Reducing Agent Metering System Delivery Unit -GX19-

The delivery system, filter, pressure pump, return flow pump, damper, level and temperature sensor is an integrated module which is in the bottom of the DEF tank. The output is a multiplexed PWM signal which includes following information: DEF level and DEF temperature.

• Reducing Agent Pump -V437-
• The three-phase AC motor of the SCR dosing pump is driven by a pulse-width modulated three-phase current. The monitoring of the supply lines works in different ways, depending on whether the SCR dosing pump is turned on or off. If the SCR dosing pump is running, the current is monitored, and if the current exceeds a defined threshold value, the voltage will also be monitored. If the SCR dosing pump is off, only the voltage is monitored, which is set via the corresponding driver power stages. One phase current is used to calculate the total current while the pump is running. The monitoring of the total current and the voltage at the output terminals of the power stage for the SCR pump takes place in the SCR control module, which transmits the corresponding fault messages to the ECM, where the fault is stored in the fault memory.
• Reducing Agent Return Flow Pump -V561-
• To avoid crystallization of urea after long vehicle parking and freezing of urea under very cold conditions the backflow pump is used to empty the urea lines after each engine stop during keep alive time.
• Reducing Agent Reservoir Sensor -G697-
• To monitor the fluid level in the reductant tank, a fluid level sensor is used, which gathers information on the fluid level with the help of a Reed switch. Each voltage value transmitted by the fluid level sensor corresponds to one defined fluid level. Some SCR systems will receive an ultrasonic level sensor, which is able to display continuously the tank level over the whole range from full to empty tank warning level. Therefore refilling detection is always possible if a minimum of about 1 gallon is refilled. This sensor sends an ultrasonic impulse and receives the echo which is reflected at the DEF surface. The time between sending and receiving the impulse correlates with the DEF level. The sensor sends a multiplex PWM-signal (tank level and temperature) to the ECU. This signal contains status and fault information about the level signal (frequency, duty cycle). The signal is evaluated by the sensor driver. The driver information is used by several monitoring functions.

• Reducing Agent Temperature Sensor -G685-
• In order to check the temperature value provided by the SCR tank temperature sensor, a comparison with the ambient air temperature is carried out. If this comparison raises doubt about the correctness of the reductant temperature value and the ambient air temperature is less than a defined threshold value, the SCR tank heater is commanded on and at the same time the increase of the reductant temperature value is monitored. This is to make sure that the comparison of the SCR tank temperature and the ambient air temperature does not produce a wrong result due to a potentially frozen reductant.