I have a th400 in my 69 468ci,373rear,ATI street fighter, transgo shift kit.
The problem I have is it wants to down shift into 1st from 3rd at about 55 mph.when given good throddle or WOT?
I can pull it into 2nd from 3rd manually, give it gas and it will pull untill it reaches the shift point.
I have a super lite corvette governor in it and that made the WOT/hard throddle shift point really good.
I also have a very large vac. modulator that is for low vacume situations or so I was told?
I can give it hard throddle from a stop and it will shift out fine 1,2,3 or I can shift it manually and it does fine.
I just cant get ridd of this downshift problem?
I is allways when I am passing someone, to much throddle and "BAM" Im on the rev limiter.
Any thoughts or advice is wanted and welcome.........
T.C.

Unhook the detent kickdown...

I would hate to loose the automatic downshift. I don't have a 400, but isn't it controlled by a cable to the pedal? Can it be adjusted or even bent to make the tranny think you are at a different trottle position?

Not on my 400, it's got an electric kick down servo that works off the gas peddle.
I have it unhooked and it still downshifts?
I also dont want to loose my auto downshift, it's just kinda scary to have it go from 3rd to 1st at 55mph.
Thanks for the reply......
T.C.

Are you sure its not downshifting into 2nd?
I don't think with 3.73's a TH400 would downshift in 1st @ 55 MPH
I will say I do not have my kickdown hooked up on TH400 and it will downshift into 2nd if I hit the gas hard and I'm cruising in 3rd/"drive"

Thanks for the replys:
This is kind of hard to explain.................
First off the kick down switch is unhooked, I have the lightest weight gov.I can find and a vacume modulator that is suposed to require less vacume to operate, in the transmition.

This is what happens, I can be cruising at 45mph, give it hard throddle and it will kick down into first instead of second and rite onto the rev. limiter.(set@5500)
On the other hand I can be cruising at 45mph and pull in down into 2nd manually, give it gas and it will pull untill I shift into 3rd.
45mph is about 2000 rpm in 3rd if I remember rite?
The transnition works and shifts fine, except for this one problem?
Could this be in the valvebody or shift kit put in wrong?
Also I only have 7" of vacume at idle, 15" at cruise and 4" at WOT if this makes a difference?
Thanks again for any help T.C.

Is it possable that a ball fell out of the shift kit during assy., or a spring in the wrong place or not strong enough?
I didnt install the kit so I dont know?
T.C.

since you changed the governor weights as well as the vac. modulator, it seems that you are too loose now, and it shifts directly into first, and not 2nd instead. increase the governor weights to change shift points back into the 2nd range.

Thanks fot the reply engineer:
Thats what the tech. at Transgo said, we went to far with the light corvette governor.
He said to use one out of a S-10, so I guess the search is on for a S-10 gov.
The only thing that conserns me is we had a stock (heavy)400 gov. in it and it did the same thing?
I guess when ya dont understand how things work ya have to trust others.
T.C.

Hmm,
Let me check my hydraulics diagram for the Th400 in the next few days. You may have too light of a 1-2 shift valve spring, so it gets backshifted when the 2-3 valve gets forcved back by governor pressure. That would be my first guess.

I may have another solution for you too though, easier one but let me double check.

Thanks for the reply Jake:
Any suggestions are welcome, I will find a S-10 gov. as the Transgo tech. and engineer suggested, Dont know if it will improve though, due to the fact it did the same thing with a stock 400 gov.
But then again, I havent a clue what I am doing?
T.C.

If it did it with a heavy governor, it will likely do it with an S10 governor. An S10 never came with a TH400, so what the TG tech is speaking of would be a 700 governor which will work but you have to change the gear.

What I think is going on is that it is doing a normal backshift caused by modulator pressure (lack of vacuum at heavy throttle) and when the 2-3 shift valve gets kicked back, there is too much pressure on the 1-2 shift valve so it is also backshifting.

What I'm thinking is... and this is totally off memory that you can grind a land on the modulator valve and it MAY fix this. It is very easy to do and if it has no effect or undesired effect, you just replace it with another. 10 minute job to do.

I don't have my books or notes here so I will have to grab them next time I get a chance...plus I've been riding my new Harley so will have to remember to grab them when I am in the truck or Chevelle....

Thanks Jake, when you say grind a land, does that mean a flat spot ?
I know that The Transgo kit directions said to grind flat spots on the modulator valve only for manual shifts?
If I remember rite they said to grind on both sides, but I dont remember what end ?
Does that make a difference, what end and/or both sides?

T.C.

OK,
find your TG instructions and grind the land as they show. You will grind a flat spot on opposing sides. I can't remember which land it is, I think it is the innermost land...
Try that and see if it fixes your problem. It does several things for the trans. It increases pressure for the direct clutches and it prevents kickdown IIRC...
try it.

Thanks I'll give it a try, next week before I get to it.
Will post results.
T.C.