Problem with KNS-80 solved.

It happened when the swithbox disconnected the
indicator resolver to connect it to the GPS.

The KNS-80 didn't like that and flagged the groundspeed.

The reason it worked with the test box on the ramp is that
the VOR portion of the KNS-80 had no input. So there
was no 30hz reference signal and the box couldn't
detect that the absence of the resolver.

There is a fairly obscure note in the KLN-90B instruction manual
that mentions this; a jumper is required.

Bill Hale