I'm going to take a wild stab in the dark as to why it won't work.  Several years ago we tried to feed GPS from an FMX to an CaseIH AFS yield monitor.  It acted like it was all working but when we went to print the maps they were all screwed up.  Turns out that the FMX output string was too long for the older device to handle.  It may be a similar situation with your stuff and the brown box can't handle the high resolution speed string.