This fuel mixture display should not be regarded as an accurate
instrument since it will not necessarily be tailored to suit the voltage vs.
lambda output curve for the particular oxygen sensor it is monitoring.
To set the unit up as a calibrated instrument would require
comparison with a known reference and subsequent adjustment of the internal
software lookup table which converts the measured voltage into an air/fuel
ratio. As published, the air/fuel mixture display is designed to follow the
Bosch 0258104002 sensor output characteristics but even then, the calibration
procedure will give approximate results only.
Other typical narrow-band sensors can vary quite a lot in their
output characteristics away from stoichiometric and, like the Bosch unit, also
vary substantially in output voltage depending on temperature.
For this reason, the Air/Fuel Ratio Meter is intended only for use as a
dashboard unit to indicate air-fuel ratio trends during driving. It should not
be relied on as an accurate instrument for tuning programmable engine management
systems on a dynamometer or for making other engine adjustments.
Work can now begin on the case. First, remove the integral side
pillars using a sharp chisel, then slide the microcontroller PC board in place
and drill two mounting holes – one through the metal tab hole of the regulator
and the other at bottom left, below the 0.1μF capacitor. These holes should be
countersunk on the outside of the case (use an oversize drill to do this), if
you intend using countersunk screws.
Two holes are also required in the rear of the case (near the
bottom) to accept the supply and sensor leads.
The front panel label can now be affixed to the case lid and
used as a template for making the display cutout and for drilling the hole for
the LDR. The main display cutout is made by drilling a series of small holes
right around the inside perimeter, then knocking out the centre piece and filing
the job to a smooth finish.
Make the cutout so that the red Perspex or acrylic window is a
tight fit. Once it’s in place, the window can be further secured in place using
several spots of super glue on the inside edges.