Thanks to Rob (magicsmoke) Finch, here’s the strategy to testing the Hall sensors:
Wire a PP3 9V battery across the Red/Blk (pos/neg) and with your DC volt meter measure across Blu/Blk, Yel/Blk and Grn/Blk when you waggle a fridge magnet or whatever over the relevant sensor. The reading should go from high to low or low to high depending on the polarity of the sensor.
If you don’t see a change, they may be ‘current output’ devices. You can check these by putting a resistor (1K to 47K range) in series with your meter probes. Set your meter to DC milliamps and probe between Blu/Red, Yel/Red and Grn/Red when waggling.
The post and discussion is here, over on the ElMoto.net.