TH400 late shifting to 3rd
 

I have a th400 that is late shifting to 3rd gear under normal driving conditions. The 1st to 2nd shift is perfectly normal. Driving around town at 30 mph it will just stay in 2nd. If I wind it up to about 40 mph, it will finally shift to 3rd. The vacuum lines on the modulator line are in good shape. To my knowledge, the transmission has never been rebuilt. Does anyone know what might be causing this 3rd gear shifting issue? Thanks

Governor weights stuck….
Kick down switch or adjustment….
Modulator, modulator valve sticking…
Weak vacuum signal….

I don’t know about the modulator being a cause.

Whenever one of was bad or not hooked up the trans would not shift out of 1st.

My brief tenure at an AAMCO shop taught me one thing…an automatic acting funny like that, without an immediately identifiable issue - low fluid, vacuum line not hooked up, etc., usually means internal problems.

I’m dumb as to the inner workings, the governor being stuck may be it, if so should be easy to diagnose/cure.

...sticking valve in valve body

Check this:

Low fluid level or clogged filter are usual suspects from my view. Then Post 2 kicks-in.

All my auto transmissions that are controlled with a vacuum modulator do that when I'm up at high elevations. Low vacuum signal to the vacuum modulator will cause a delayed shift, most noticeable on the 2-3. As soon as I drive back down to sea level it's back to normal.

Not sure what elevation you're dealing with but it's something to look at. Could just be a simple adjustment on the vacuum modulator (screw inside vacuum nipple) If someone was messing with yours there is no telling what they might have set it at.

When I adjust mine down to shift normally at high elevation, it causes very quick short shifts when I'm at sea level, so it's a balancing act. I just have to live with it one way or the other or find a happy middle spot that's good enough.

What is hilarious is that the huge, black-canister modulator assembly that everyone tosses in the trash in order to install a crappy, cheap aftermarket "adjustable" modulator...is altitude compensated. They're also adjustable in most cases, provided you're careful about breaking the spot-weld on the adjustment screw.

Yup those things are almost non existent anymore.

I had similar symptoms on a th400 years ago. Cause was a bad governor flyweight pin, the ones that hinge the weights and look like crimped panel nails. New governor fixed it. $20 and a gasket at AAMCO's parts counter.