MacDon FFT FD75 Assembly Instructions Manual page 281

SETTING UP AUTO HEADER HEIGHT CONTROL
In addition to power (A) and ground (B) wires, a signal
wire (C) is connected internally to a movable wiper that is
attached to an external arm and sweeps high resistance
filament band. As external arm is rotated and wiper is
moved toward or away from power wire connection,
measured resistance at signal wire (C) changes.
The resistance measured across signal and ground wires
should increase uniformly from a low 80–100 ohms
(.08–0.1 k) to a high 800–1200 ohms (0.8–1.2 k). This can
be observed if an ohm meter is connected across signal
and power wires and sensor shaft rotated. When an input
voltage is applied to high resistance filament band through
power wire (A), output (or measured) voltage in signal
wire (C) is changed by this variable resistance.
Figure 5.288: Power, Ground, and Signal Wires
NOTE:
Ground and power wires may differ depending on combine.
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