I have a 1987 v6 firebird with 700r4. and it has an electric speedometer. I have checked my vss up to the 4 out buffer and the purple/yellow have continuity and put out ac voltage (I even tried a new vss. From the box all the dash wiring is connected correct in the dash. I'm thinking either the box or speedo is dead. Is there any tests I can do

 

Use a multimeter to test continuity from VSS to buffer box then buffer box to speedo

Next check that the cluster has power and ground on the proper pins

If those check out, then a bad part is somewhere in the system

Usually, multiple gauges/lights will be inop when power/grd is off on the cluster, but not always. There are multiple pins for each at the cluster

 

For everyone whos electric speedos arnt working if you Bridge brown ecu wire with green/blk speedo wire at the buffer box. That's what mine needed. I couldn't find any info on here in my searches.