Fig 1.

Common people

Diagnosing a common fault is not as simple as it might seem; You need to test as if you are seeing it for the first time

Fig 2.

Published:  01 December, 2021

How do you go about diagnosing a common fault that you have seen before and all the symptoms match? Do you go ahead and fit that new part with no testing? Do you go straight to where you think the issue will be or do you test to be sure regardless of the situation?

You may or may not recall several articles ago, in the May 2020 issue of Aftermarket, I had a Land Rover Discovery 3 which would not start after being jump-started incorrectly and was fixed by reflashing the engine control unit software. Well, strangely enough, I was recently presented with a Range Rover Sport with near enough the exact same initial symptoms and fault codes. I want to show how starting afresh and testing, instead of jumping to the same conclusion, prevented a misdiagnosis.

Customer complaint
The customer’s complaint was that the vehicle would crank over but would not start. They said previously that the vehicle had started showing an intermittent no-start condition after sitting for a short period of time, for example to go into a shop. Once they returned, the car would crank and not start. The customer had discovered though that if they then waited five minutes and tried again, the vehicle would then start and be okay for the rest of the day. However, by now the symptoms had slowly become worse and no amount of cranking would start
the vehicle.
    
As always in my diagnostic process, the first step is to confirm the customer complaint and look for any tell-tale clues along the way. Yes it seems silly on the face of it to crank the engine over knowing it will not start, but an experienced technician may pick up a clue which will give direction where to go next so it always pays to always confirm the complaint. On this occasion confirming the complaint revealed no clues so it was on to the next step and to check for fault codes and review some live data.

Multiple fault codes
As can be seen in Fig.1, we have multiple fault codes stored for all different circuits and systems on the vehicle so where do we start? As in previous articles I have written, I always like to split them up into a list and put the most likely causes at the top and start there. Looking at the list we have five fault codes and I felt three could cause the no-start.
    
There are a number of likely causes. It could be a lack of fuel pressure, as the fault code states it is too low. The DC/DC converter fault also is another clue, as this converts the 12V supply from the battery and boosts it up to 60/70v to open the fuel injectors. The fact that code is stored could be another reason the engine will not start and the system voltage low fault code as this could indicate the control unit isn’t receiving the voltage it should to operate correctly.
    
The other two fault codes I felt could be put to the bottom of my list. An EGR fault most likely would not cause a no-start issue on this particular engine and there are two fitted due to the the engine being a V configuration. Having plenty of experience with this engine, I have seen many stuck open and closed EGR valves not cause the customer’s complaint due to the pipework configuration so it could be ignored for now. Lastly, there is the control box fan fault. This is a small fan mounted next to the engine ECU to control its temperature and would also not cause a no-start complaint.

Live data
My next step was to consult live data and look at module voltages and fuel pressure as these were at the top of my list. Cranking the engine while monitoring rail pressure showed there was next-to-no fuel pressure being generated, so this is one of the reasons the engine will not start. In that case, why do we have a low system voltage code and a DC-DC converter fault logged? With reference to Fig.2, looking in the module voltage section in live data showed why we have 0v for battery voltage and 3v for the DC-DC converter. As I mentioned, this should be around 60-70v on this particular vehicle so this explained the reason for the other fault codes. I then decided to pull up a wiring diagram and look at how the engine ECU was supplied power to formulate a plan of attack for these faults.

Test plan
The main power supplies were from several fuses in the engine bay fuse box, most of which were fed power when the engine control relay was energised. This also then turned on the fuel pump relay and allowed the low-pressure fuel pump in the tank to operate. I now had information to write up a test plan. My plan was to check low fuel pressure as it was easily done by a Schrader valve on the top of the engine. If there was nothing I could now be confident to test the fuses for power and the engine control relay and the fuel pump relay for correct operation as these items linked all my faults together. Upon connecting a gauge to the fuel system and turning on the ignition, I had no fuel pressure coming from the tank, so I was happy to now test at the fuse box. I checked the fuses which were supplied power from the engine control relay and all had nothing, so I moved onto the engine control relay. Upon touching the relay to remove it to test its inputs, a click was heard and the engine started to buzz indicating there was power being supplied to components. A quick circuit check showed the relay to be faulty and intermittently latching correctly internally so a new relay was fitted and the system retested. I now had power at all the fuses required and the low fuel pressure was now correct as the in tank pump was being energised. Looking to Fig.3, a check of live data now showed the correct voltages. A fault code clear was then done and upon cranking the engine it now fired into life, with only the EGR fault code remaining. After some checks this was found to be seized. The vehicle was then rebuilt and handed back to the customer.

Starting fresh
To summarise, a faulty relay caused all the problems. Having seen similar fault codes on the previous vehicle I mentioned, it would have been very easy to go down the same path.
    
However,  starting fresh and applying methodical thinking and a plan of action with technical information found the fault quickly and accurately. Always confirm the cause of the problem and don’t rely on what you have seen before as it may not always be the same.

Fig 3.

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