Th400 wont go into 3rd

[Tim219]

I recently purchased this 1983 K10. It has a new 6.0 LS motor with a TH400. The truck will not go into 3rd gear. Sometimes I think it's trying to grab 3rd when the tranny is cold, but slips right out. I feel I have done all the recommended suggestions on YouTube with no results:
1. Changed the fluid
2. Adjusted the Vacuum modulator; had no change in either direction, so I changed it. Still no changes in shift strength or point when it shifts.
3: I removed the governor and found the valve was stuck. Disassembled and cleaned. Still no change. I ordered the kit to adjust the weights. I put the heaviest weights on and was going to adjust if I noticed a change, but it made absolutely no difference.
4: I verified the shifter was aligned correctly.

I am at a loss of what to try ot look at next.

 

[Sad Sack]

Throw a "Hello" greeting and pics of truck here and hopefully someone will chime in, intros bring comments.

 

[squaredeal91]

Got pictures. Transmission cooler could be plugged up. But if it was that it would feel funny in other gears and shifting. Possibly a shot transmission. Pictures?
How did the transmission feel when you first got it?

 

[squaredeal91]

Will it manually shift from 2nd to 3rd?

 

[Matt69olds]

Unplug the electrical connector near the shift linkage. If it goes into 3rd, the kickdown switch is defective or not adjusted correctly.

 

[Tim219]

I know the tranny cooler isn't plugged because I opened the return line after the cooler and flushed it when I changed the fluid. Thanks for the quick replies. I'm going to go try the electrical connector now and hope to reply in 20 minutes. The truck had a few issues when I purchased it, such as the 3rd gear. I figured it was an easy fix when I purchased it because the guy was mechanically ignorant. Such as the Governor. It shifts nicely from 1-2 and will not shift to 3rd even when doing it manually 1-2-D.

 

[Tim219]

Unplug the electrical connector near the shift linkage. If it goes into 3rd, the kickdown switch is defective or not adjusted correctly.

Unplugged the one wire and no change

 

[Rustisbest]

One of the springs in the valve body broke in mine on the test drive after a rebuild earlier this year. Shifted a couple times, the spring broke, then got wedged in there where the 2-3 valve couldn't move. Fun times lol

 

[Tim219]

One of the springs in the valve body broke in mine on the test drive after a rebuild earlier this year. Shifted a couple times, the spring broke, then got wedged in there where the 2-3 valve couldn't move. Fun times lol

I dont mind the work, its a hobby truck for me. I dont know crap when comes to trannies though. Fun to learn. Is a Valve body a pain? I also seen a video there is a filter to the governor. Wondered if that could be clogged? Either way I think the VB has to come off to change that.

 

[Rustisbest]

I dont mind the work, its a hobby truck for me. I dont know crap when comes to trannies though. Fun to learn. Is a Valve body a pain? I also seen a video there is a filter to the governor. Wondered if that could be clogged? Either way I think the VB has to come off to change that.

Couldn't tell you since I've only done a shift kit on a TH350 a time or 2. I had paid a local guy to build it because I really dont have a good space to rebuild one. I just pulled it back out and let him figure out what it was. $hit happens sometimes lol

 

[Sad Sack]

I tend to ask CoPilot a lot of questions.....I sit at a computer all day so the opportunity to use AI is very doable and then develop a plan of attack if needed.




If your GM TH400 transmission won't shift from 2nd to 3rd gear, there are several common causes and troubleshooting steps you can take. Here's a breakdown based on expert sources and community experiences: [chevelles.com], [monstertra...ission.com], [itstillruns.com], [chevelles.com], [67-72chevytrucks.com]

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Common Causes & Fixes for No 2-3 Shift in TH400​

1. Vacuum Modulator Issues​

• The vacuum modulator controls shift timing. If it's faulty or disconnected, the transmission may not shift properly.
• Fix: Check the vacuum line for cracks or disconnections. Replace the modulator if it's not holding vacuum.

2. Governor Problems​

• The governor regulates shift points based on vehicle speed. A stuck or dirty governor can prevent the 2-3 shift.
• Fix: Remove and clean the governor. Ensure the weights and spool valve move freely.

3. Valve Body Malfunction​

• A misassembled or dirty valve body can block fluid flow to the 3rd gear clutch pack.
• Fix: Inspect the valve body, especially the 2-3 shift valve. Reassemble carefully and clean all passages.

4. Direct Clutch Pack Failure​

• The 3rd gear relies on the direct clutch pack. If it's worn or burned out, the transmission won’t engage 3rd.
• Fix: Drop the pan and inspect for clutch debris. A rebuild may be necessary if the clutch pack is damaged.

5. Incorrect Dual Feed Mod​

• If you've modified the transmission for dual feed and didn’t plug the reverse feed hole, it can cause 3rd gear failure.
• Fix: Verify the case modifications and ensure all necessary passages are properly sealed.

6. Low or Contaminated Transmission Fluid​

• Dirty or low fluid can affect hydraulic pressure and clutch engagement.
• Fix: Check fluid level and condition. Replace with fresh ATF if needed.

7. 2-3 Accumulator Issues​

• A plugged or incorrectly assembled accumulator can block fluid flow to the 3rd gear.
• Fix: Inspect the accumulator piston and spring. Ensure it's installed correctly or consider removing the spring for firmer shifts.

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Diagnostic Tips​

• Air Test the clutch packs to verify engagement.
• Use a pressure gauge to check line pressure at various gears.
• Test drive under different throttle conditions to see if the shift occurs manually or at higher RPMs.

 

[dragstews]

4. Direct Clutch Pack Failure​

• The 3rd gear relies on the direct clutch pack. If it's worn or burned out, the transmission won’t engage 3rd.
• Fix: Drop the pan and inspect for clutch debris. A rebuild may be necessary if the clutch pack is damaged.

Shoot, 400's are fairly easy to rebuild ..
A few upgrades can be performed while it's guts are spread out on the table ..

Go for it ... It'll be fun ..
We'll help ya, won't we boys ?

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No 'YoYo'
Man, if that ain't been a few years ago ..

 

[idahovette]

Got one of "Mr. Shift" kits in my stash.....it's in a Green box

 

[dragstews]

Now a days it's Superior that seems to have a handle on the aftermarket kits ..

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'Gill Younger' days was in the 50's, 60's and into the 70's ...
RIP - A definite legend in the transmission industry.

But, what I'm getting at is the tranny sounds tried to me ?
With the governor being stuck, debris is evident ...

A full tear down, rebuild might be in order ?

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THM 400 big brother ^^ 4L80E ..

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I turned it into a 4L85E with a world of mods ..

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Did retired this ^ ... Auto took over ..

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[dragstews]

Ya know, might be a idea to upgrade to a trans with overdrive and lock-up converter ?

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Better gas mileage would be a huge plus ..

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