How to use a multimeter to quickly determine the fault point of the SCR post-treatment urea pump?

The components of SCR post-treatment are:

Urea tank, urea level temperature sensor, urea pump, urea nozzle, upstream temperature sensor, ambient temperature sensor, NOx sensor, corresponding pipes and wiring harnesses.

Without diagnostic equipment

How do we determine the fault point?

The answer is based on the voltage at the pins of each harness connector.

Next, we will introduce in detail the pin voltage measurement of each component.

Urea sensor

First of all, we need to understand the corresponding positions of the sensor pins and the engine ECU. Let's first look at the four pins of the urea liquid level temperature sensor.

The measurement is divided into two steps: the first step is the non-working state (sensor unplugged), and the second step is the working state (sensor working).

The voltage of the urea liquid level and temperature sensor is measured when it is unplugged. The liquid level signal voltage of port 1 K57 is 4.7V, and the temperature signal voltage of port 3 K80 is 4.7V.

When the urea level temperature sensor is connected normally, when the urea level is 80%, the signal voltage of interface K57 is 1.3V, and when the atmospheric temperature is 22 degrees Celsius, the temperature signal voltage of interface K80 is 2.2V.

Exhaust temperature sensor

The exhaust temperature sensor is installed upstream of the post-processor to monitor the engine exhaust temperature. The lead wire of the exhaust temperature sensor is 1 meter long and has a metal braided layer on the outside. Next, let's look at the upstream temperature sensor:

When the exhaust temperature sensor is unplugged, the signal voltage of K81 on the left side is 4.8V. When the sensor is normally connected, the signal voltage of K81 on the left side is 0.9V.

NOx Sensor

The NOx sensor is a very important sensor, which mainly detects the NOx value in the exhaust pipe. After receiving the value, the ECU performs logic control for OBD monitoring. Unlike the first two sensors, this sensor is an active sensor. The NOx sensor assembly consists of two parts: the sensor and the NOx sensor controller. The nitrogen oxide sensor needs to be heated to a certain temperature before it can work properly. Otherwise, the condensation of water droplets in the exhaust pipe may cause damage to the sensor.

The voltage is measured when the NOx sensor is unplugged. Interface 1 is the power supply voltage, interface 2 is the power supply voltage 0V ground wire, interface 3 K76 voltage is 2.4V, and interface 4 K54 voltage is 2.5V

The voltage of the NOx sensor is measured when it is connected normally. Port 1 is the power supply voltage, port 2 is the power supply voltage 0V ground wire, port 3 K76 voltage is 2.4V, and port 4 K54 voltage is 2.5V

Urea pump pin definition diagram