Connecting to Miata Fuel Sensor

[kiwiracer]

Hi, so AnalogX reads as as 0-5V signal, I am not a radioshack, build a component type guy.. I like to buy things

Anyone know of something I can put inline to map Miata Fuel tank Level back ?

Here's the Miata Fuel Sensor specs and research from a few forums..

Gauge 110 ohm adjustable fuel converter

Looks like 12V
Gauge is 110-0 ohm fuel sender (Mazda Miata)

Engine Running vs. Not can change Ohm's (there's a calibration), you can check on your voltmeter.

Full: 3 ohms
3/4: 15.8 ohms
1/2: 32.5 ohms
1/4: 64.2 ohms
E: 110 ohms

notes:
i believe the fuel sender is a variable resistor, so the signal to the gauge in the cluster is a resistance value, something in the neighborhood of 0 to 90 or 100 ohms. I think there is a tap out on the back of the cluster where you can read the voltage from the gauge, but if you remove the cluster you wont have it.

[brentp]

The original cluster has a pull-up resistor, which translates the fuel gauge reading into a voltage divider network.

https://learn.sparkfun.com/tutorials/voltage-dividers

You can enable the built-in 2.2K pullup resistor for shiftX2 by bridging the solder pad, or you can add a pull-up resistor externally. The 2.2K resistor is mostly designed to work with high resistance temperature sensors - the more common application. So, an external 220 ohm pullup resistor might work better for you.

There's also a handy voltage divider calculator here that will show you the output voltage based on the varying resistances:
http://www.ohmslawcalculator.com/voltag ... calculator

Hope this helps

[coma]

Hi there,

We have been trying to make the Miata fuel gauge input work according to the instructions found here on the Forum but didn't succeed. For some reason nothing in the RaceCapture Fuel Level gauge reading changes when we apply different settings in the Analog Sensor config (raw, linear, mapped). No matter what we choose the gauge always reads like 500 gallons. Since the tank is almost full, our voltage reading at the fuel level sender is ~0.5V, so we assume what we really are reading at the gauge are the Millivolts coming from the sender. For some reason it does not get translated according to the mapping defined in the analog sensor setting. Any ideas what might be configured wrong?

We are racing at 24 Hours of LeMons at Thunderhill in California this weekend (car on track already).

Thx much in advance.

[brentp]

Hi,

We have a specific how-to video on connecting and calibrating a fuel level sender - see https://www.youtube.com/watch?v=Cb3eBNv ... 5j&index=9

This should help you get started. let us know how it goes!

[l8apex]

I'm about to tackle this and just want to make sure I'm on the right path. And I don't see that this was answered. This might be a dead thread so I might just be checking my own sanity.

The Miata fuel level sender is just two wires in which resistance is measured; one grounded and one straight to the dash. I don't know the voltage that the dash was running on, but I suspect 5v. I also don't know what resistor, if any, is in the cluster or if it does convert it to voltage. The miata dash gauges were kinda "dumb" so it wouldn't surprise me if it was just reading the ohms.

As kiwiracer pointed out, this is the typical readings for the fuel levels:
Full: 3 ohms
3/4: 15.8 ohms
1/2: 32.5 ohms
1/4: 64.2 ohms
E: 110 ohms

So to convert these ohms to voltage, we need to know the voltage reference. For that we should refer to the AnalogX, which is 5v, and not the 12v system.

Using a 220 pull-up resistor, we'd get:
Full: 4.933v
3/4: 4.665v
1/2: 4.356v
1/4: 3.871v
E: 3.333v

I think I'd want a larger spread between voltage readings, so I might use a 22 resistor.

[brentp]

Don't use too small of a resistor, otherwise the current consumption will be high. Use an ohms law calculator to ensure you don't exceed more than 100mA or so.

[l8apex]

Used a calculator and it showed a 22 resistor would be 180+mA. So I'll stick to a 220 resistor. Thanks.

[indianapolis77]

I think I'd want a larger spread between voltage readings, so I might use a 22 resistor.

This is a good idea.

[Chappy]

I’m just about to tackle this on my Miata. Wondering if anyone has a success story on how to add/calibrate for AnalogX. Did the 220 ohm resistor work?

Is there a way to do this and still keep the stock gauge working also?

[brentp]

You'd have to check the voltage range of the original stock gauge to see if it has a 0-5v sweep from empty to full.

We have a general guide here which should be helpful:
https://wiki.autosportlabs.com/RaceCapturePro_Sensors

However, please note AnalogX works with linear sensors. A fuel level sender will likely have a curved response, which is more suitable for a RaceCapture/Pro analog input.