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
- 2 LM3915
- 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
- 1 pcb
You can place the LEDs as I do,
from left to right, LED 1 to 20
00 <- MAX POWER (LED 15-16)
00 <- BETTER FUEL ECONOMY (LED 5)
Less than (to the left of) LED 5 = TOO LEAN
Greater than (to the right of) LED 15 = TOO RICH
From 1 to 5: Red
From 5 to 15: Green
From 16 to 20 : Red
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,
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 acceptable.
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.
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
Dick Lague (email@example.com) adds this additional
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. I believe it is around $100.00
The gauge is also available separately.
Riverside CA 92502