To confirm correct operation while sitting on ground, turn the transponder mode knob to alt and the display should say grd, then you can increase pitot pressure to above the stall speed setting and the transponder display will switch to alt.
If you fly in wave without this switch your ground speed may be very low and ATC will think you are on the ground.
AIR PRESSURE SWITCH/Normally open, 42 knots, adjustable.
