Tech Tip: Diagnostic Thinking from A to Z – UnderhoodService
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Tech Tip: Diagnostic Thinking from A to Z

A customer brings their car to your shop with a concern. How do you get from broken to fixed? I’m sure you’ve had many people ask you over the years a question that goes something like this, "Sometimes my car doesn’t start, we already tried a new this and a new that, but it stills does it. What causes them to do that?" What’s worse, they could say that the shop down the street tried a new this and that and didn’t fix it. Yeah, maybe it was the shop down the street, but it still put a bruise on the same repair ….

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By Glen Beanard
Contributing Editor

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A customer brings their car to your shop with a concern. How do you get from broken to fixed? I’m sure you’ve had many people ask you over the years a question that goes something like this, “Sometimes my car doesn’t start, we already tried a new this and a new that, but it stills does it. What causes them to do that?”

What’s worse, they could say that the shop down the street tried a new this and that and didn’t fix it. Yeah, maybe it was the shop down the street, but it still put a bruise on the same repair industry that we are all in, so we all still feel it a little bit. Not only does guesswork lower customer confidence, it can be costly to a repair shop. So how do you get from point “A” to point “Z” efficiently?

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Go with the Flow
Well, there are always the factory flow charts. They usually get you the right answer if you read and answer correctly. These charts are written by the people who designed the vehicles and sometimes they have certain specs found in them that you might not find anywhere else, such as internal resistance values of some components.

For example, if you were to look up the Ford factory flow charts for an Evap purge valve fault circuit fault (P0443) on a 1998 Taurus, you’ll find that at some point, the flow chart will ask you to measure the internal resistance of the coil windings in the solenoid and it will ask if it is between 30 to 38 ohms. A “yes” answer passes the solenoid and a “no” answer fails it, so obviously the spec of that solenoid is 30 to 38 ohms. Also, flow charts are good to use as a guide into unfamiliar territory. So there are some handy advantages to having access to the flow charts.

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On the other hand, flow charts bring their own pitfalls. One of the biggest pitfalls is that they often only offer a temporary solution to the underlying issue. That being that the technician may be using them because he or she is in unfamiliar territory.

In this case, the technician may become dependent on them because what to do is stated in them, but rarely why you’re doing them. Or at least, the why is not always spelled out. The reasons why a flow chart instructs a technician to perform certain measurements can be found by reading between the lines and applying reason.

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For example, if a flow chart step instructs a technician to measure the resistance of a given wire at a given connector to chassis ground, and the question it asks is “Is the resistance 10,000 ohms or higher?” and a “yes” answer moves the technician on to the next step, whereas a “no” answer results in instructions to repair the circuit, then obviously that wire is not supposed to have a clear path to ground. Now, I realize that is a very straightforward example, but all other steps of a flow chart hold the same information within them if the technician looks for it. System-specific resistance and voltage values, frequencies, as well as generic basic electrical testing can be found within a flow chart. A technician can use this information to hone his or her own diagnostic skills.

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Get a Good Reading
To further understand the hidden training class that is contained within a flow chart, the technician can read the appropriate system description and operation before starting any flow chart testing. In the case of a fault code, the technician can also review the code set criteria to get an explanation of what it was that the PCM was looking at to reach a particular code.

Whether a technician is using a flow chart or using his own methods, the process may be very similar. It resembles a funnel, with all of the possible causes flowing into the mouth, and only the root cause falling out the bottom. The symptoms are first linked to a system(s).

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For example, a symptom of a check engine light on would point to the powertrain control system as a cause. Since the powertrain control system is comprised of many subsystems with a vast number of components collectively, the focus is very wide as to what could be at fault here. So, further inspection of the symptoms reveals a P0402 (EGR Excessive Flow) in the PCM’s memory.

Is this the cause of the check engine light? No. This is still a part of the symptom. The cause has not been diagnosed yet, we’ve just taken a closer look at the symptom, which narrowed down the list of possible causes. After all, you wouldn’t consider a loose gas cap as a cause of a P0402 now would you? However, a loose gas cap was on the list of causes back when the only known symptom was a check engine light on. So the funnel is closing down a little. Now the possible causes have narrowed down to the components and wiring in that system.

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The next thing to consider is any PCM strategies that may link one subsystem to another.

For example, certain Nissans had a PCM strategy that if the engine was misfiring with the EGR open and the misfires stopped when the EGR is commanded closed, then it is assumed by the PCM that the EGR was flowing excessively and set a P0402 based on that. In that case, we know we should include the ignition system as well as the injectors when testing for a P0402 on a vehicle with such a strategy. That’s where reading the description and operation information (as well as the code set criteria) can provide good insight into operating strategies that are unique to certain vehicles.

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Pinpointing a Decision
So now that a subsystem has been chosen to focus on, pinpoint testing can be performed. This is probably the most critical part of testing. Unfortunately, this is also where mistakes are often made. Following a flow chart does not make a technician any more mistake-resistant. Questions can be misread, wrong wires can be probed and wrong testing answers can be followed, to name a few. And who says the flow charts are written perfectly anyhow? Flow charts can contain mistakes, as well as statements that may be misunderstood.

For example, if you were to follow the factory flow chart for an early ’90s Town Car for noise vibration and harness and answer accordingly, it will give you some unusual sounding instructions. After removing the wheels to check hub run out and finding nothing wrong, it will say “replace vehicle and test drive.” As funny as that might sound, the chart doesn’t mean “replace vehicle” in the manner we say to replace a part. It means to put the wheel back on the vehicle and lower it to the ground.

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Though I’m sure no one has replaced a vehicle over that word choice in the flow chart, I’m sure that there has been a lot of confusion generated from word choice in other flow charts.

Another one that I like is, after repairing a wiring fault in a wiper circuit of a Taurus, the technician is instructed to drive the vehicle through a car wash. Hmm…wax or no wax? All they really want is that the repair is fully tested. This would provide a chance to not only test the wiper motor, but also to test the repair against the original root cause, which was water getting into the harness and bridging from pins at a connector.

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In the End…
OK, so let’s assume that the P0402 is on a ’99 Explorer. Code set criteria indicates that the engine must be at idle to set an excessive EGR flow code. Because what the Ford PCM is looking for is EGR flow at idle. If you use the flow chart to test this code, it will have you attempt to check for EGR flow at idle by seeing if the engine stalls during the KOER test when it idles down. It has you do this twice. If the engine doesn’t stall, then it passes and sends you to the next step.

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What if the EGR isn’t flowing enough to make it stall? In the next step it has you test the DPFE sensor directly by applying vacuum to the sensor and watching the voltage on the data PID. If the sensor’s voltages are not right, it says to replace the sensor. It does not have you test the wiring, just replace the sensor.

What if the voltages are still not right with a new sensor? It doesn’t say that. Though it is not perfect, and has you repeating same steps over like running the KOER test twice, you will most likely find and fix the cause of the P0402 eventually. If a technician were to test this vehicle with his own methods, how might it go? Perhaps he’d first observe the engine’s idle quality and check the temperature of the EGR supply tube to determine if the EGR valve is leaking exhaust gases into the intake. Maybe he’d also compare KOEO voltages of the DPFE against KOER voltage to see if the DPFE is reacting. If he suspected the EGR valve was flowing, maybe he’d block of the EGR valve by fabricating an EGR base gasket that is solid and without any means of flowing EGR.

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So what is the difference between the flow chart and the technician using their own method? Both methods seemed to start with the same goal. It’s just that one took more advantage of the fact that the vehicle was in the hands of the one deciding what to test next, and one was written by someone who could not see the vehicle.

Regardless of what method is used, the idea is similar. To work your way down that funnel and get that much closer to the small opening. Starting with the symptom, then to the suspect system, then to the component, to the root cause, then confirming the fix.

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