The circuit simply consists of a crystal oscillator and divider (74HCT4060) which generates a very accurate 600Hz square wave at its output (Pin 1). This signal is then divided by 10 by the 74HCT90. The square wave at the output (Pin 8) drives a transistor that removes any DC offset and is suited to drive the Fiero speedometer.
It is also of course a nice reference generator for those of you that are into electronics... just short the capacitor at the output and you get a nice square wave for instance.
Using the generator is simple: Disconnect Vehicle Speed Sensor (VSS) or Tach lead. Connect a 9V battery as the power source. Connect the output of the generator and the generator ground to the VSS connector or Tach lead respectively.
The speedometer should read:
The tach should read (4cyl/6cyl):
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