Older AT50's and some AT60A's strobe the output from the encoder, and unless
the transponder is modified the output is not always present as required by
the GPS. There is a Narco service bulletin that addresses this problem.

"Victor" wrote in message
m...
I have installed a KLN-89B installed but the altitude comming from the
xponder is not working. The xponder is an old Narco AT50. I wonder if
other pilots have another model of xponder installed that actually
interfaces with the GPS.