Re: fuel mixture guage

[Brad Sheridan <brads@mindspring.com>]

At 10:02 PM 8/18/98 -0700, you wrote:
>Okay, here's what I did, and my gauge and car are working great. I got the
>cyberdyne gauge from Summit. It has 3 wires, one ground, one positive, one
>o2 hookup. 
>From what I've seen on the net and stuff is you HAVE to get full 12v to the
>gauge, and have a good ground. 
>I used a relay to get straight power off the battery for the gauge. I
>grounded it at the steering column. 

 The best possible ground would be at the O2 sensor. Remember, the sensor
creates a voltage referenced to ground. It is grounded on its case. To keep
the voltage reference accurate, you should use as the ground for your meter
the same ground that the O2 sensor is referenced off of. You don't need to
relay the gauge. Depending on the internals of the gauge(and the few that
I've read about are all crap), they have an internal 1.25V reference. So
all you have to do is get enough voltage to keep that up. Now they may have
an internal voltage divider on the input, so you might need somewhere close
to 12V, but the voltage should not be overly critical.

>The main thing to remember is that your working with .01 voltage increments
>so you have to make sure you have as little resistance possible in the
>wires and _joints_ as poosible.

  Most of the gauges I've seen schematics for are dealing more in 0.1V
increments. While you do want good connections, the resistance in the
sensor signal wire is not necessarily bad. The gauge should be op-amp
buffered, with a very high internal impedance(resistance). If not, it can
affect the sensor output to the cars ECU. If for some reason, the gauge
doesn't have a large input impedance, it would be wise to add a resistor
inline so that it has a sufficiently high impedance.

 I split off the o2 wire with a solderless
>crimp-on and soldered EVERYTHNIG as well. make sure the o2 wire(s) have
>good connections.

If you're using a 3 wire sensor, it may be just as easy to get all 3
(12V,ground, and signal) from the O2 sensor directly.

  And once you get it all 100% working, realize that due to the nature of
the standard automotive O2 sensor, the output you are getting is quite
inaccurate, and take any absolute AFR numbers you get from the gauge with a
grain of salt.

Brad

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