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

[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.

[57taskforce]

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.

[Getter-Done]

Originally Posted by Shifty One

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

Originally Posted by HeavyHauler

I did not mess with the filter.

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.

[Joyridin]

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?

[SCOTI]

Originally Posted by Joyridin

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?

>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.

[MARKDTN]

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.

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.

[SCOTI]

Originally Posted by MARKDTN

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.

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

[Joyridin]

Originally Posted by SCOTI

>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.

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.

[HeavyHauler]

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

[HeavyHauler]

Originally Posted by SCOTI

>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.

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.

[HeavyHauler]

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.

[Steeveedee]

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.

[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.

[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.

[HotWheelsFan]

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

[Shifty One]

Originally Posted by HeavyHauler

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.

Over-filling will cause foaming. I'd drain some out (cooler line would be easiest) until the level is correct, and start there. Until you rule out the things that you know are wrong, you're only going to frustrate yourself.

[Rust_never_sleeps]

Originally Posted by Shifty One

Over-filling will cause foaming. I'd drain some out (cooler line would be easiest) until the level is correct, and start there. Until you rule out the things that you know are wrong, you're only going to frustrate yourself.

If it's too full, wouldn't it foam out the vent?

[Shifty One]

Originally Posted by Rust_never_sleeps

If it's too full, wouldn't it foam out the vent?

It might if being driven. I'm just throwing theories here....

[3drburb]

Originally Posted by HeavyHauler

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.

Did you check the vacuum hose and the modulator on the right side of the trans?

[Rust_never_sleeps]

Originally Posted by Shifty One

It might if being driven. I'm just throwing theories here....

Ah, good point

[Shifty One]

Originally Posted by 3drburb

Did you check the vacuum hose and the modulator on the right side of the trans?

Good one, it's even happened to me a few times, I should have thought of that. Check the vacuum line for splits or tears as well.

[HeavyHauler]

Originally Posted by 57taskforce

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.

Yesterday I put your theory to the test and added 2 more quarts. It did nothing but at least I tried. Most likely it is overfilled now but it doesnt matter because I will be pulling it anyway this summer.