An Air/Fuel gauge allows you to monitor the running condition of your engine at all times. The ECM computer that is the heart of your engines electronic system uses an Oxygen sensor (o2) to monitor the exhaust gasses. It is constantly checking and adjusting the Air/Fuel mixture for good performance and low emissions. For a completely stock car there is no need to mess with anything as the ECM will make sure your car is running the best it can no matter the conditions, however once you begin to add performance parts to your car the computer begins to get confused. A typical 4 stroke piston engine is in its simplest form just an air pump. So that being said the more air you can get in and out of the engine the more performance you will be able to attain from said engine.

However as you increase the airflow through the engine you also need to increase the fuel flow to maintain the optimal Air to Fuel ratio of 14.7:1. Bolt on performance parts are designed to increase the airflow in and out of your engine far beyond what the stock system could ever offer, therfore the ECM has to compensate and add more fuel. In many cases it does not take much to reach the limits of fuel flow for a stock ECM and the system needs to be retuned to attain maximum power output and reliability. Therefore you will need to be able to monitor the running condition of your car under various conditions to retune it properly. Some popular methods for this are by using an (EGT) Exhaust Gas Temperature gauge, an emissions analyzer typically found on a chassis dyno, or an Oxygen Sensor.

For most the easiest choice is to use the oxygen sensor since they can be found on all computer controlled cars. An Air/Fuel Gauge allows you to see the signals that the ECU is receiving about the fuel mixture from the oxygen sensor. Oxygen sensors found on %99 of all computer controlled cars are narrow band type oxygen sensors. This means that at or near that perfect 14.7:1 mixture the sensor is very accurate, however the downside is that the further you get from that perfect mixture on either side of the ratio the less accurate the readings are. The other type of sensor is called an Wide Band oxygen sensor and is accurate throughout the entire range. More info on these can be found on the web as I am not experienced with these at all.

To monitor these readings an electronic gauge is used with typically 10 or 20 LED's to indicate the mixture in real time as the computer sees it. For this installation I chose Autometer's Air/Fuel gauge part number 4375. Aluminum bezel with aluminum face. This gives a nice clean look I think and works well with the 80's styling of the car.

First I needed to choose a good location to mount the gauge. I wanted to avoid cutting into the visible areas of the interior if at all possible. I finally realized that the ashtray in the console was doing me no good and it looked like I could get some gauges mounted there. I finally decided to cut out a mounting plate in the shape of the ashtray in 1/4" plywood. It looks like this.

Some of the plastic bits that used to be under the ashtray had to be cut away also for proper clearance of the guages. For this I used a Dremel rotary tool with a drum sander attatchment. The end result was this.

Ok now that the physical mounting aspect of the gauge has been taken care of it is time to make the wiring connections and put it all back together. The unit will need a 12v lead wire that is on with the ignition and a ground wire . The nearby cigarette lighter is the prefect place to source these leads from. I used some 3M connectors that allow tapping into wires without removing insulation. So all that I needed were a pair of pliers to crimp the connectors into place like this.

Last but not least you will need to tap into the signal wire from the oxygen sensor. The best place to do this is where the wire goes into the computer which is located behind the glove box. First remove the screws that attach the glove box hinge and the arm on the right side. Next remove the screws in the top section of the glove box. After you have removed all the screws slowly pull out the glove box taking extra care not to damage it as it is only cardboard. Once you have it pulled out un-clip the plug from the light socket on the back of the glove box. Now you should have access to the computer. In my supra it is mounted high under the dashboard and has 3 yellow plug going into it. You next need to figure out which wire is the o2 sensor feed wire.

On my 85 Supra it is the light tan wire in the middle plug however it is very important that you consult the shop manual or wiring manual for your car to be certain you have the right one as you will destroy the gauge if you connect the wrong wire. I went the extra mile and made sure by using a voltmeter and a short piece of wire. Somehow attach this wire temporarily to the wire that you think is the o2 wire. Then plug the harness back into the computer and start the car. Once the car has warmed up for a few minutes ground your voltmeter to a known good ground and take a reading from the o2 sensor wire. If it is the correct one it should fluctuate between 0 and 1 volt. Once you are certain you have the right one use one of the 3M connectors to connect the signal wire to your Air/Fuel Gauge, on the Autometer gauges this wire will be Violet.

*A Note About Proper Grounding*

Since this gauge reads the output directly form your o2 sensor which is only between Zero and One Volt it is very important that the gauge and your o2 sensor have a good ground.  When I first installed the gauge I was getting very wild readings on the gauge, sometimes 5 or more lights would be lit up at one time.  This was caused by a weak ground.  I replaced the negative battery cable which runs from the battery to the grounding post on the drivers side of the engine block directly below the distributor. There is also a wire that runs from the battery to the fender that was replaced as well. Another ground wire was connected to the exhaust manifold near the o2 sensor and is bolted to the firewall.  Fixing bad grounds is one of the best things you can do for your car, it makes all your electrical items work better, your engine will run better, be more powerful, and get better gas mileage with good grounding.  Your stereo will sound better, your lights will be brighter, and will stay that way even when idling. 

Now put everything back together making sure none of the wires could be chaffed or put under stress. I created a second hole on the passenger side for another gauge which will be added shortly. There is wiring for the ashtray light that can be used if you would like to add a backlight to any of your gauges. The finished product looks like this and works quite well. I mounted the gauges at a slight angle which makes them very easy to read.

Any questions or comments should be adressed to wjh@ufl.edu