic gear on the govenor isn't stripped.

If you want to check the govenor/govenor gear, it is the little 4 bolt cover located at the rear of the transmission, on the passenger side by the tail housing. Take the 4 bolts loose, the cover will pop off, and the govenor should just slide out.

[81_Chevy]

alright, thanks engineer ill check that today after school

[81_Chevy]

well, i have some bad news    its not the guvnor gear either, pulled it out and it looked fine and there was fluid all on it so its getting fluid.

Engineer what do you mean by an internal leak? hopefully this is my problem. not looking forward to sinking in a couple hundered plus that i dont have into a newly rebuilt trans. 

thanks -Nic

[Engineer]

An internal leak would most likely be caused by scoring or galling of the governor shaft where it rotates in the case. If it is smooth then I doubt that is your problem.

Anything more than what we have discussed is going to require that you drop the oil pan.

[travisr1988]

I'm glad a came across this thread. My th400 shifts hard sometimes and tends to shift earlier than I want sometimes. When I fixed my timing and tuned my carb better the shifts got softer unless I hit a sweet spot, and they also came a little later. I plan on doing a shift kit soon, at which point I plan on replacing the vacuum module and kickdown solenoid too. I'll let you know how it shifts after. But seriously make sure your timing is good, carb is tuned good and vacuum is off of manifold behind carb. I also had a problem with the vacuum line being loose and coming off then slipping back on which ofcourse new line fixed that.

[81_Chevy]

alright, thanks for the reply engineer and trav.

well i talked to my dad about it. he looked at our old receipts of the rebuild and my trans is still under warranty so hes gunna take it back and see if they can fix the problem. ill keep everybody updated!

[Engineer]

Let us know what they find out.

[81_Chevy]

well this has no turned from some Modulator questions, to modulator problems!

i adjusted the screw again, i turned it out a little too far and it let a BUNCH of trans fluid into the engine. this thing smokes like crazy right now, like to the point that its no drivable anymore. i went under and screwed the adjustment screw back in and unplugged the vaccum hose and let all the extra fluid drain out, i started it up and went for a drive and the problem didnt get any better.

could i have ruined the diaphragm in the modulator? or is it just the extra fluid buring off thats still in the engine? how could i get it to stop smoking?

oh and its also smoking only from the passenger side exhaust(duals) so i know its trans fluid coming from the modulator through the vacuum hose.

and help would be appriciated as this is my only form of transportation right now :/

[travisr1988]

Correct me if I'm wrong but the vacuum line should go to the manifold vacuum port on the intake, which should be drawing vacuum from both banks, in which case if you get trans fluid in the engine from that it'd be from both pipes. I don't see how fluid would be pulled through the vacuum line so easily though, I would think the vacuum part of the modulator would be sealed from the rest so that couldn't happen. How late has it been shifting? I know when it's colder out my truck will shift at pretty high rpm if I get on it at all and I can smell burning oil, maybe you're burning a little oil on that bank? maybe the high rpms all the time are just too much for your rings (maybe worn out and letting oil blow by into the cylinder) What color smoke is it and what does it smell like? It could also be fluid got on the exhaust which would make it smoke at that area, unless there's a hole in the pipe, a substantial amount of fluid on the exhaust can smoke for quite a while. Oh, I've been looking into the shift firmness, the faster the shift the harder it typically is, so if you want a smooth soft shift and not a snappy hard shift then you want to slow the shift down, the modulator should change the speed at which it shifts (the one I'm buying is +/- 4mph) it's the internals that change the harshness, odds are the trans got a shift kit put in to increase performance and longevity, the down side for you is snappy hard shifts. I prefer a snappy shift personally. It's the inconsistancy that bothers me.

[81_Chevy]

my trans has already had probelms before it happened, so i adjusted the screw on the modulator way to war out and it let trand fluid go up the line. its not as bad as it was before but its still smoking. im pretty sure its just left of trans fluid in the muffler still burning off.