Changed trans fluid in my TH400 and now it wants to slip and not get into gear.

[HeavyHauler]

I recently changed the trans fluid in the TH400 of my 1968 C10 shortbed fleetside and now it wants to slip and not get into gear. I feared this was going to happen. I decided to fix a trans pan gasket leak and changed the fluid in the process. The trans has alot of miles on it and most likely needs rebuilt but I was hoping to get some more cruising in before swapping my factory 396/TH400 for a 454/4L80E combo. So that I could rebuild my original drivetrain and set it aside if I wanted to return it to factory specs in the future.


Any suggestions about my TH400.

[Joyridin]

Just checking, but you didn't put synthetic fluid in it by chance? That would definitely make it slip.

[Stocker]

No clue if this might apply to your TH400 -- my TH350 acts exactly as you describe if it is low on fluid.

[MARKDTN]

Good suggestions. You do want Dextron III not VI. Needs to be full warm running in neutral on flat ground. Back in the day we used to put some type F fluid in to make them shift a little harder. Might try that if all else fails. Is it possible that the filter is not on fully?

[Rust_never_sleeps]

Everyone's on this, but how much fluid did you put in vs the rated capacity?

[Shifty One]

Is it sucking air maybe (filter not tight or similar)

[BDC90]

Low on fluid. Needs to be filled while running.

[HeavyHauler]

Originally Posted by Joyridin

Just checking, but you didn't put synthetic fluid in it by chance? That would definitely make it slip.

Definitely not. I put in Type F. Thanks for asking as that is a good question.

[HeavyHauler]

Originally Posted by Shifty One

Is it sucking air maybe (filter not tight or similar)

I did not mess with the filter.

[HeavyHauler]

Originally Posted by BDC90

Low on fluid. Needs to be filled while running.

I did not fill it while running. I put in 4 quarts of fluid. When it wouldnt move after I started it up and let it run for about 3 minutes or so I shut it off and just left it parked in my garage and havent tried again since.

[HeavyHauler]

Originally Posted by Stocker

No clue if this might apply to your TH400 -- my TH350 acts exactly as you describe if it is low on fluid.

I put in 4 quarts of fluid. The only thing I did was drain and remove the trans pan to replace the trans pan gasket because it was leaking.

[Stocker]

Originally Posted by HeavyHauler

I did not fill it while running.

You got good advice above. To check for correct fluid level, park on a level surface, engine warmed up & idling, trans in neutral (actually, Park is OK too). Otherwise you are just guessing.

[MARKDTN]

Likely low on fluid. Depending on pan could be 4-8 qts. If you can't see any on the stick on level ground and cold you don't have enough yet. I'd think at least 5 if not 6 qts.

How did the old fluid look? If I am looking at a filter in a transmission, I'm going to change it except in some very rare set of circumstances.

[68Gold/white]

Drop the pan and make sure the filter is properly attached to the trans.

Something happened there, theres no other answer...

[70STOVEBOLT]

The other answer is that it is low on fluid. There is going to be more than 4 QTs of fluid that comes out. It will likely take 9-10 QTs. Remember, fluid drains out of the trans back into the pan when the pump is not turning. Just because the pan only holds 4 QTs of fluid does not mean that that is all that is in the trans.

[70STOVEBOLT]

On another note, Type F will not hurt the trans, it will cause firmer shifts. The issue is, you didn’t drain the torque converter, so if you had dexron in it before they are now mixed, AND, you now have old dexron mixed with new Type F, which kind of defeats the purpose of changing the fluid. The torque converter holds a few QTs of fluid. I would start it up on level ground let it warm up and add fluid until you see it at the full mark on the dipstick, I think you will find it cures your issue.

[Rust_never_sleeps]

Does Dexron mix with Type F ?

[HeavyHauler]

Originally Posted by 70STOVEBOLT

On another note, Type F will not hurt the trans, it will cause firmer shifts. The issue is, you didn’t drain the torque converter, so if you had dexron in it before they are now mixed, AND, you now have old dexron mixed with new Type F, which kind of defeats the purpose of changing the fluid. The torque converter holds a few QTs of fluid. I would start it up on level ground let it warm up and add fluid until you see it at the full mark on the dipstick, I think you will find it cures your issue.

It has always been Type F that I have used and only Type F.

[HeavyHauler]

Originally Posted by MARKDTN

Likely low on fluid. Depending on pan could be 4-8 qts. If you can't see any on the stick on level ground and cold you don't have enough yet. I'd think at least 5 if not 6 qts.

How did the old fluid look? If I am looking at a filter in a transmission, I'm going to change it except in some very rare set of circumstances.

I changed the filter and fluid using type F approximately 6 months ago. I am the one who probably caused the leak around my pan due to installing a rubber trans pan seal that never did seal very well and or my pan not being flat as I didn't check that.

I will double check the fluid this weekend with it running. I will need to find a good cork gasket instead.

[57taskforce]

Dex 6 won’t hurt it. All this anti synthetic bs is amusing. Dex 6 is so far superior in every way to dex 3 and its reverse compatible meaning when the converter is still full of 3 you can safely run 6 on fluid and filter change. I run it in all my old automatics. It’s far more resistant to heat, the viscosity is more consistent, and it doesn’t foam near as much as 3. Sorry had to vent lol, I see these kinds of posts on some of the other boards I’m on and it drives me nuts how some of these opinions and understandings are skewed.

[72c20customcamper]

Originally Posted by 57taskforce

Dex 6 won’t hurt it. All this anti synthetic bs is amusing. Dex 6 is so far superior in every way to dex 3 and its reverse compatible meaning when the converter is still full of 3 you can safely run 6 on fluid and filter change. I run it in all my old automatics. It’s far more resistant to heat, the viscosity is more consistent, and it doesn’t foam near as much as 3. Sorry had to vent lol, I see these kinds of posts on some of the other boards I’m on and it drives me nuts how some of these opinions and understandings are skewed.

When I changed out the converter to a 2400 stall (th400) the instructions said it would void the warranty if synthetic was used . Only type F and Dextron III were acceptable. Converter was built for me by Yank.

Not saying you’re incorrect just my experience.

[SCOTI]

Originally Posted by 72c20customcamper

When I changed out the converter to a 2400 stall (th400) the instructions said it would void the warranty if synthetic was used . Only type F and Dextron III were acceptable. Converter was built for me by Yank.

Not saying you’re incorrect just my experience.

There are similar disclaimers for not allowing synthetic rear-end lubes as well as there's apparently 'too much' slippage on clutch material causing more harm than reducing temps so fluid is based on the differential unit.

[57taskforce]

Originally Posted by 72c20customcamper

When I changed out the converter to a 2400 stall (th400) the instructions said it would void the warranty if synthetic was used . Only type F and Dextron III were acceptable. Converter was built for me by Yank.

Not saying you’re incorrect just my experience.

I run it in an original factory built (minus fluid and filter change) 700R4 in my IROC-Z, The th400 in my square body and the 4l80E which is also still a factory build in my K2500 HD. All originally spec’d for Dex2 and 3. They run cooler and the fluid says cleaner. I’ve got 100,000+ miles collectively on these with 6 and not a single negative thing to report, And I would if there was issues. It dropped the average towing temps in my K2500 by close to 10*.

I understand the warranty concerns for sure, and don’t blame you for not voiding that for what it’s worth.

[70STOVEBOLT]

Originally Posted by Rust_never_sleeps

Does Dexron mix with Type F ?

Shouldn’t cause any issues, but I wouldn’t do it deliberately and only short term.

[Steeveedee]

B&M Trick Shift was about the same as Type F except it was dyed blue. I ran it for years in my hot rods after putting in shift kits. Don't know if it is still blue; it's been awhile.