Reducing Agent Heater Control Module -J891-

WARNING: This page is about a different car, the 2010 Audi Q7, 2009 Audi Q7, 2008 Audi Q7, and 2007 Audi Q7. However, it is still accessible from the selected car via links, so may be relevant.

Reducing Agent Tank Heater (Heating Circuit 1) -Z102-
Reducing Agent Pump Heater (Heating Circuit 2) -Z103-
Reducing Agent Line Heater (Heating Circuit 2) -Z104-

The SCR system contains a heater for the tank and the dosing module, including the supply line, since the reductant freezes at temperatures below 11 degrees below zero Celsius (12 degrees Fahrenheit). The electric resistance heater thaws the reductant so that it is ready for dosing. The SCR dosing line heater circuit has a high-side driver and a low-side driver, the current of which can be monitored separately. The advantage of this is that, when a short circuit is detected in one, the other driver can be switched off to ensure component protection. The SCR heater additionally has pull-up and pull-down resistors, which are used to monitor the lines when the heater is not active. For this purpose, a transistor is used to switch defined voltage levels via high Ohmic resistors (pull-up, pull-down) and the voltage of the corresponding lines is monitored. The circuit monitoring of the SCR dosing line heater takes place in the SCR control module, which transmits the fault messages to the ECM, where the corresponding fault is stored in the fault memory.