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:
- 60min x 60sec x 60 cycles / 4000 pulses per mile = 54 mph.
The tach should read (4cyl/6cyl):
- 60 cycles x 60sec / 2 = 1800 rpm
- 60 cycles x 60sec / 3 = 1200 rpm