TH400 upshift problems - Hot Rod Forum : Hotrodders Bulletin Board -- Hot Rod Forum

Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Transmission - Rearend
User Name
lost password?   |   register now

LinkBack Thread Tools
  #1 (permalink)  
Old 03-28-2007, 08:17 PM
Registered User
Join Date: Sep 2006
Location: Doylestown, Pa
Posts: 6
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
TH400 upshift problems

I have a TH400 behind a mild 350 sbc with a 4.11 rear. When I ran a 0 to 60 recently the tranny would not shift from 1st to 2nd without letting up on the throttle. As soon I put the hammer down again it then downshifts from 2nd to 1st. At this point the engine is screaming past 6000 rpm (I progressively adjusted the tach shift light until I stopped at 6000 rpm). I have the tranny out of the car now and the governor seems to be O.K. I do have a Lokar downshift kit on it and I am wondering if this is causing the problem. It seems to be adjusted according to the instructions. The only other issue may be the vacuum line to the modulator. It Y's off to a vacuum gauge at one point. I was wondering if that may be slightly reducing the vacuum to the modulator. Thanks in advance.


Reply With Quote Quick reply to this message
Sponsored Links
  #2 (permalink)  
Old 03-31-2007, 12:49 AM
Registered User
Last photo:
Join Date: Mar 2007
Location: canada
Posts: 30
Wiki Edits: 0

Thanks: 1
Thanked 2 Times in 2 Posts
sounds to me like you have too high line pressure causing the governer to down shift
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 03-31-2007, 01:00 AM
Brian_B's Avatar
Lost in the 50s!

Last journal entry: time to start body work
Last photo:
Join Date: Jan 2006
Location: Mountain Home, AR
Age: 50
Posts: 759
Wiki Edits: 0

Thanks: 0
Thanked 1 Time in 1 Post
Are you sure the modulator valve is indeed OK? They can cause some wacky shifting conditions. Check the end of the vacuum line (that went to it) and see if it looks moist up in there.

Also, before you pulled the trans, did you disconnect the lokar downshift "kit" to make sure it was not the problem?

We had issues with a C5 (ford) trans and simply unhooked the kickdown for our testing. Yep...that was it (in that particular case).
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 03-31-2007, 06:50 AM
Registered User
Join Date: Feb 2007
Location: MN
Posts: 46
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts

I would agree with the vacuum to the vacuum modulator. That's what they do.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 03-31-2007, 07:42 AM
Crosley's Avatar
Global Moderator
Last wiki edit: T-350 rebuild tech
Last journal entry: 1962 Morris Minor 1000
Last photo:
Join Date: Nov 2002
Location: ... USA
Posts: 9,273
Wiki Edits: 1659

Thanks: 960
Thanked 301 Times in 265 Posts
every body has addressed the manifold vacuum feed directly to the modulator so I will not.

What is the history of this trans? Recent build? Old timer?

Did the trans operated fine then start this problem?

It sounds like the kick down switch could be activated , leaking , broken .

Next place I would look is the shifter linkage for proper adjustment

Governor has no problems as you mention? Plastic gear is tight , springs and weights are OK?

Dont worry how old you are... it is the mileage on your body that adds up.
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 03-31-2007, 03:34 PM
Registered User
Join Date: Sep 2006
Location: Doylestown, Pa
Posts: 6
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Th400 upshift problems

Regarding the history of the trans I purchased the rod a few years back. In the paperwork supplied with the car it said TH400 trans rebuilt heavy duty. Don't have clue what that entailed. It has a stamped tag on the side which says 086 in the middle and below it says 86-FMA and 58459. I think the rod was built about 1990. I have no contact with the original owner/builder.
I never really did any 0 to 60s until last fall.
I added the Lokar downshift kit during the summer. The downshift kit is just a plunger connected to a cable which in turn connects to the throttle on the carburetor. When you push the throttle to the floor the plunger activates a switch which then energizes the solenoid on the side of the trans. I spoke with one of the local rodders and he seems to think its being caused by the downshift kit. He ended up taking off his kit. I checked the archives here and at one point someone suggested that the engaging the downshift solenoid somehow raises the shift point. I just can't understand how this would have worked in its original car/setup.
As far as the shift linkage goes there may the possibility it could be moving the linkage with the power on and power off. The engine mounts are new but poorly designed with lots of free play. I will look at that possibility when I put everything back together.
The governor appears to be in excellent condition. No wear on the gear and the plunger down the center and the weights seem to operate smoothly. Last fall before I pulled the engine & tranny apart I bought a B&M governor counterweight shift kit. I presumed before hand that someone already modified the weights to get the high shift point. Much to my dismay the governor appears to be stock. The real problem with this rod is that it is very difficult to do any mods to the trans (do to the enclosed cross member) once its in the car. So I hope to get it right before it goes together.
Anyway thanks for everyones advice. I think I will implement the changes one by one until I get to the bottom of the problem.

Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 04-01-2007, 05:32 AM
coupe66's Avatar
Registered User
Join Date: Oct 2005
Location: Stockholm
Age: 64
Posts: 82
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
From the "Ron Sessions" manual...
No 1-2 shift, Governor malfunktion.
-Check free movment
-Check for jammed weights
-Check for worn or loose nylon governor gear
-Check for loose or worn stop pin

-Could also be valve body, loose feed pipes, loose bolts, clogged pipes, stuck valvetrain.

-Or, faulty detent solenoid operation

Reply With Quote Quick reply to this message

Recent Transmission - Rearend posts with photos

Quick Reply

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:
Please select your insurance company (Optional)


Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
Thread Tools

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
TH400 problems Silver00V6 Transmission - Rearend 5 03-14-2007 07:57 PM
TH400 Shifting Problems vwmusky Transmission - Rearend 3 02-22-2007 07:44 AM
4L80 to TH400 swap Jason Transmission - Rearend 4 06-14-2003 06:46 PM
67 firebird th400 info speedfrk3 Transmission - Rearend 2 12-10-2002 05:31 PM
TH400 in firebird v/s TH400 in bonniville help speedfrk3 Transmission - Rearend 11 11-04-2002 11:19 AM

All times are GMT -6. The time now is 02:20 AM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999 - 2012. All Rights Reserved.