Fiero LED Gauge Prototype

Fiero Multigauge

  • Coolant Temperature: 0 – 300 degrees Fahrenheit with warning flash at 240°F
  • Fuel Level: 0 – 99 % with yellow warning light at 10% or less capacity
  • Battery Voltage: 0.0 – 20.0 volts with warning flash at 11.5 or 15.0 volts
  • (optional) Boost/Vacuum: 29.1in/Hg – 14.7PSI
  • Built in light sensor for auto dimming, but also dims with OEM dimmer switch
  • Price: “More than you can afford pal … Fiero”
  • Can be modified for other sensors like MAP and Lambda

How Does It Work

This gauge is a drop in replacement for the original Fiero coolant temp and fuel level gauge without the need for any cutting or splicing into anything on the car.  There are two digits for the fuel level on the bottom half and three digits for the coolant on the top half.  The three digits on the top will start off as the coolant display and can be changed to show battery voltage.  The fuel level reads from 99 (full) to 00 (empty).  When it is 10 or lower, a yellow LED to the left of the number lights up.  The coolant gauge reads from 0 degrees Fahrenheit up to well past 260F.  The display has a light sensor also to the left of the fuel level display.   It will adjust the brightness of the LED’s automatically without the use of the parking light dimmer switch, but when the dimmer switch is turned on, it will dim the display with the dimmer switch.


How to Use the Dimmer

Turn the parking lights on, take the dimmer switch from high, to low, and back to high within 2 seconds, now the coolant temperature changes to show the battery voltage.  When you are looking at the battery voltage and the coolant temp rises past 240F, the gauge will automatically switch to coolant temperature and start flashing.  The same can be said for battery voltage.  If you are watching the coolant and the voltage drops below 11.5V (or above 15.0V), the gauge will switch and begin flashing.  If you toggle the dimmer switch, it will stop flashing and not warn you about that gauge again for several minutes, so as to not distract you.  It is very user friendly.

How to Install

In the basic setup with only fuel level, coolant temperature and battery voltage there is no cutting of any wires for the gauge cluster.  With the cluster apart this device drops right into place.  The link to the dimmer switch is also a plug in to one of the cluster lights controlled by the dimmer switch, no cutting.  The extra button (optional) is the only thing that someone could possibly chose to drill a small hole to install.  That could go in the panel below the steering column for example.  Two of the cluster light bulbs behind the gauge are removed so that they do not give it unnecessary back lighting.

How To Make One

Click here for a list of materials (BOM)

Click here to download Express PCB software to view other files

Click here for the circuit schematic

Click here for the lower printed circuit board file

Click here for the upper printed circuit board file

Click here for the Arduino development environment

Click here for the software that is programmed onto the processor

The BOM has extra tabs with more information about the sensor and voltages in use.  The software also has quite a bit of commenting with more details.  The chip can be programmed as it sits in the gauge cluster using the Atmel AVRISP mkII.  The upper board with the LED’s must be removed to program it.  It is not the easiest way to program anything, but you could also take the chip out and use the Arduino UNO developer board, then put the chip back in the gauge.

Atmel AVRISP mkII ready to program gauge with ignition powered up at test bench

If there were enough demand for these, then I would be selling all the blue items in the BOM and letting everyone purchase the items in yellow from Digikey.  For the time being, that is not the case.


Because an LED gauge for the Fiero is old news, I don’t think there will be enough demand for this one to justify a bulk order of the printed circuit boards. I wanted something interesting to brush up on my circuit/software skills so this is what I opted for.  All the parts are through hole so that anyone with with a soldering iron and not much skill can assemble it.  Building it myself also let me customize it.  My first version displayed coolant, fuel, voltage and the oil pressure.  I will probably do something later on to ditch the light sensor and the dimmer input so that I can show MAP, coolant temp, voltage, fuel level, oil pressure, and lambda.  The switch input can be programmed to toggle the gauge, but then also change the brightness if held longer.  It seems like having only 2 settings or maybe 3 at most would be enough anyways.

If you are interested in a straight forward, reliable drop in gauge that is LED, order one from Tom’s Digital Electronics.  If you want something you can hack up, leave me a message.  With enough interest I would be willing to front the cost for a batch of boards (@$500 for 30).  If you want me to build a custom one for you and you have a ton of money to burn, I accept large amounts of cash via paypal.  I also take cash for doing nothing if you like giving it away.

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