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.

[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.

[mike16]

Originally Posted by Joyridin

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

I doubt it.

Any transmission fluid has the characteristic of being able to clean out varnish and particles in a transmission. On older transmissions it's more pronounced. That's why it was used to " clean" lifters in noisy valve trains.

When you change over to new fluid it will flush out or loosen that varnish or debris.

The particles loosened when the new fluid circulates can jam up the operation of valves and pistons . This will cause shifting problems.

Time for a rebuild and proper periodic servicing in the future.

Synthetic transmission fluid is not your problem. That may have been a cause for engine seal leaks back through the mid 90's but how it became a transmission problem currently is unclear.

[Stocker]

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

[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.

[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)

[HeavyHauler]

Originally Posted by Shifty One

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

I did not mess with the filter.

[BDC90]

Low on fluid. Needs to be filled while running.

[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.

[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.

[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.

[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.

[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.

[Rust_never_sleeps]

Does Dexron mix with Type F ?

[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.

[HeavyHauler]

Well I got the trans pan sealed tight with zero leaks. I started the truck and put in 5 quarts while it was in neutral. I let the truck warm up and made sure the dipstick was reading at the full mark.

Truck still doesnt move. I shifted through all of the gears and nothing. The trans is original to the truck and has ALOT of miles on it. i cant think of anything else to check. I guess it need to be rebuilt.

Instead of rebuilding it though I might just get a rebuilt 4L80E instead and just put the TH400 in storage.

I am bummed out though as I was hoping to cruise it around some more this summer.

[67 twins]

Originally Posted by 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.

I would have sworn it was green.

[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.