Fiero Speed Signal Converter
The Fiero uses a gear driven variable reluctance magnetic type VSS that produces 4000 pulses per mile traveled in the form of a high voltage sine wave. The driven gear can be changed to compensate for different tire sizes. The VSS signal that is sent to the instrument cluster where it is buffered and distributed to the meters (speedometer/odometer) as well as the cruise control module and engine control module. Most other GM vehicles, at least the ones that are commonly used for Fiero engine swap donors, operate a bit differently. An approximate 24000 PPM speed signal is generated by the VSS and sent to the ECM first. The ECM is calibrated for the specific tire size. The ECM buffers and divides the signal and distributes it to the appropriate modules and the speedometer in the form of a low voltage square wave. Calibrations are made by changing constants in the ECM programming. The signal that is output by the ECM will not directly drive the Fiero speedometer. If you are doing a Fiero engine conversion and using this type of system, you will need a signal converter to make the speedometer work. Here is a simple circuit that will convert the speed output signal from the new ECM to a signal that can drive the Fiero speedometer. It will not provide a way to correct the speedometer reading for different tire sizes, that must be done through the programming in the ECM or by using a more complex module that I will have available soon.
