It controls the shift points under part throttle conditions.
Under full throttle vaccuum is not present in the manifold (where the modulator sources vaccuum). The modulator valve then allows the governor to control the shifts.
Under part throttle (such as highway and in town driving) manifold vaccuum can vary and the valve keeps the shifts in their appropriate places.
Many modulator valves are adjustable.
An extremely late shift is usually because of a broken vaccuum line, a plugged valve, or a poorly adjusted valve.
__________________
93 Thunderbird 5.0 HO
89 Firebird L03
95 F150 XL
Get ready for the 351 boss build this summer, 4v closed chamber heads, 2.25/1.75 valves, .750 lift, 12.5:1 piistons, wieand tunnel ram, nitrous, I think its gonna roar!
| 
general question: what does a modulator valve do? 
Linear Mode
