• 2021

DEF Pump Replacement

Diesel engines have seen a refinement in efficiency over the years that has resulted in not only a significant increase in fuel economy, but an impressive reduction in tailpipe emissions, including oxides of nitrogen (NOx). The development of the SCR catalyst system is the main reason for the success in controlling NOx.

DEF System Basics

The primary emissions control component within a diesel vehicle’s emission control system is the selective catalytic reduction (SCR) catalyst. This technology converts NOx from the engine exhaust to inert nitrogen and water vapor by injecting small quantities of a special urea-based fluid (DEF) into the hot exhaust gases stream (see Figure 1), where the DEF vaporizes and decomposes to form ammonia and carbon dioxide. The exhaust gases and ammonia/carbon dioxide gas mixture continue to the SCR catalyst, at which the catalytic conversion of NOx into inert nitrogen and water vapor — the carbon dioxide passes through — occurs. The SCR system includes two NOx sensors and special software that monitor and optimize the system’s NOx control efficiency.

FIGURE 1 | SCR CATALYST WITH INJECTOR NOZZLE

FIGURE 1 | SCR CATALYST WITH INJECTOR NOZZLE

Ram trucks equipped with either the 3.0L V6 diesel or Cummins 6.7L I6 Turbo Diesel are equipped with the SCR system to reduce NOx emissions. These trucks will have two fill tubes: one for diesel fuel and the other for DEF (see Figure 2). The diesel fill tube, which is larger in diameter than the DEF fill tube, has a light green cap, labeled in white letters, with the word DIESEL.

The DEF fill tube, which is smaller in diameter than the diesel fill tube, has a blue cap, labeled in white letters, with the word DEF. Depending on the model of truck, these fill tubes might be side by side, as shown in Figure 2, or the diesel fill tube is at the back of the truck frame and the DEF fill tube right behind the cab.

FIGURE 2 | DIESEL/DEF FILL CAPS

FIGURE 2 | DIESEL/DEF FILL CAPS

Note: It is important that diesel fuel never be poured into the DEF tank; if this occurs, damage to the DEF pump and/or SCR catalyst can occur. DEF is supplied in a plastic jug with a special nozzle that is screwed onto the top of the jug (see Figure 3) for ease of pouring. Similarly, it’s important not to put into the diesel fuel tank.

FIGURE 3

FIGURE 3

DEF System Problems

Performance problems can occur with DEF systems; for example, excessive fuel or oil entering the exhaust system due to a failed engine component or fuel injector can cause the temperature to rise and therefore damage the SCR catalyst. While it might not be readily apparent from a performance standpoint, what will be affected is the NOx emissions level from the exhaust.

The DEF dosing control unit (DCU) is a smart device that is used to control the operation of the SCR system by monitoring a variety of DEF system operating conditions and communicating that information to the powertrain control module (PCM) over data link circuits.

The effectiveness of the SCR catalyst in converting NOx is monitored by a NOx sensor probe and module assemblies located upstream and downstream of the SCR catalyst. These modules are smart devices that are calibrated to each other in order to communicate to the PCM over data link circuits the upstream/downstream NOx emission levels measured by the sensor probes.

In addition, the NOx sensor modules perform their internal diagnostics and report malfunctions back to the PCM. The two NOx sensors are not interchangeable, and if a sensor probe is damaged, the entire assembly must be replaced. As mentioned above, excessive fuel entering the exhaust system can result in damage to the SCR catalyst. While a damaged engine component might be the culprit, this can also occur if diesel fuel is added directly into the DEF tank. While this seems very unlikely, it does occur from time to time, so care should be taken when filling the fuel tank as well as when filling the DEF tank. As shown in Figure 2, in many applications, the DEF and diesel fill tubes are side-by-side which can lead to accidentally filling the DEF tank with diesel fuel. If diesel fuel does make its way into the DEF tank, the DEF pump will inject the fuel directly onto the SCR catalyst, which can damage the catalyst outer layer, or washcoat, as well as other DEF system components, such as the DEF pump (see Figure 4). Excessive exhaust gas temperatures (> 225°F) can result. Also, if this situation were to occur, it is possible that a diagnostic trouble code (DTC) will be stored and the engine trouble light will be illuminated. A likely DTC to be displayed is P0422 — Main Catalyst Efficiency Below Threshold Bank 1.

FIGURE 4 | DEF PUMP (USED)

FIGURE 4 | DEF PUMP (USED)

Replacing the DEF Pump

When the DEF pump is damaged by diesel fuel or otherwise, it must be replaced. This is a rather straightforward procedure; however, care must be taken. DEF is highly corrosive. If the wire harness connector is exposed to DEF during service, the harness must be replaced, as well.

Disconnect and isolate the negative battery cables. Next, raise and support the vehicle, then drain the DEF tank. This is done by disconnecting the DEF fill tube from the DEF tank, then draining the tank using a commercial hand pump. Store the DEF in an appropriate container. After the tank has been emptied, remove the pump cover bolts and the pump cover. Refer to the DEF pump ② in Figure 5. Disconnect the supply line ③ from the DEF pump. Disconnect the wire harness connector ① and tie the wire harness connector away from the pump to prevent exposure to the DEF fluid. Using the SAE Fuel Pump Lock Ring Wrench 9340, remove the lock ring from the DEF tank. Remove the DEF pump and level sensor. Remove and discard the O-ring.

FIGURE 5 | DEF PUMP REMOVAL

FIGURE 5 | DEF PUMP REMOVAL

Before installing the new DEF pump, clean the O-ring surface and install a new O-ring. Install the new DEF pump and level sensor into the DEF tank. Using the SAE Fuel Pump Lock Ring Wrench 9340, install the lock ring onto the DEF tank. Untie the wire harness and connect it to the wire harness connector (see Figure 5). Connect the DEF supply line to the pump. Install the DEF pump cover and bolts. Tighten the bolts to 10 ft-lbs. To finish the replacement procedure, remove the support and lower the vehicle. Fill the DEF tank with fresh clean DEF fluid. Connect both negative battery cables.

Using the appropriate scan tool, verify that the DEF tank level is at least 50%. Then, perform the DEF Reductant Doser Prime Override Test to its completion. Perform this test two times. Start the engine and test drive the vehicle for at least 15 minutes. After the test drive, use the scan tool and check for DTCs.

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