Speedo signal buffer

91 Astro, 91 k3500 truck. Any help appreciated, have the AC voltage signal input to the buffer, what am I supposed to see on the output wire to the dash, AC or DC ??? At this point I have nothing at the output wire & have changed the buffer with no positive results. The parts guy at the dealer told me these things have to be bought, sent away & "burnt" to the individual specs of the vehicle that they came out of or from the old one. I'm thinking that thats just for calibration rather than function but tried 3 different ones from various vehicles in the lot with no results. Thanks.