What does this valve do, and what would be signs of it going bad?
I don't know if this related, but most of the time when I mash the pedal, my C4 doesn’t down shift when in 2nd or 3rd gear. I don't think it's related; so if you could comment on both topics I would appreciate it.
The modulator valve is connected to an intake manifold vaccum source, and is used to control the throttle valve in the transmission. The strenght of the vaccum tells the transmission how fast the engine is spinning, and the transmission uses this information in conjunction with the speed of the governer valve to determine when to shift the transmission. If it were going bad, your transmission would not shift well on up shifts OR down shifts.
Massing the pedal to the floor and having it down shift to 2nd is usually a function of the kickdown lever. At slower speeds, the throttle valve (remember ... controlled by the vaccum modulator) will kick it down at lower RPM's but will not kick it down at higher ones as it thinks your going fast enough to be in 3rd, so sees no need to make a shift. The kickdown arm is mounted on the side of the transmission where the shift linkage attaches (and where the neutral start switch is), and is a mechanically controlled device. In other words, you must make sure that your kick down linkage is all in tact, and adjusted correctly. When it is, stomping the pedal to the floor will pull on the kickdown arm, which in turn opens a valve in the valve body and causes the downshift.
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