Electrically all sensors can either be resistive (eg a temperature sensor) or voltage (a suspension potentiometer). The problem with fuel tank sender units is that they are normally resistive sensors, and AIM products require voltage signals to work. To convert a resistance sensor to a voltage, a pull up resistor is required, see figure 1 below
V signal = Vref * Rfuel
Rpullup + Rfuel
Equation 1 – voltage divider
This circuit is called a voltage divider, as the supplied voltage (Vref ) is divided between the two resistors (Rfuel – resistance of the fuel tank & Rpullup – resistance of the fixed pullup resistor). Rpullup is a fixed voltage, and Rfuel varies depending of the volume of fuel in the tank. The equation for the voltage output is shown above, in equation 1.
Therefore if two resistors are equal in value, the output voltage would be half the supply voltage. Similarly if the fuel tank resistor is half the value of the pullup, then the output voltage would be two thirds the supply voltage.
The first job to do is to check whether the sensor is a resistance type or a voltage type. A voltage sensor would have three wires or contacts, compared to a resistance sensor would have two wires or contacts. Double check this using a electrical multimeter.
If it is a voltage sensor you are in luck and can miss this stage and go to the next section. If it is a resistance sensor you unfortunately have a little more work to do.
If it is a resistive sensor you need to determine the maximum resistance seen by the sensor. To do this monitor the resistance between the two poles of the sensor through the whole range of the tank. Once you have done this select a pullup resistor of value about the same value as the maximum measured in the tank.
The voltage reference, supplied by the AIM logger, should go to one side of the pull up resistor ( pin 4 ). The other side of the resistor should go to signal, ( pin 1 ) and to one side of the fuel tank sensor. The other side of the fuel tank sensor should go to analogue ground. (Pin 2 ) The pinout stated here is for a standard Aim 4 way 719 Binder conector.
Now you have a voltage type sensor which will give a value understood by the AIM logger for varying levels of fuel load. The next stage is to record the voltage output for the fuel in the tank. To do this I recommend that you use the AIM logger as a multimeter. To do this create a custom calibration with values as below
X (mV) Y
1 0 0
2 5000 5000
Compute the curve, save the channel as “mV output”, and assign this sensor type to the channel for the fuel tank. This will give you the real voltage output (in millivolts) for fuel entered.
Now work out sensible steps of fuel to put in, keeping in mind the maximum number of steps allowed is twenty. Also be aware if the tank has an odd shape, if so group more samples around the step. For example if you have a rectangular 50 litre tank, measure and record the voltage from empty to full in 2.5 litre steps.
Now you have the voltage for individual steps of fuel load. The next step is to enter these values into Race Studio 2 for the logger. Simply create a new custom configuration and enter the values you recorded previously. Compute the curve and save the sensor as “Fuel Tank”. Select this as the sensor type for the fuel tank channel.