Pretty sure it was always the same sensor on the fan as on the wheels. And I thought this same sensor was used on the old pull-type sprayers as well for ground speed. As far as I remember they were always at least $300.
I built my own sensor that worked on the old sprayers but last I tried it on the air cart it didn't like it, but that might have been because the real problem was under voltage from a failed alternator. I guess you could build one and try it. They are basically bog-standard PNP hall effect sensors with some added circuitry to let the monitor detect shorts and missing sensors.
See "ISO-compatible" Speed Sensors
. You'd want to use the PNP inductive proximity sensor and a normal NPN transistor. The sensors themselves are quite cheap. For example: https://www.automationdirect.com/ad...uctive_proximity_sensors/18mm_round/pnk-ap-3a
. Adding the circuitry takes a bit of soldering.
It might work, it might not. Perhaps I just have the resistor values not quite right.