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

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.

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

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

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

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

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

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?

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

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

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

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

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

Low on fluid. Needs to be filled while running.

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

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

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

I did not mess with the filter.

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

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.

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

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.

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

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.

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

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.

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

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

Something happened there, theres no other answer...

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

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.

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

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.

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

Does Dexron mix with Type F ?

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

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

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

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.

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

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.

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

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.

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

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.

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

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.

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

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

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

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.