Fuel level sensor calibration
Fuel level sensors are relatively simple devices with a few basic types that differ in how they measure fuel level. Regardless of the style, these sensors share a common feature: they send out a resistive signal (measured in ohms), which a Haltech ECU or Dash display can interpret to determine the amount of fuel remaining in the tank.
Wiring up a fuel level sensor
Most fuel level sensors have two pins or wires, with the resistance between them varying based on the fuel level. You can test this by using a multimeter set to measure resistance (ohms). These wires should be connected to an analog voltage input (AVI) and sensor ground. In most cases, the connections are not polarity-sensitive, so the wires can be connected either way. However, it’s recommended to consult your fuel level sensor’s documentation or the vehicle's wiring diagram to confirm if one wire is specifically designated as the fuel level signal and the other as the sensor ground.
On some fuel level sensors, there may only be a single pin that needs to be connected to an AVI, while the sensor itself grounds through the fuel tank. For these types of sensors, it is important to check and verify that the fuel tank is properly grounded to the chassis, and thereby to the battery, to ensure the fuel level sensor circuit functions correctly.
Wiring to a Haltech Dash display - Preferred inputs
Haltech Dash displays feature one AVI equipped with a 240-ohm internal pullup resistor specifically for wiring a fuel level sensor. On a Haltech iC-7 dash, this is AVI 4. On a Haltech uC-10 dash, this is AVI 1. Using this AVI with the internal 240-ohm pull-up resistor would be the easiest way of wiring a fuel level sensor in, as you don't have to physically wire an external resistor between the signal wire and the 5V supply. The 240-ohm internal pull-up resistor is used by enabling it in the NSP software.
Wiring to a Haltech ECU
If you're wiring the fuel level sensor directly to a Haltech ECU, it is recommended to use an external 240-ohm resistor like the one shown below. This is because most AVIs on a Haltech ECU are designed with an internal 1K-ohm pull-up resistor, which is more suitable for other types of engine sensors. Using the internal 1K-ohm pull-up resistor with a fuel level sensor will result in a very narrow voltage range, making it difficult to calibrate the signal accurately.
Pull-up resistor
A pull-up resistor is required when using fuel level sensors. This resistor has a fixed value to serve as reference as the resistance from the fuel level sensor varies. It is wired between the signal wire and the 5V output from the ECU or Dash display, converting the sensor’s varying resistance into a voltage range (0-5V) that the ECU or Dash can interpret and calibrate. The pull-up resistor's value should be selected to provide a wide voltage range (0-5V) when paired with the sensor's resistance range. A good choice for a pull-up resistor is 240 ohms, as most fuel level sensors output resistance signals between 0 and 300 ohms.
How to check if the wiring is correct and working
Once everything is wired, you can verify that the fuel level sensor is functioning by checking the AVI voltage in NSP. The voltage should change between 0-5V as you move the sender (e.g., the float) up or down, or as fuel is added to or used from the tank if you don't have direct access to the sensor. Ideally, the voltage range should be significant, such as a change of 3V or more between 0 and 5V. If the voltage remains fixed, too narrow, or only reads 0V or 5V as the float moves, the wiring, pull-up resistor value, or settings need to be rechecked.
Calibrating the fuel level sensor in NSP
Calibrating the fuel level sensor in NSP is straightforward. Start by recording the AVI voltage readings in NSP when the tank is empty and when it is full.
Use these readings to define 0% and 100% in the fuel level sensor calibration table in NSP.
If you prefer to calibrate using volume (e.g., liters), you can record additional voltage points by gradually filling the tank with known amounts of fuel. For example, begin with an empty tank, then add 10 liters at a time, recording the voltage reading at each step until the tank is full. Note that the calibration may differ from the example below and may form a curve, depending on how the float moves relative to the shape and size of the fuel tank.
For more practical tips on calibrating fuel level sensors, check out the video below.
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