Sensor, Diesel Exhaust Fluid Level/Temperature: Operation

The Diesel Exhaust Fluid (DEF) Tank Level Sensor and Diesel Exhaust Fluid (DEF) Tank Temperature Sensor are both on the Printed Circuit Board (PCB) in the DEF supply module. The temperature sensor reads tank temperature and reports it to the PCM via a PWM signal. The PCM uses this information as part of its heating strategy. The level sensor is ultrasonic and has no contact with the DEF. It uses a focus tube built into the supply module to direct sound impulses through the DEF. The height of the DEF is determined by the Time of Flight (TOF), the amount of time it takes for the echo from the impulses to return to the sensor. This calculation is based off of known good DEF (ISO 22241) and can be sensitive to contamination or an obstruction in the flight path. The signal sent by the level sensor is a PWM signal, which is decoded by the PCM into millimeters and a percentage, and displayed to the driver. This calculation will vary based on tank shape and size. The level sensor is sensitive to temperature, thus the tank must be thawed before sensor can take an accurate reading. The DEF/air transition area is critical to the level sensor operation. The tank is specially designed so that an air gap is maintained above the sensor, even when the tank is full. Overfilling or "topping off" the tank can cause the level sensor to not read correctly, due to the DEF/air transition area not being present. The level sensor does not update like a level sensor in a fuel tank. Certain parameters have to be met before the level sensor will update in both the PCM and on the gauge for the driver. The level sensor needs to be free of foreign materials, and cannot have any ice in its path. Tank level may not update immediately after adding fluid when temperature is below 12°F (-11°C).