ECU and diagnostic strategy with James Dillon

James Dillon is a practicing Master Technician. He owns a specialist vehicle diagnostic workshop and is a world renowned seminar host and technical training instructor. 

I am covering a problem with a SEAT Ibiza 2010 MY which was initially presented with a flashing MIL and the vehicle was unable to rev to more than 2000 rpm. The Siemens/Continental Common Rail Diesel ECU had set trouble a trouble code for Fuel Pressure Regulator Short to Ground and Fuel Pressure Regulator Electrical Malfunction.

This job which seemed to look straight forward at the outset. A quick visual inspection of the diesel control system showed that it employed a technique known as two point fuel control; This is where the system ‘throttles’ the fuel intake to the high pressure pump via an inlet metering valve, and may also control the rail pressure via a pressure control solenoid.

The main reason for inlet metering technique is that the pump is allowed to pressurize the appropriate quantity of fuel for the engine’s speed and load conditions (i.e. it isn’t over working). Compared to the older ‘pressure control only’ system, where the entire pump intake fuel quantity is pressurised and the excess is dumped back to the tank, the inlet metering is much more efficient as it reduces unnecessary fuel pump engine drag, and the fuel temperature (and therefore its density) is relatively speaking much cooler.

The second part of two point fuel control is a pressure control solenoid. This device is located within the high pressure part of the system and essentially ‘dumps’ a quantity of pressurised fuel back into the fuel return. Normally, this occurs during load transitions, for example from throttle open to throttle closed. This device can affect the actual rail pressure more rapidly than through simply adjusting the inlet metering valve in isolation.

Now that we have a working understanding of the system’s key components, let’s get down to the diagnosis.

Fig. 2

The trouble codes were indicating an issue with the pressure control solenoid. But how does the vehicle ECU know there is a problem with this device? In particular, how does it know that there is a short circuit in the system? And if there is a short circuit in the system, why hasn’t the wiring or the component suffered a meltdown? In some cases, the supply voltage may come from a relay or a fuse, and the switching or control side is managed by the ECU. In our case (and relevant to the initial diagnosis), the ECU is responsible only for the switching. This is confirmed after consulting the wiring diagram for the vehicle (figure 2). N276 is the fuel pressure control solenoid and N290 is the inlet metering solenoid. The supply for both of these devices comes via fuse SB14.                      

As we’d had a diagnostic trouble code for this component, and the symptom the vehicle was exhibiting seemed plausible and to relate to a malfunction with pressure control, it seemed sensible to evaluate this circuit. Also, as there was a common fuse, if this was blown because of a short circuit, both the inlet metering and pressure control solenoids would cease to function. I attached the scope to the ground wire and placed an inductive current clamp in the circuit. I found that the circuit was functioning correctly. It was receiving a pulse width modulated ‘signal’ on the ground side of the circuit (provided by the transistor shown in the diagram).

cThe ground element of the pulse width was taken fully to ground, so no earthing or resistance faults were evident. The supply was good, and didn’t drop when the current flowed, so no obvious resistance faults in the supply. As current was flowing and the valve was operating – why was the ECU setting a code for this component? I decided to take control of the solenoid (via the ground wire) by applying my own ground (safely), and checked to see if the solenoid operated (which it did). With the ground applied, the measured rail pressure increased significantly, so to top it off, mechanically, the valve appeared to be operating as designed.

Fig.3

Figure 3 shows a diagnostic and evaluation circuit on the ground wire, within the ECU. The purpose of this circuit is to measure the voltage on the control wire. It’s a sort of on-board multimeter. If the circuit functions

correctly, this device, via the protection resistor, measures 12 volts, initially, during Key on Self-Test. This proves that the solenoid and circuit is intact: Essentially, 12 volts passes from the fuse, through the supply wire, through the solenoid, back, via the ground wire, past the diagnostic and evaluation circuit and into the control transistor. When the transistor turns on (and the wire is grounded), current flows and the solenoid turns electrical energy into mechanical movement (it operates). The diagnostic and evaluation circuit continues to measure the voltage and is anticipating a drop to almost zero volts (the volt drop within the circuit) when the solenoid is actively working. The diagnosis of a short circuit to ground would normally be indicated by zero volts being measured at the evaluation point within the ECU. It is possible that some ECUs may measure circuit current, rather than voltage.

In our case, the solenoid was functioning as designed, yet the ECU was complaining about the component; or was it? There are some cases where the diagnostic executive software within the ECU gets it wrong, and the DTC doesn’t actually relate to the right ‘malfunctioning’ component. A quick check in this case to save wasting time checking the wrong thing, is to force the component into a different fault condition, and then to check that the ECU is complaining about the actual component which the code indicated. In order to do this, we have to get the component to fail in a new way.

 So, if the component is failing ‘short to earth’, we will have to get it to fail in another way to prove the ECU’s internal logic. In this case, it’s sufficient to open circuit the device and check to see that a different failure condition DTC is set. Unplugging, then cycling the key (to reset the ECU and get it to re-evaluate the circuit) showed that the ECU registered a different fault on the same circuit. This time I had short to ground and an electrical malfunction fault. This proved that we were at the right component, but it rather muddied the water as to the nature of the root cause, as everything appeared to be working as it should.

The facts are that the component was working (and could be forced to work), however, the ECU was setting a short to ground fault. The ECU seemed to be monitoring the correct circuit, as it set a different code when the circuit failed in a different way (i.e. open circuit), but even when the component appeared to be functioning correctly, the ECU was setting a short to ground code. The possible root causes of this concern are 1) The ECU is faulty (maybe), 2) That either the solenoid or circuit are faulty (I seemed to have proved the circuit and its function) 3) A different component or circuit is in trouble and the ECU is being confused by the other failure (maybe).

After a quick visual inspection of the engine bay wiring, motivated by the fact that I had a short to ground fault, as wiring chaffs and rubs are normally responsible, I found something interesting (Fig 4). This happened on the EGR control solenoid wiring. At the time of writing the job is still underway, and I’ll update you of the outcome.

More testing is required to prove exactly what is at the root of this problem. It goes to show (as all experienced technicians already know) that a fault code is merely one element of a set of data which is required to accurately conclude the root cause of a fault. I hope to have shown some elements of a logical data gathering and analysis process, which is critical to quick and accurate diagnosis. In the same way that owning a cooker doesn’t make you a chef, owning a scan tool doesn’t make you a diagnostician.

James runs a series of diagnostic training courses from his base in Somerset. He also provides train the trainer sessions for colleges. See his website www.techtopics co.uk or contact him (0044 (0)1278 428699) for more details.