TH400 shift problems
Thread Starter
Heel & Toe
Joined: Feb 2002
Posts: 18
Likes: 0
From: Slagelse, Denmark
TH400 shift problems
Hi
I'm having problems with my TH400, it's been leaking alot and one day when I was driving the following happened :
I accelerated and after that the tranny would stay in 1.st gear and would not upshift. I checked the oil and it was low, so I added oil but the problem was still there.
I can now get it to upshift from 1st. to 3, if I accelerate, but it will not go into 2 gear.
What's the problem ?
P.S.
I have replaced the modulator.
I'm having problems with my TH400, it's been leaking alot and one day when I was driving the following happened :
I accelerated and after that the tranny would stay in 1.st gear and would not upshift. I checked the oil and it was low, so I added oil but the problem was still there.
I can now get it to upshift from 1st. to 3, if I accelerate, but it will not go into 2 gear.
What's the problem ?
P.S.
I have replaced the modulator.
Le Mans Master
Joined: Sep 2000
Posts: 5,635
Likes: 4
From: Orlando Floriduh
Re: TH400 shift problems (Jesper)
The TH400 is modulated two ways.. The vacuum modulator for low speed up and downshifts. Then at wide open throttle it is solenoid detent controlled via 12 V soleinoid that is in the transmission. It is energized by switch at the carb or under the gas pedal. If this switch is stuck, mal adjusted or otherwise sending 12v to the solenoid the trans will not shift out of first except at the extreme upper rpm and then it usually goes into third. This is what also controls high speed forced downshifts that mant TH400 owners complain are non existant
[Modified by bluevetteman, 9:38 AM 4/5/2004]
[Modified by bluevetteman, 9:38 AM 4/5/2004]
Safety Car
Joined: Jun 2001
Posts: 4,004
Likes: 2
From: Severna Park Md
Re: TH400 shift problems (Jesper)
Sounds like maybe its not getting vacuum to the modulator. Check to see if the hose slipped off. Also if the modulator diaphram goes bad it will act up. It can even suck trans fluid up thru the hose and into the intake and burn it in the combustion chamber.
Instructor
Joined: Mar 2003
Posts: 217
Likes: 1
From: Buffalo MO
Re: TH400 shift problems (Jvette73)
hey - on second thought---- second is applied w/ a "sprag" (or, one way roller clutch), not a band as i was thinkin b4..... i'm gonna say that ur problem is that the sprag has "givin it up". if that is the case, ur in line for an overhaul in order to get it repaired. ~brian~
Thread Starter
Heel & Toe
Joined: Feb 2002
Posts: 18
Likes: 0
From: Slagelse, Denmark
Re: TH400 shift problems (Jesper)
Thanks for the help guys.
I will check the solenoid detent switch.
Here is a little more info on what is happening :
The transmission will not upshift from 1 to 2 gear at any speed, I can get it to upshift to 3, if I accelerate harder.
When in 3 gear the transmission will drop into 1 gear if I get below a certain speed.
Anyone else got any ideas ?
I will check the solenoid detent switch.
Here is a little more info on what is happening :
The transmission will not upshift from 1 to 2 gear at any speed, I can get it to upshift to 3, if I accelerate harder.
When in 3 gear the transmission will drop into 1 gear if I get below a certain speed.
Anyone else got any ideas ?
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
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
Melting Slicks
Joined: Jun 2003
Posts: 2,802
Likes: 241
From: Niles, MI
Re: TH400 shift problems (Jesper)
I'll just second what was mentioned above. It's quite common for the diaphram in the vacuum modulator to start to leak. When this happens the engine will suck transmission fluid into the intake manifold and burn it. The transmission will also stop shifting. You should take care of this right away. If let go it will burn up the clutches in the transmission.
Just thought of something else. Shift points are also controlled by a governor. If the gear gets stripped on the governor the trans will stop shifting
[Modified by Kid Vette, 8:49 AM 4/9/2004]
Just thought of something else. Shift points are also controlled by a governor. If the gear gets stripped on the governor the trans will stop shifting
[Modified by Kid Vette, 8:49 AM 4/9/2004]
Thread Starter
Heel & Toe
Joined: Feb 2002
Posts: 18
Likes: 0
From: Slagelse, Denmark
Re: TH400 shift problems (Kid Vette)
Hmm, I have changed the modulator, so it's not that.
So what I've got now is : governor or switch at the gas pedal.
Thanks I'll check that.
So what I've got now is : governor or switch at the gas pedal.
Thanks I'll check that.
Instructor
Joined: Mar 2003
Posts: 217
Likes: 1
From: Buffalo MO
Re: TH400 shift problems (Jesper)
im sure u think im nuts, since i stated that 2nd was applied w/ a band, then changed my mind... but, im sure that u will find that since it "skips" 2nd all together, that your roller clutch (sprag) is broken. when this happens, intermediate(2nd) is not workable, so therefore, it shifts on to high gear(3rd). IF u had a modulation/gov/kickdown problem, 2nd would be that at some point and time. since it NEVER shifts into 2nd,,,,the sprag is your problem. im sure that if you call any reputable tranny shop, they will verify what i am telling you about no second gear. it is a common problem in 400's.
Instructor
Joined: Mar 2003
Posts: 217
Likes: 1
From: Buffalo MO
Re: TH400 shift problems (Jesper)
"hmm, okay but 2nd is working if I shift it manually ?"
i didnt realize that---- so...
well then, in that case.... u have two things it could be.... 1) modulation problem>>> lack o vac etc, or..and MOST LIKELY #2) governor problems... the problem w/ gov problems, is the main reason they "hang up" is because of "debris" in the trans. metal/material floating around in the trans can only come from one source, the trans itself. (this is because the friction plates/ reaction plates never actually touch....the fluid keeps this from occuring) that usually (not always) means, time for an overhaul.
OR.. u can remove the gov by simply removing the cover and pulling it from car. unfortunatly, in a vette, this usually requires the removal of the crossmember to allow the rear of trans to drop enough to remove gov. if u hold the gov vertically, w/ weights at bottom, (gear @ top) then push weights up, the valve inside should raise and lower w/ ease. then, w/ weights in up postition, u should be able to push up on the inside lever, and then it should return to its original postion by no other force than gravity.
gl-bk
i didnt realize that---- so...
well then, in that case.... u have two things it could be.... 1) modulation problem>>> lack o vac etc, or..and MOST LIKELY #2) governor problems... the problem w/ gov problems, is the main reason they "hang up" is because of "debris" in the trans. metal/material floating around in the trans can only come from one source, the trans itself. (this is because the friction plates/ reaction plates never actually touch....the fluid keeps this from occuring) that usually (not always) means, time for an overhaul.
OR.. u can remove the gov by simply removing the cover and pulling it from car. unfortunatly, in a vette, this usually requires the removal of the crossmember to allow the rear of trans to drop enough to remove gov. if u hold the gov vertically, w/ weights at bottom, (gear @ top) then push weights up, the valve inside should raise and lower w/ ease. then, w/ weights in up postition, u should be able to push up on the inside lever, and then it should return to its original postion by no other force than gravity.
gl-bk
Le Mans Master
Joined: Oct 2002
Posts: 5,714
Likes: 6
From: Silver City NM
St. Jude Donor '05-'07
Re: TH400 shift problems (Jesper)
Had exact same problem in my '72 w/TH400. Wouldn't shift out of 1st gear. Replaced modulator...poof...problem solved. Check to make sure you have vacuum at modulator. You can disconnect the electric kickdown switch on the gas pedal and run without it. Just downshift manually if you want to downshift.
Dep
[Modified by DJ Dep, 9:46 PM 4/11/2004]
Dep
[Modified by DJ Dep, 9:46 PM 4/11/2004]
