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.

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

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.

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

Did you check the fluid with it level, running, and in park? I see you made mention of neutral but not park. I don’t think you’d loose forward and reverse both because of clean fluid, and definitely not right away on start up. I could see maybe after a few hundred miles of the clean fluid circulating thru the system but not immediately on start up. TH400’s are tough as nails. I bet it’s still low on fluid.

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

Just a Suggestion.

I would check and change the filter and the O-rings on the filter tube.

Those filters are what I call a Floating filter because it is mounted with a Shoulder bolt in most applications.

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

Sorry, but this makes zero sense. Even if you changed the fluid, the truck would still move if you put the correct amount of fluid back in. It doesn't just quit working the minute the fluid is changed.

Keep adding more fluid. Maybe the dipstick is wrong for some reason. I would have never dropped the pan without changing the filter. Maybe the filter is plugged?

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

>50% of the time.... If the fluid & filter is changed on a trans that did not receive 'regular' maintenance (~80-100k miles), it will start slipping/acting-up. I heard this as a youth & called BS. But I learned. I did 'proper' maintenance on new additions to my fleet (3 different rides), inevitably the trans started slipping within miles when I did trans service.

After No.3, I learned to leave them alone. You can get away with a slow-mo fluid change on extended milage transmissions (drain & add fresh fluid every X-miles) but a filter w/complete fluid flush seemed like the beginning of the end-o-life from the units I experienced.

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

It does appear that 6 is backwards compatible BUT the viscosity is lower so it may not pump as well in a worn transmission. I'll have to consider that next time I need to change fluid in an older trans.

As far as the post above, I have not had that experience with immediate failure. I too would have changed the filter while I was looking at it, but I have changed fluid (and filter) in several older transmissions without any failures. Guess it's the luck of the draw.

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

Probably correct (luck of the draw)....

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

Yes, I know that, but that isn't what happened here. He didn't even really change the fluid except what was supposedly in the pan. There is no reason if all he did was drop the pan, seal it, then glue it back in place and add some fluid the trans won't move or at least jar the vehicle. Considering the amount of fluid in the trans and in the converter, which is 11.5-12 quarts, that fluid wouldn't even be 50% new. Something else was either done or is wrong.

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

Here is the exact order of what I have done so far. My truck ran fine last summer all the way into October 2025 and it had a small trans pan leak and I tried to fix it replacing the gasket, fluid and the filter and it still leaked. I then parked it in my garage but it had a bigger trans pan leak over the winter and it leaked ALOT of fluid out during storage.

Now the following is what I did recently.

1) Replaced trans pan gasket because it was leaking badly but I did not replace the filter because it is like new. The filter bolt was secure and the rubber Oring looked fine. The pan sealed and does not leak anymore.

2)I put in 5 quarts of Type F (truck was not running)

3) I started the truck up, let it idle for about 5 minutes and then checked the fluid level with the truck idling in neutral and the fluid level reads slightly above full. Truck does not want to move.

4) Shifted truck through all the gears while it was still running and it will not move at all.

5) I started the truck up again today, let it run about 5 minutes and checked the fluid while idling in neutral. Fluid level reads slightly above full and truck still doesnt want to move

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

I have had the same experiences in the past and yes this trans has been slipping some in the past and I think it was due to the trans pan leaking when prior owners had it and not keeping enough fluid in it which I think shortened the life of the trans but regardless the truck still ran and moved when I parked it over the recent winter.

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

Went out and started my truck up again. Letting it warm up and checked the fluid level while parked and the level still reads above the full mark. Still no changes.

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

It has not been my experience that servicing a transmission led to failure soon after, and I worked as a mechanic for decades. I guess it's down to the luck of the draw? FWIW, my shift selector shaft seal leaks, and the trans gets low on fluid every once in awhile. I just put more fluid in it. It's done it for years, but adding fluid always brings it back. Trans was rebuilt about 40 years ago and now has about 100k miles on it. OP, I think that your trans was just ready to die.

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

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.

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

I would have sworn it was green.

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

GM Hydromatic service guides are helpful, they show fluid routings, diagnostic charts for items to check for and pressure test readings.

As I don't have the books I have used in the past, a quick search did not point to a single failure point. Problem might be as simple as a broken spring in valve body or gasket failure, or anything up to needing the removal for repair.

If trans is old, and has old gaskets and seals, rebuild or replace would be the way to go.

Pressure gauge readings can be found online and would be a great test to perform. If readings are off, the service manual would then be useful.

Bob