Your SCR system and AdBlue FAQ’s answered by Forté

Always wondered what AdBlue is? Or perhaps how the SCR system works? We’ve compiled a collection of your most frequently asked questions surrounding the SCR system and AdBlue. In this blog, we’ll be answering all these questions and more using our expert knowledge and years of experience in helping and advising garages on how to make a noticeable difference to a vehicle’s performance and enhancing customer satisfaction.

Read more

Let’s start off simple with what the SCR system and AdBlue are.

What is SCR – Selective Catalytic Reduction?

Selective Catalytic Reduction (SCR) is an advanced active emissions control technology system that injects a liquid- reductant agent through a special catalyst into the exhaust stream of a diesel engine. The reductant source is usually DEF (Diesel Exhaust Fluid) – AdBlue. 

What is DEF / AdBlue?

Diesel Exhaust Fluid (DEF) or AdBlue is used to reduce harmful gases being released into the atmosphere. AdBlue is a 32.5% solution of high-purity, synthetically manufactured urea in demineralised water. It is a safe-to-use fluid.

AdBlue causes a chemical reaction that converts nitrogen oxides into nitrogen, water and tiny amounts of carbon dioxide (CO2), natural components of the air we breathe, which is then expelled through the vehicle tailpipe. It works to protect the vehicle from any contamination that could cause serious damage. 

Now moving on to how these both work… 

What is the consumption of AdBlue?

AdBlue is injected in a percentage of 3-5% compared to diesel, so consumption is approx 1.5 litres per 650 miles.

How does the SCR system work?

As exhaust gases pass through the SCR cat, the system reads gas temperature and NOx amounts. It calculates the exact quantity of AdBlue to be injected into the system. AdBlue is injected in the exhaust gas before passing it through the catalyst. At a temperature between 300-400°C, a chemical reaction between AdBlue and NOx in exhaust gases reduces NOx (NO and NO2) to N2.

The majority of questions around SCR systems and AdBlue focus on the problems that may occur. Let’s address these… 

What problems occur with SCR Systems?

If the temperature is above 400°C, urea will burn rather than reacting with NOx, which will lead the system to be ineffective. If the temperature is below 270°C, the reaction rate will be low and the ammonium sulphates formed will destroy the catalyst and block the injector.

What problems occur with AdBlue?

AdBlue is a mixture of water and 32.5% of urea, so it shows us four problems; the fluid is very corrosive, it freezes at -11oC and can form crystals when the temperature of operation is below 260oC. It also expires after 12–18 months of manufacture. Remember that by using incorrect or low quality AdBlue that does not follow manufacturers standards, can cause costly repairs to the vehicle. 

To help prevent AdBlue system failure, we’ve created the Exhaust Crystal Preventer. Here’s a few questions and answers we’ve addressed surrounding our product and how to properly use it in order to get the most out of it.

What is the dosage rate of the product?

One 150ml bottle will treat all light vehicle tanks up to 20 litres.

Can I overdose the system?

It is recommended to only use one 150ml bottle of treatment per tank of Adblue. Increasing the dosage is not necessary.

What is the minimum dosage rate of the treatment?

When adding Exhaust Crystal Preventer at service, this can be added directly to the Adblue tank at any level. Working on an average, when the -mile warning has triggered, there should be around 3.5 litres of Adblue remaining in the tank. Where the warning has been triggered the AdBlue tank should be filled, and one treatment added.

I’ve topped-up the Adblue tank, but the warning light is still present on the dashboard. What should I do?

When the tank is low, crystals can form around the level sensor preventing the movement to register the tank has been filled. Fill the tank with Adblue and add 1 x Exhaust Crystal Preventer. Over the next hundred miles the level sensor should clear and the system should register the fill. If not, further investigation of the tank sensor will be required.

Will Exhaust Crystal Preventer remove crystals already in the system?

Yes, adding Exhaust Crystal Preventer to the Adblue tank will remove light crystals already present in the Adblue and exhaust system. Where crystals have formed and the engine management light is illuminated, the fault must be rectified first and Exhaust Crystal Preventer is added as a preventative treatment, after repair. 

I think this month’s case study will have HD techs very interested. As everyone knows Diesel Exhaust Fluid (DEF) Pumps are now in use on every aftertreatment system out there today on the modern heavy-duty vehicle. They are used to supply pressurized fluid to the doser injector, so it can deliver the fluid in an atomized spray into the selective catalyst reduction (SCR) chamber. Within the chamber, a chemical reaction takes place, and the result is a reduction in oxides of nitrogen (NOx). We understand the theory of how the pump is supposed to work, but how do we diagnose a DEF pump problem?

For more information, please visit DONZHEN.

In this article, I am going to show you how to diagnose a faulty DEF pump with a scope. This method is much faster than doing all the resistance checks the manufacturer wants you to do on the harness, according to service information.

These DEF pumps are not your average DC electric motors anymore (ones containing brushes/commutator that are energized through a switch). These electronic-controlled pumps need digital signals to work. Depending on the model, some will have a pressure sensor built internally to monitor pressure and will have valves in the pump to redirect flow during a purge cycle.

The other design I have seen (which seems to be common in many aftertreatment systems) is one having a pump without check valves. In this design, when the system shuts down, the DEF just goes back to the tank by gravity and depressurizes over time, but every DEF pump I have seen recently indeed has a brushless DC motor.

These brushless motors work by having north and south pole magnets mounted on a circular rotating assembly that attaches to the mechanical shaft of the pump. These magnets pass near field coils. When a set of coils is energized with current, the magnetic field of the coils causes the magnets to attract and then repel, which then give mechanical motion to the shaft of the rotor.

The field coil current is controlled by a microprocessor inside the pump, which controls when each coil is energized, so in order to know RPM of these types of brushless pumps, you have to know how many field coils they have. Also, on other systems, including the one I am going to talk about, there is a pressure sensor built into the doser.

My advice to everyone here is to make sure you look at service information and understand the system before you formulate a test plan. Unfortunately, from what I have seen from different manufacturers, their troubleshooting information on these pumps is certainly lacking. Don’t get me wrong: most of the time you can fix these issues with the available information. However, the challenge is understanding how to efficiently fix a problem like this if they don't tell us how the system works, or the signals the system relies on to function. Many times I like to figure out how systems work with a scope, so I may become more efficient in my diagnostic process.

The patient

We had a New Flyer 40 ft. Excelsior Low-Floor Bus with a B6.7 Cummins Engine. It has two DTCs

• — Diesel Exhaust Fluid Pump Command Circuit-Voltage Below Normal or Shorted to Low Source
• — Aftertreatment Diesel Exhaust Fluid Dosing Unit Relay Feedback-Voltage Below Normal or shorted to low Source

Next, I got an electrical schematic that I used with permission from diesel laptops, a diagnostic tool vendor that specializes in the sale of HD diagnostic tools and service information, as well as diesel technician training. There is a voltage, ground, control signal and a speed sensor signal to the pump. The Cummins ECM also gets a feedback voltage signal from the DEF dosing unit relay. This voltage signal (of 24V) on this model goes to the DEF pump and the Cummins ECM, which determines if the relay has turned on. This is one of the inputs the ECM uses to turn on the pump.

The speed sensor operates at 4.6 to 4.8V, and this signal is pulled to ground by the pump. To indicate the speed to the computer, please refer to Figure 1 again. I have also taken this pump apart to determine how many field coils it has, and on this model year with the B6.7, the pump has 12 field coils and is a diaphragm-type pump.

You will see in the images captured two voltage transitions of the speed sensor signal. This indicates one full revolution of the pump (Figure 2). We can now measure this frequency and multiply it by 60 to get a RPM reading. Frequency is measured in Hertz (Hz), which reflects cycles per second (why I multiplied by 60 — there are 60 seconds in a minute). This ratio is equal to Hz.

Scope time

Here I am going to show you how to test this pump by monitoring/understanding the following:

• Command signal
• Speed signal
• Verifying good power and ground to the pump

To test this pump promptly, you need to have a scan tool with bi-directional controls. You could take the vehicle for a test drive to get the pump to come on. However, if this is an intermittent problem, it makes it hard to find.

If you use bi-directional controls with the scanner, you can cycle the pump on and off to stress it. Doing it this way works better, so I hooked up my scope with a breakout lead I made to look at voltage, ground, pump current, command, and speed of the pump (Figure 3). I then initiated the dosing unit leak test with Cummins Insite, the factory diagnostic software. It took a few tries of cycling the pump, but I finally saw what I wanted to see.

Clearly, we can see the drop in voltage, so now the question I bet everyone will ask is what about the rest of the capture? As I zoomed in on the capture in Figure 7, I noticed that as the voltage dropped, the current increased to 12.5A (which is way too much for this pump), and at the same time, it kept the command signal low. By this, I mean that when the current went high to 12.5A, it brought the command signal voltage down, which caused the computer to see a low voltage when there should not be a low voltage.

Instead, it should show a 20V square wave command signal. The excess current of 12.5A from the pump did this to the command signal. On initial prime of this 24V pump, it should not go over 4.5A. As it steadies out, after it reaches prime, it should be no more than 800mA to 900mA.

By these readings, it is indicating to me that the pump is internally shorting to ground, causing this problem. Also, it makes sense with the DTCs because the voltage feed goes to the pump, as well as the feedback signal. The pump is the common point that is bringing all these circuits down.

New pump

I installed a new pump, and the voltage did not go below 21V on the initial turn on, and there are no more DTCs stored for the relay feedback circuit or command circuit (Figure 8). I have also marked rulers on the current waveform to get the RPM of the pump (Figure 9). This pump has 12 field coils on it, so, I counted out 12 coil current humps and then multiplied frequency by 60 (because there are 60 seconds in a minute), which equates to 13,677 RPM for this pump.

For more information, please visit scr pump(tr,es,fa).