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Fuel Mixture Display For Cars, Pt 2

All the constuction details and calibration procedures.

By John Clarke


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.

Click for larger image

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.

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