TH400 won't stay in 3rd under 50mph
 

Hello everybody,
I've got a slight problem with the functioning of my TH400. Here's what happens:

When cruising not very fast at 40mph, the tranny stays in 2nd.
If I accelerate to over 50 and then back off the throttle, the tranny goes into 3rd.
However, if I let the speed drop under 50 (49 / 48 / 47 mph) the tranny will start "slipping". It "slides" constantly between 2nd and 3rd and is rather annoying to drive.
Considering that before I modified my motor, I could cruise easily in 3rd at 40 mph. (Of course if you "floored" it would kick down.)
Apart from this above mentioned problem, the tranny seems to work OK everywhere else - nice tight gear changes when accelerating hard etc…

If anyone has an idea, I would be most grateful as going around town in 2nd all the time is starting to bug me. (Not to mention the extra fuel costs!)

Regards,
Rafe

Possible broken seal ring on the center support? Worn center support bushing? Could be more. I could be wrong but one I built did the same thing and it has a broken top seal ring....Broke on install

Doe the trans hold 3rd gear without slipping and under load once it finally shifts???

If so.............

First, I would check the modulator to make sure it is hooked up to the proper vacuum source (manifold vacuum) and that there are no leaks in the supply to it.

If that checks out, then pull the governor cover and remove the governor to inspect the gear teeth. If they look good, then put it back in and replace the modulator.

It sounds like by what you indicate, that the trans operated OK before the engine mods.....chances are, if things were unhooked, that maybe they didn't get re-connected properly..............

Depending how extensive your engine modifications were, you could have radically changed the vacuum signal that the modulator is seeing. (Also as mentioned make sure the line is still attached and not kinked.)

Some TH400 modulators are adjustable by inserting a small, flat bladed screwdriver inside the vac. port. I would experiment with that first since it is an easy fix. Sorry but I can't remember which way turning the screw does what to your modulator function.

JS

possible clogged filter, or/and low fluid level.

Turning it clockwise reduces vacuum signal thus raising and friming shift point. Turning counter clockwise.. well you can figure that one out...
I usually go 1 to 1 1/2 turns at a time and test to get it where I want...

I also agree with checking the vacuum source and put a gauge on it to make sute it is within specs...

Good luck!

X2.

It's possible that your kickdown switch is stuck.