Theory Of Operation
The Diesel Exhaust Fluid (DEF) System uses a solenoid style pump. There are two separate solenoids inside the pump, one for the pressure pump and the purge pump. The pressure pump is used to pressurize the system and deliver a supply of DEF to the DEF injector. The purge pump runs for a brief time after the vehicle has been shut down to remove DEF from the supply line. The DEF pump operates in three main stages: Pressure Build, Metering Control, and Purge.
There is no pressure sensor in the system, rather pressure is a modeled calculation based off the amperage draw and the duration of the pump stroke. The higher the amperage and the longer the stroke, the higher the pressure is. Pressure seen on the scan tool is a calculation performed by the Powertrain Control Module (PCM).
Once the SCR Catalyst reaches operating temperature of 134°C (273°F), the PCM will begin the pressure build phase. The PCM will activate the DEF Supply Pump to refill the line. As the line fills, the pressure builds to the set point. Then, the PCM turns on the DEF Injector to bleed air out of the Pressure Line. Each time the injector is turned off, the PCM monitors the amount of time it takes for the pressure to rise back to the set point to determine when all of the air is bled out of the line. During Metering Control, the PCM adjusts pump run time to maintain a steady pressure between 4.5 - 8.5 bar (65 - 123 psi).
When the engine is shut off, the Purge pump will activate. The purpose is to draw the DEF back into the tank to protect the line. DEF will freeze at -11°C (12°F) and its concurrent expansion has potential to damage the pressure line.