TH350 shifting problems... (boost related I think)


New Member
Aug 18, 2001
Hopefully my logic is right on this situation, here goes:

There is a vacuum line that goes to the modulator on the tranny. With a straight line the tranny shifts unbelievably high under boost. It spun to 6200 RPM before lifting on 10 PSI of boost and hit the rev limiter of 6500 RPM when we turned the boost up to 12 PSI... So apparently more boost equals later shifts. I can only imagine what would happen if we decided to turn the boost up to 30?

A friend suggested putting a check valve in the line so the modulator can only see vacuum, which didn't make sense to me. I did it anyways and sure enough the truck shifted at around 3000 RPM at WOT, way to low. I believe this was because the check valve allowed air to be sucked out of the modulator, but since no air could return, it stayed in vacuum at WOT...

Hence is my dillema. I'm leaning towards putting a valve that completely shuts off that line when we're at the track. So at the track it will always be at 0 PSI. Then we can open it up for regular driving. Is this feasable and will it work?

I heard of a conversion to TV cable as well. Anyone done this?

We will be going reverse manual valve body with a ratchet shifter when money permits, but I need a fix for the way it is now.

My GN is doing the same thing, I want it to shift but it does it after I want it to, Although it doesn't reach the rev limiter I don't like it winding that tight. Will adjusting the kick down cable change the shift point at all?
I don't think it will. The kickdown cable is just to shift it into 2nd if you're cruising in 3rd and put your foot in it....
check out b+m's trans pac for t-350

not the shift kit but the trans pac

unless they've changed it the kit does away with the modulator all together and comes with a plug to put in its place

Reds- do you know where they sell this Transpac? jegs? summits?Also-what size 9/11 converter are you running with the 70 turbo?
hey mad psi,there is a mechanical modulator available for forced induction is a cable that moves the modulator instead of vaccuum.i think atc sells it.we have a nice forward pattern full manual valve body for 99 dollars if you would like to try it out.why buy a kit when you can just bolt in a comoplete valve body?
I'll definitely go with a bolt-on valve body when the time comes. What is this ATC company you speak of?
atc distribution group.they are a transmission parts distributor.there should be one in your area.
yea, put a mechanical modulator on my 400, only "right" way to do it. Also, i dont think you wanna be runnin mad boost into the modulators (vacuum ones anyway). Pop the diaphram on these, you be suckin tranny fluid through your engine. Anyway, the mech. modulator looks like the vacuum ones but uses a TV cable just like a 2004R
I called ATC and they told me they didnt list anything for the 400 as far as the mechanical modulater goes, just for like the allison transmission and more along the lines of diesel trannys. I did though get a hold of my tranny guy here in town and he found me one in CA i believe for 100 bucks. I just put a 400 in my car and after reading this post, i didnt want to go through what some of you have already. Hopefully this works well...
yea, i had to special order mine - a few companies make em - i know B&M used to make one and thats the one i got - it was round 100bucks. Havent taken the car out with themodulator but should work fine - one other thing - depending on what mech. modulator you get, you may need to modify the throw of the throttle arm tv cable post on the throttle arm.. took me a few minutes with ruler and welder handy.
Just got my mechanical Modulator and the company my tranny got it from is Transstar Industries out of Cleveland Ohio.
Part # 40M
I just got a universal kickdown cable or whatever you want to call it so i didnt have to make any major mods.
Hope this helps
one way valve

use a one way vacuum valve in line with your vacuum source so that your modulator can see engine vacuum but not boost. on the modulator side drill a .030 hole to let air in when on boost.
That's what I was going to suggest. A built in leak with the check valve. :)

I wonder how Buick handled this on it's eariler Turbo V6's which used the 350 tranmission?
Well, you guys beat me to it. I thought of using a check valve, but didn't want to say anything, because I had no idea if it would work or not. I didn't want to be responsible for a bad tranny. I know that when I first converted my waGoN over I was running the 350, and when we took it out, the vacuum modulator was leaking tranny fluid into the lines.
Well, anyway, I was just reading through 'Turbochargers' by Hugh MacInnes, and it suggests the same thing. It suggests putting a 0.050-inch orifice in the line leading to the tranny. After that orifice, install a 'T' with a check valve in it to vent excess pressure. It suggests that the other side of the check valve be vented to the air cleaner, but this is because they are talking about a draw through carb system. In that case, you would be venting fuel as well as air.
It also mentions the B&M mechanical modulator called Modulink. This book is a bit old, so it may no longer be available, or may be called something different by now. Good Luck.


no longer in production. lf you have to use one gm has one for the allison 545 diesel, but it is big jk
john im happy to see you here.i would again like to thank you for all the information you have given me over the years ,especially the planetary gear have allowed me to go further with what you gave me for free.everybody if you dont know this guy ,you should.he is one of the smartest people i have ever met in respect to automatic transmissions.
Welcome John Kilgore- i ve read and used several of your transmission mods and installations.
Here are a few I've seen on the net
TH400 Mechanical modulator, Teckpak Fitzall®,
"#4X. 350 / 400 Cable Operated Modulator, no vacuum needed. This works well for superchargers and on diesel motors without a vacuum regulator. This modulator uses a 700R4 throttle valve cable. Cost $120.00 "
400 Transmission 350 Transmissions 400 Transmission 350 Transmissions 400 Transmission 350 Transmissions 400 Transmission 350
I've talked to the Raptor guys in the past and they had it in stock so its available or was about 8 months ago. This constantly sits on my backburner as a backup to the TH200-4R if I have to scrap the nailhead early due to actual or impending engine failure in time for Powertour 2008.