I tried following script:

ODBSET /Equipment/ArduinoTestStation/Variables/_S_, 10

LOOP 10
  WAIT seconds, 3
  ODBINC /Equipment/ArduinoTestStation/Variables/_S_
ENDLOOP

and it worked as expected. So I conclude the problem must be in your script. Probably a typo in 
the ODB path pointing to a 32-byte string instead to a 4-byte float.

Stefan