Quad 4 w/eprom ecu, speedo wiring help (Page 1/1) Motive123 JAN 27, 09:27 AM This is a very old swap that merged a 89 Grand Am ECU complete with the merging of the wiring harness to the 84 wiring. The original ECU did not output to the speedo, the signal came directly from the VSS to the speedo. The Grand Am VSS goes directly to the ECU and then to the speedo. There was a guy in Durango CO that specialized in Turbo Quad 4 swaps (20 year ago!) and as I remember it required adding a resistor to the stock fiero speedo. I know this is a long shot but anyone that has used that ECU probably knows how to wire it. Any help would be much appreciated. Steve Will JAN 27, 10:17 AM In original form, there's a speedometer buffer in the instrument panel. This buffer accepts the sinusoidal signal from the Fiero gear-driven VSS, then outputs a half frequency square wave to the ECM. In later transmissions, the magnetic reluctor VSS signal went straight to the ECM, which output a 0-5V square wave signal for the instrument panel. The RC circuit you've seen conditions the 0-5V square wave to have zero crossings so that the Fiero instrument panel can read it correctly.