We’ve all heard the saying “knowledge is power,” but it can be quite profitable, too! A rock-solid technician is one who has honed his/her craft as being a problem-solver. We often think of that person as one who can “fix” anything. Although that may be true, there is certainly more to it than being good with your hands. Having an understanding of the systems and the components that they are comprised of, along with the goal of that system, is what it takes to be successful in today’s world of automotive service, repair and especially diagnosis. Learning to leverage service information (SI) in your diagnostic approach will surely bring you success and efficiency. I’ll show you how it works for me!
Developing the diagnostic game plan
You’ve likely heard me speak of this many times in the past, but it’s worth repeating because I live by this approach: Having a plan (rather than just attacking the vehicle) is a great way to approach a diagnostic dilemma.
I know it seems counter-intuitive to step back and tread slowly (especially from a flat-rate perspective), but by first separating one’s self from the vehicle then creating a game plan and seeing it through, it will keep you focused on the facts. This, in turn, will create efficiency without missing any steps. When I approach a vehicle, I never do so without having a game plan. For me, this means gathering some pertinent data about the components that make up the system I’m to address:
• How are they configured to carry out a goal?
• What is that goal?
• How do we know the goal was achieved or not achieved (how is the system monitored)?
• What are some PIDs that can give me easy-to-grab insight into the system’s functionality?
Examples like the ones above can yield plenty of preliminary information about any problem on any system, in any vehicle, you may be facing. Think for a moment how powerful of a statement that is. Regardless of what is in your bay, it’s this information that will allow you to efficiently solve the riddle. Think of the game plan as nothing more than a series of questions. I invest my time (away from the vehicle) developing the questions I wish to ask the vehicle. The tools I use and the tests I perform will carry out the questioning process. The results of those tests are the answers I need to make the necessary diagnostic decisions.
Calling the huddle
Just like players in a sport, it’s the individual components that make up the team. The team is the system being addressed. It is our job as diagnosticians to know what players on the team are responsible for winning the tournament (The goal that the system is trying to carry out). And how do we know what the goal is? Through research.
This is why I separate myself from the vehicle. It takes time to familiarize oneself with the players on the team and how the sport is played. Thus, I spend my preliminary/initial diagnostic time understanding the information I mentioned earlier. This key information can be found in the service information (or SI). Many of us rely on aftermarket sources like ALLDATA, Mitchell, IDentifix, MotoLogic, etc. Some of us purchase access to OE service information provided by the manufacturers. It’s a common assumption that OE service information is the most complete. This is true in many instances, but I will warn you — even the information that comes from the manufacturer can be wrong at times. So when it comes to answering the age-old question: Which source of information is the best? The answer is simply “All information is the best.”
In many instances, I have found missing information from the factory side (or OE) that required my referencing an aftermarket source to find what I was looking for.
To demonstrate the importance of referencing SI, I want you to recall another recent Motor Age contribution of mine, “Diagnosing the causes of a ‘no communications’ concern,” March 2020, demonstrating two different vehicles carrying out the same goal (sounding the horn), but in two totally different ways (Figures 1, 2). The point is the horn sounds milliseconds after the horn pad is depressed…in both cases. If you weren’t aware, you may assume both vehicles function the same. You’d be sorely mistaken and assumptions tend to be costly. When developing a diagnostic game plan, I always seek out at least two key pieces of information — the theory and operation (sometimes called description and operation) of a system as well as the wiring diagrams. Together, those two pieces of information allow me to create the diagnostic game plan for whatever vehicle or system I’m addressing.
Troubleshooting flow charts — are they of value?
I recall at one point in my career being let down by the troubleshooting flow charts. I was then confident that any issue could be solved, so long as the flow charts were followed stringently. It was only to my dismay that many a time I was left with only the option to substitute a known-good component (like a PCM — I’m sure we’ve all been down those winding roads a few times).
I recall being discouraged by the fact that not only was the vehicle’s ailment still present, but I had invested a lot of time/money and was no further along than I was hours ago. How could this have happened? What was the missing element that prevented me from being successful? After all, the flow charts were written to help me fix the car by the same people who designed the vehicle, right?
WRONG! The flow charts are there to help the average factory-trained technician repair a vehicle (that is under warranty), in the most financially efficient way, the majority of the time. The flow charts were written concerning the most likely failures the system(s) might encounter. They were not written concerning our wallet. This is why we may spend so much time with disassembly/reassembly, rather than a pursuit with logic. Of course, we realize that the engineers who design the flow charts aren’t necessarily the same engineers that design the systems, either. There tends to be a disconnect between the two at times. My point is my frustration grew to the point that I viewed the flow charts as nothing more than toilet paper.
My point of view has changed over the years, though. I’ve learned that there is great information/key information within those flow charts that I use every day to streamline my diagnostic approach. Reading the steps and following them blindly is never recommended. Understanding what it is each of the steps is asking for will give you a good idea of what the ECU (monitoring the system) is looking for and anticipating, as well as a logical approach.
For instance, let’s look at resistance specifications. Keep in mind that voltage drop, resistance and current flow all relate to one another. Knowing what the resistance specification is calling for will allow us to anticipate how much current flow the circuit being monitored should draw. This is how circuits are being evaluated for performance and the reason why related DTCs are set when components’ ohmic values fall too far outside of specification. I’d much rather monitor a circuit’s current flow dynamically than to open the circuit and measure for resistance statically. A comparator circuit is used to carry out this task for the ECU’s self-diagnostic strategy (Figure 3). It serves as a DVOM (but within the ECU) to measure voltage in the circuit under various states of operation. In the example drawn here, a few things can be seen:
• This circuit is of a pull-down design (ECU provides the ground-path, to energize the circuit)
• The circuit is open and no current should be flowing
• The DVOM should be measuring/indicating source-voltage (12volts) in the circuit’s current state of operation
(This is what is typically is occurring when an ECU sets a DTC about “circuit low” faults).
The basic building blocks of diagnostics
By viewing not only the wiring diagram, but also the theory and operation of the circuit, this provides for a solid understanding of the circuit functionality and anticipation of what the ECU expects to see on that circuit during its current state of operation (energized or de-energized circuit). The DVOM represents not only where the ECU is monitoring the circuit, but also where we would place our DVOM to monitor the circuit ourselves. In the example in Figure 3, it should be obvious that the intended state of the circuit would have us anticipate source voltage at that point under the circuit’s current state of operation. A lot can be derived from just these few pieces of data.
Another statement you’ve heard me stand by many a time is: Having fundamental knowledge of the individual component’s functionality (at the most basic level) can be applied to any vehicle or system out there.
The point is that nearly 85 percent of what occurs in any automotive circuit can apply to any vehicle or system out there. This is due to the physics involved. The other 15 percent is how each manufacturer chose to make that circuit function (like what was described earlier in Figures 1 and 2).
Here, we see a diagram representing the magnetic field that builds within a device (like a solenoid or ignition coil) when it is energized (Figure 4). Assume, if you will, the circuit is configured similar to the circuit shown in Figure 3. As the solenoid is energized, the current begins to flow within its coil windings. Some of you will recall from science class that if a coil of wire is wrapped around a conductor and an electrical current, then passes through the wire, a magnetic field is created. This magnetic field allows the pintle (in the solenoid) to move or shuttle.
This is because electricity and magnetism are very similar to one another. The time for the magnetic field to build is referred to as dwell. If the circuit is opened and the current flow is interrupted, the magnetic field will collapse almost instantly. The energy that was created (over the dwell time) will have to dissipate instantaneously. The magnetic field will transform back into electrical energy and will be many times higher in voltage then the initial source voltage. A basic waveform representing the voltage signature of a healthy solenoid (magnetic/inductive device), when viewed on a lab scope is seen in Figure 5. Some of the characteristics exhibited display:
• Available source voltage (on the switched-side of the solenoid) with the circuit de-energized
• Very little voltage available with the circuit energized (indicating a good path to ground)
• A healthy inductive-kick nearing 100 volts (counter-voltage produced as the magnetic field collapses). This couldn’t have occurred if the ECU driver wasn’t functioning correctly or there was a lack of current flow/magnetic field, due to a voltage-drop or high resistance issue
Combining these basic fundamental concepts (from years of practice) along with the tools/testing techniques you’ve learned to employ (again…with prior practice) can help yield a diagnosis quite efficiently.
A shift in strategy
To demonstrate how I carry-out the process, I will use an example: 2008 Dodge Grand Caravan experiencing a stored DTC P0760 “Overdrive-solenoid circuit fault,” along with a transmission functioning in a defaulted state (no upshift from 2nd gear). The point isn’t necessarily that the vehicle was fixed, but more so how the circuit is being monitored and what techniques I recruited (from my experience and fundamental knowledge) to prove the fault and repair the vehicle.
As can been seen in Figure 6, I built my game plan away from the vehicle by referencing the service information for DTC P0760. The description/operation of the system and the wiring diagram proved to be all I needed in my arsenal to approach the vehicle and ask of it the questions I would like answered.
• Why won’t this vehicle upshift?
• Why is the DTC set?
• Is the solenoid functional?
• Does the solenoid have everything it needs, to function?
We can see that the ECU is anticipating seeing an inductive kick (sound familiar?). If you’ve made it this far through the article, you should realize that to have a healthy functioning solenoid, the resulting inductive kick (from its magnetic field collapsing) should be present. Said another way: If a weak inductive kick is present, the solenoid cannot do its job properly.
The service information and wiring diagram together tell us where to test, how the circuit functions, what we should anticipate seeing during a test and (most importantly) what the ECU is looking for to either verify or condemn the circuit, for functionality (Figure 7). By placing a lab scope at the point indicated on the wiring diagram and referencing it to ground, we could then, compare the signature to that derived from testing one of the known-good solenoids controlled by the PCM.
The PCM was located within the left-front fender well of the vehicle (Figure 8). The appropriate connector/circuits were identified and then probed to be monitored while using the scan tool to carry out a bi-directional control of the individual solenoids. The test results proved that the suspect solenoid circuit DID NOT produce a healthy inductive kick, like that of a healthy solenoid circuit (Figures 9, 10). It also displayed that the ground-side of the circuit was compromised and the PCM was likely at fault. This conclusion was made because when energized, the ground path still had significant voltage available on it. That, coupled with the fact that we tested directly at the PCM terminal, concluded that wiring was not an issue. If the PCM’s ground was compromised, the other solenoids’ circuits would’ve suffered as well.
The only logical explanation was a poorly functioning solenoid driver within the PCM itself. To further prove the fault, I continued to monitor the suspect circuit, but this time, I supplied an external ground, which allowed me to bypass the PCM. This enabled the solenoid to function properly. This resulted in a voltage signature that pulled very close to the ground and an inductive kick, similar to the known-good solenoid (when the circuit was de-energized). What isn’t displayed is a subsequent test I conducted. I followed the above test with a measure for current flow from each of the solenoids using the lab scope and a low-amp probe. Seeing that they all drew about the same amperage reassured me that the failed PCM driver hadn’t anything to do with a shorted solenoid winding.
Being an efficient and accurate diagnostician isn’t about having the fastest hands in the shop. It’s more about using logic. Having fundamental knowledge, built from mastering the basics (the 85 percent) and learning to utilize the tools you have properly means practicing on known-good vehicles and investing your free time to better yourself. Taking the time to develop a diagnostic game plan is derived from the goodies provided by service information. A combination of the wiring diagram and description/operation will yield you the arsenal you need to combat the vehicle and win the battle. It all starts with a little bit of discipline and patience; but, it ends with the rewarding feeling of a job well done and in a timely fashion.
