T 400 shifting problems
Thread Starter
Intermediate
Joined: Jun 2018
Posts: 48
Likes: 5
From: Detroit
T 400 shifting problems
Hey all.
Just bought a '69 with a 427,.. a TH-400,... and a 3.08 Posi
The trans has good looking fluid in it,.. right in between the marks.
When I drive it it stays in each gear a good while,... not shifting into high until you're at 55 I think. (But the engine was only at 2,500 rpm in second when it shifted,.. so it's fine I suppose.
BUT,. when it does shift,.. it's quite firm.
And when you lift off the gas,... it doesn't up-shift like you'd expect,... rather, the engine practically goes to idle. As if the torque converter was one-way, with a free-wheeling clutch. Get back on the gas and the engine revs up to where it was before and you resume as normal.
(Car has only been driven 290 miles in the last 14 years)
Is this a modulator issue? Kick-down vacuum line?
Thanks in advance for any help.
Just bought a '69 with a 427,.. a TH-400,... and a 3.08 Posi
The trans has good looking fluid in it,.. right in between the marks.
When I drive it it stays in each gear a good while,... not shifting into high until you're at 55 I think. (But the engine was only at 2,500 rpm in second when it shifted,.. so it's fine I suppose.
BUT,. when it does shift,.. it's quite firm.
And when you lift off the gas,... it doesn't up-shift like you'd expect,... rather, the engine practically goes to idle. As if the torque converter was one-way, with a free-wheeling clutch. Get back on the gas and the engine revs up to where it was before and you resume as normal.
(Car has only been driven 290 miles in the last 14 years)
Is this a modulator issue? Kick-down vacuum line?
Thanks in advance for any help.
Team Owner
Joined: Jan 2008
Posts: 30,894
Likes: 1,189
From: chicago
pull the vacuum hose off the modulator,,is it wet with trans fluid?
you couldn't take a few pics top and bottom of the metal vacuum line could you?,i need the routing for my 69 427,and where the bracket that holds it to the trans [which hole is it in too]
you couldn't take a few pics top and bottom of the metal vacuum line could you?,i need the routing for my 69 427,and where the bracket that holds it to the trans [which hole is it in too]
2026 Loser of the Year
Joined: Sep 2013
Posts: 36,559
Likes: 7,006
From: New Or-leens Loo-z-anna
I had the same problem with a Ford transmission. It had an intermediate band adjusting screw which I was able to tighten according to the repair manual. It shifted CORRECTLY after a few trial adjustments. I do not know if TH400 bands are adjustable. If the previous owner of your car installed a "shift kit" it is possible one of the "valves" in the valve body has a weak spring which is causing the valve to "stick" and it is not moving far enough to allow good fluid flow.
Do you have any idea what the "stall" rpm on the torque converter is....A high stall may affect the off-throttle response.
OR......It could be as simple as a leaking seal inside the port that the filter tube fits into.
That' my rookie guesses.
Do you have any idea what the "stall" rpm on the torque converter is....A high stall may affect the off-throttle response.
OR......It could be as simple as a leaking seal inside the port that the filter tube fits into.
That' my rookie guesses.
Last edited by doorgunner; Jul 8, 2018 at 08:36 PM.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
Modulator would make it think you are hard on the gas. Shift late and hard. Vacuum pulls on a thingy. If diaphragm is blown or stuck, it doesn't move the thingy. It assumes low vacuum which means heavy throttle. Also, worth checking is a solenoid and kickdown valve in valve body. I had a 65 Catalina vert got for 300 bucks cuz the valve was stuck open. Stayed kicked down. 1st to 2nd about 50, never hit 3rd til I popped valve back in place. Sold it for 500 bucks...
Last edited by derekderek; Jul 8, 2018 at 09:00 PM.
Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
I'd definitely check the modulator adjustment first, make sure the vacuum hose to the modulator is dry etc... Then I would likely pull the governor out and check/replace it as it may be sticking. Ofcourse there could be an internal trans issue as well but, I'd check the easy things first.
Le Mans Master
Joined: Sep 2013
Posts: 6,192
Likes: 750
From: near the thumb in the mitten
Transmission uses vacuum modulator to shift. Make sure strong vacuum is present at modulator under car. If you have vacuum pump you can check operation and if it holds vacuum. Behind cover held on by 4 bolts is governor inspect weights, springs, and gear for sticking, missing, or wear. Shift kit? Transmission uses " kick down" solenoid for forced downshift.
Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
Transmission uses vacuum modulator to shift. Make sure strong vacuum is present at modulator under car. If you have vacuum pump you can check operation and if it holds vacuum. Behind cover held on by 4 bolts is governor inspect weights, springs, and gear for sticking, missing, or wear. Shift kit? Transmission uses " kick down" solenoid for forced downshift.
This just reminded me!!
Check your kick down switch/solenoid. If its stuck in the "kick down" position, it WILL do weird **** like you say. It will act like it free wheels in neutral and shifts to 3rd at really high rpms.
I'd bet you lunch, this is your problem. Its stuck in "passing gear". A quick test for this is unplug the kick down wire off of the left side of the transmission and drive it. This will tell you real quick.
Last edited by ajrothm; Jul 8, 2018 at 09:42 PM.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
Thread Starter
Intermediate
Joined: Jun 2018
Posts: 48
Likes: 5
From: Detroit
Thanks guys. I was thinking it might be the kick-down tube. If that has a vac leak,.. it would think you're harder on the gas than you are,.. and shift late,.. and hard.
The 2" piece of rubber hose that connects the tube to the governor on the trans was split. I replaced it with a couple inches of fuel line I had around,. and now it shifts perfectly. (Well, it seems to. I don;t have plates yet,.. so I'm just cruising around in my sub).
The 2" piece of rubber hose that connects the tube to the governor on the trans was split. I replaced it with a couple inches of fuel line I had around,. and now it shifts perfectly. (Well, it seems to. I don;t have plates yet,.. so I'm just cruising around in my sub).
Thread Starter
Intermediate
Joined: Jun 2018
Posts: 48
Likes: 5
From: Detroit
I only see one bracket of any sort on mine. It's a small thing crimped onto the line down where the bell-housing meets the block. It is held on by one of the trans to engine bolts. Then at the top and bottom it is just held on with the hose couplers. I couldn't get a focused pic of that bracket.
Last edited by Duckstu; Jul 8, 2018 at 10:57 PM.
Race Director
Joined: Jan 2000
Posts: 13,014
Likes: 2,257
From: Corsicana, Tx
2020 C2 of the Year - Modified Winner
2020 Corvette of the Year (performance mods)
C2 of Year Winner (performance mods) 2019
2017 C2 of Year Finalist
That would do it. That line actually goes to the modulator...not the governor.
Now go have some fun with it!
JIM
Now go have some fun with it!
JIM