I did a solder splice on mine and the mph were very erratic. Assumed it was the splice, so I ordered another sensor from Baja Designs, 20 bucks. That fixed it up. BUT, not sure if it was the splice or the sensor.
What you could try, is take the supplied wheel sensor and just tape (ie. scotch) the magnet to the rim, then tape the pickup to ?. Then see if you get any indication. That will tell you if it's the sensor or the instrument/wiring.
Actually, maybe you could just tape the magnet to a drill chuck and hold the pick up close enough to get a reading?
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