TH400 shifting problem

[sgt1022]

Ok guys, it's been awhile since posting. I have recently run into some problems with the TH400 in my 71 c10. It's not the original tranny, but the same kind that was in the truck when I bought it. The current one is a replacement from FTI performance in Deland, FL. It is just about 1 1/2 yrs old. It recently started shifting differently/highrer that normal. I found that the hose tot he vacuum modulator came off. I replaced the modulator, hose and line to the manifold. I checked manifold vacuum and it fluctuates between 10-12 inches of vacuum. The tranny shifts from 1-2 at 4300 and 2-3 at 4100. Which is about 1500 more than normal at regular acceleration. It won't hold in any particular gear, ex. if I'm below 3000 it'll downshift to 2 from 3. I cant shift it manually through the gears. If I'm in 2 running at 3500 the tranny won't shift into 3. I am at a loss. There is plenty of fluid, all vacuum ports are closed. everything is new from the intake manifold to the modulator. I even adjusted the screw in the modulator out some. no change.

[Jayred]

Maybe the new modulator is junk. Sounds like it is still operating in a low vacuum condition.

[sgt1022]

I put a vaccum pump on it and it moved.

[sgt1022]

It feels like its operating in low vacuum.

[AussieinNC]

10-12 at the manifold is too low...you should be looking for somewhere around 15-16...did the replacement modulator exactly match the original one? As the trans came from a performance shop, it may be the valve body has had a performance kit installed...and a matching modulator...if its usable, put the original modulator back in, probe the port into the manifold with a piece of wire, like a paper clip...check your ignition timing and try to get as much vacuum as possible...hope this helps...

[sgt1022]

10 - 12 hg vacuum is what gm performance says the motor should
be pulling. I found the problem. The kickdown switch is malfunctioning. Its making the tranny operate in passing gear. I disconnected the switch and tranny shifts normal. Just have to adjust modulator to shift a little higher rpm

[AussieinNC]

Glad you found the issue...I was just about to update the post with a check the kickdown function...

To be honest, most of my T-400 installs and builds usually delete the kickdown function...they can cause big issues if it goes crazy at 8- mph on a freeway...

Have a great evening

[mattfranklin]

Looks like you found it. I was going to say that I had that exact same thing in a '74 Jeep Cherokee with 360 V-8 and TH400. It was super fun and super embarrassing to drive all at the same time. It turned out to be a hardened and broken vacuum hose between the intake and the modulator.