Gabriel offers us this great tip on building an in dash air-fuel monitor.
This can be potentially used to monitor your car's performance, and may also
allow mixture adjustments while driving. Gabriel has just recently begun to
learn English, so I have edited the text for better readability.
The lambda sensor outputs a voltage signal that represents the instantaneous
composition of the air-fuel mixture, from near 0 Volt (too lean) to 1 V (
too rich). See Figure 1.
The sensor must have reached a temperature of above 350°C before it outputs
a reliable signal. In my case, my Porsche 911 has 1974 headers, and I
installed the O2 sensor approximately 15cm before the muffler connection.
Don't install the O2 sensor too close to the exhaust valve cover, otherwise,
you will have to remove the o2 sensor every time you do a valve adjusting
job. A generic O2 sensor has a M18x1.5 threads, but I didn't use them. I
found it more practical to measure the sensor diameter (this is measured on
the electrode end, approx 10.25mm), then drill a hole with the same or
smaller diameter and install it using a "pressure fit". Then, to completely
constrain it I used a wire type bracket.
I used (for more accuracy) a four wire
(internally heated) O2 sensor. There are two signal cables (wire it to the
cockpit) and two for the heating element (connect one to ground and other to
+12V). In my NGK O2 sensor, the heating wires are white. You can check it
with a multimeter, as the heating element has aprox. 6 ohm resistance. The
other two (black & gray on mine) are the signal cables.
When you have the signal wires routed near
your dashboard, you can connect a generic digital voltimeter. However, as
you can see in Figure 1, voltage from the O2 sensor is not a linear
function; it rises quickly near lambda factor 1 (14.7:1 air/fuel) and slowly
at rich & lean ends, so you have to compensate this if you wish more precise
readings. I used two LM3915 units to drive 20 leds, and to compensate for
the lambda curves at the same time. See Figure 3 for the schematics.
20 LEDS (RECTANGULAR, BETTER LOOK), 8 RED, 12 GREEN
2 180 OHM RESISTOR
1 22K OHM PRESET
1 4.7uF 16v capacitor
1 7805 (5v regulator) with heat sink
The following information was obtained by me by playing with this A/F Meter.
You can start with this information and practice. If you have found anything
wrong or incorrect in this article, please email me.
Apply 0.92 Volt (check it w/ a digital voltimeter) to the signal input and
adjust P1 until led 16 just comes on.
Turn on your engine and wait about 15 or 20 minutes to warm up the O2
sensor. If you have CIS ( K-jetronic fuel system) just apply light pressure
on the sensor plate with your finger until the engine reach maxium RPM. At
this point, led 16 should be on, if not, adjust P1.
On carburated engines, move the idle mixture adjusting screw to obtain
maxium rpm and move P1 to turn led 16 on.
DRIVING WITH IT (also see Figure 2):
On Idle: Values between led 6 to 15 are acceptables.
You will have smoother idle around led 11.
Partial throttle: Between LED 6 to 11
Full throttle: LED 16
Suggestions & questions: write to:
John Francis Duncan wrote:
People who already have an o2 sensor installed in their car can use the
existing o2 sensor by tapping into the signal lead. Yes, you'll have to find
the correct wires. Some lambda sensora have one, two, three or four wires;
it depends if it has a heating element or not. I don't know exactly what
kind of o2 sensor Porsches use (I think, 1981 year onwards), but I hope that
somebody can provide more info about that.
John Rodgers (firstname.lastname@example.org)
has the following hints and tips on making this Air-Fuel Monitor:
20 LEDs take a lot of room, plan accordingly for a case 4 to 5 inches x 1 to
2 inches so they can go across the face nicely.
The 4.7 microfarad capacitor may not be
necessary. According to National Semi-conductor it is needed if LED wires
are over 6 inches in length. I left it out and it works okay.
The 22kohm preset was too low in value. I ended up using a 47k preset set to
full resistance. to make light 16 come on at 0.92 volts signal.
Use old/wornout 1.5 volt batts to supply
the signal voltage or make a variable DC power supply as I did, but it is
hard to find one that goes below 1.25 volts. Adding a large pot across the
signal connections will drag the signal voltage down less than 1 volt so tou
can calibrate it.
The long wire on the LED is the Positive side, won't work if backwards.
Use 18 pin sockets to mount to the pcb and
insert the ICs after all soldering is done.
Use the minimum heat to solder or a heat-sink, I overheated some parts and
had to redo them.
Dick Lague (email@example.com) adds this additional thought:
I read the tech piece on installing an
oxygen sensor. I have used such a setup in my 1955 Speedster. It is a great
tool for tuning. There are at least two commercially available units. K&N
[the air filter people] and C.B. Performance make air/fuel Ratio meters.
I used the C.B. Unit in the Speedster. It
is a 2" round analog electrical gauge. It is heavily dampened and reads from
12:1 to 17:1. You can either buy it as a complete kit of the gauge only.
Gauge only part#2910
Complete kit 2905
1715 N. Farmersville Road
Farmersville, CA 93223
K&N makes a gauge with 10 LEDS. It is
also a 2" round instrument. The part # for the complete kit, which includes
the gauge, oxygen sensor and weld on sensor adapter is 85-2441.
The gauge is also available separately.
You can find the K&N Air/Fuel Ratio monitor kit at our online store!
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