💻 The diagnostic pc, use it or lose it

Diagnosing faults in a vehicle can be a daunting task, and many technicians and fleet owners believe that a diagnostic computer will solve all their problems. To the contrary, a diagnostic computer is just a tool to assist in diagnosing faults. Take Volvo’s Tech Tool, the name speaks for itself Tech(nicians) “Tool” or more pertinently VCADS, “Volvo Computer Aided Diagnostic System”. As the name suggests, “Aided Diagnostic System” is there to “Aid” the technician, it does not mean it will fix the problem. The technician still needs to interpret what the diagnostic system is telling them and then go find and repair the fault.

When reading Diagnostic Trouble Codes (DTC’s), many technicians focus on the “Active / Present” codes. This is very wrong! Firstly, because they are so focused on the Active DTC’s, they prematurely clear the inactive ones, whereby the inactive DTC’s are just as important, if not more. Reading and interpreting ALL the DTC’s is what makes using the diagnostic pc worthwhile, and separates a technician from a good technician. If you do not use it correctly, you may as well chuck it in the cupboard and forget about it.

📜 Relevance of the DTC

In many instances, inactive DTC’s directly relate to Active DTC’s and clearing them only makes the diagnostic process more difficult. This is where understanding the various systems in modern electronic vehicles comes into play. Once we understand the system in question, we can then interpret all the DTC’s and make clear informed decisions on our fault tracing strategy. Think of DTC’s as clues to a murder case that are stored and categorised for later analysis.

Example 1: The vehicle suddenly loses accelerator and the engine switches off.

We connect our diagnostic pc and read the fault codes. We browse the codes but there are no active codes in any of the ECU’s. So now what? OK, lets see.. We look at the inactive DTC’s. We notice that all the control units in the vehicle have stored, inactive, J1939 data link fault codes. A hasty technician’s first instinct is to clear all the codes as they are inactive. No! This is where we analyse and notice that the engine ECU (EECU – MID 128) has not stored any J1939 codes. Immediately this tells us that all the control units are losing communication with the EECU and we should concentrate on figuring out why.

But why you may ask. The system is designed to shut down with a slight delay. In the event of a sudden loss of power, the ECU’s do not have time to log any faults, the same as if you disconnected the battery with the ignition on. In this particular instance, only the engine ECU is shutting down suddenly while the others are still powered up. This gives them adequate time to log the DTC but not the EECU. Basically the other ECU’s have said, “oh shit, where did MID 128 go”, and stored the note “lost communication with MID 128” in their memory for us to see later. When the EECU powers up again, it is none the wiser that it suddenly shut down and therefore does not give itself a DTC, in its mind it’s done nothing wrong.

As no J1939 faults are stored in the EECU, along with the symptoms of losing accelerator and the engine switching off, we can assume a power supply fault to the EECU. If it were a datalink (J1939) issue, the EECU would also log a J1939 fault, as it too would lose communication with the other control units. Further, if it were a datalink fault, we would lose accelerator yes, but the engine would continue to run. The only rational explanation to our symptoms would then be an intermittent EECU power supply fault.

In the example above, by not deleting the inactive DTC’s, we have eliminated many hours of fiddling and struggling and immediately narrowed it down to an EECU power supply fault, so we concentrate there and start looking at the power supply circuit. If we understand the power supply circuit, this should not take longer than 30 minutes to isolate the fault.

Example 2: Miss firing engine

We connect our diagnostic pc and read the fault codes. We browse the codes, and notice an active fault for injector number 2, mechanical system failure (MID 128 SID 2 FMI 7). Immediately our attention is drawn to injector number 2 and we waste hours and days guessing, replacing injectors checking the wires, fiddling and faffing, but the fault remains on number 2. Again, knowing how an engine and the electronic injection system works and with the aid of the diagnostic pc, it can assist us in determining where the fault lies.  

All mechanical engines will have 2 pistons at top dead center (TDC) at the same time. For a 6 cylinder engine they are, 1 & 6, 2 & 5 and 3 & 4. With common rail injection systems, we know that for every cylinder that “plus compensates” there will be 1 that “minus compensates” to achieve smooth running of the engine. This means if one cylinder is over fueling, its counterpart will under fuel. So, if number 2 minus compensates, its more than likely that number 5 will plus compensate as these 2 work in tandem. The same goes for 1 & 6 and 3 & 4, and vice versa.

In our example above with active fault on number 2, we can do 2 diagnostic tests before dismantling anything, compression test and cylinder balancing test. We start with cylinder balancing test and notice number 5 plus compensates to 100% and number 2 stays around 0% or minus compensates slightly. This doesn’t give us a definitive answer, but it tells us to start looking at number 5. We then do a compression test and notice that number 5 compression is low. Now we have solid information or clues suggesting that number 5 is our culprit and not number 2. Why do I say this?

Remember, if 1 plus compensates another will minus compensate. Generally, if 1 cylinder plus compensates, it means that cylinder needs more fuel to achieve even operation in the engine. If, in this case, number 5 has low or no compression, it will need more fuel to run correctly, and because number 5 and number 2 work in tandem, number 2 will give less fuel. The EECU then interprets this as a fault with number 2 (as if there is a problem with the injector not delivering any fuel), when in fact the problem lies in number 5. In a case like this we focus on the low compression of number 5, by checking valve clearances and worst-case scenario removing the cylinder head to inspect the piston, sleeve and valves.

P.S This example is very rare, as normally the EECU gets it spot on, but hey, electronics… what can I say. That is why it is so important to have a clear understanding of what you are working on. An auto electrician with no mechanical background called in to diagnose this fault would probably run out out the door with his tail between his legs.

📦 Conclusion

In both examples above, its clear to see that fault tracing 1 thing can be misleading if one does not have a thorough understanding of the systems, and how the diagnostic pc can guide us in the right direction if used correctly. The advanced section articles will delve deep into diagnostics, help you understand the various systems, as well as give you detailed fault tracing steps.