Maybe. The modulating valve and the detent are hydraulically connected and work together to control upshift/kickdown shift points.
From GM (see the bold italics for your issue):
Description
The engine vacuum signal is provided by the vacuum modulator which consists of an evacuated bellows, a diaphragm and a spring. These are so arranged that the bellows and spring apply force that acts on the modulator valve so that it increases modulator pressure. Engine vacuum and a spring oppose the bellows and spring to control modulator pressure.
To reduce the effect of altitude on shift points, the effective area of the diaphragm is different than that of the bellows. Atmospheric pressure acts on the resulting differential area to reduce modulator pressure.
Operation
A vacuum modulator is used to sense engine torque input to the transmission automatically. The vacuum modulator transmits this signal to the pressure regulator, which controls line pressure, so that all torque requirements of the transmission are met and proper shift spacing is obtained at all throttle openings.
Vacuum Modulator Valve
Provides modulator pressure that senses engine torque and vehicle speed. The vacuum modulator speed is used to vary the shift points according to throttle opening and to raise line pressure proportional to engine torque.
2-3 Modulator Valve
Senses modulator pressure to apply a variable force that tends to hold the 2-3 shift valve downshifted.
3-2 Valve
Shuts off modulator pressure from acting on shift valves after direct clutch has been applied. This allows fairly heavy throttle operation in third speed without downshifting. In third speed detent pressure or modulator pressure above 105 psi can be directed to the shift valves to provide downshift forces.
A nice flow diagram of the TH400 shifting controls:
You must be registered for see images attach
Some explanation:
TH400 has two downshift mechanisms, vacuum and electric. The electric is the "passing gear" which kicks in near full throttle. The electric is dependent on the vacuum to work.
First, check the simple things. Is the vacuum line kinked or flattened? Are the hoses connecting the line to the modulator and the intake manifold tap in good shape and tight fitting?
The manual says to first check vacuum modulator and lines for leaks, also check for a stuck modulator valve. I have also seen the tap on the manifold plugged with carbon on older vehicles. An easy way to check this is to pull the hose off the tap fitting and check for vacuum with the engine running.
Next step is to remove pan and valve body. Check for stuck 3-2 valvetrain by shaking valve body. Valves should move under their own weight, also check for broken spring.
Next is to remove pan and disassemble pressure regulator and check for worn or stuck parts.
