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Loaded Calipers: It’s a Good Thing

Adapted from Gary Gom’s article in Underhood Service


Many “nuts and bolts” technicians, both young and old, have now begun to realize that the “black art” of diagnosing and repairing motor vehicles has become immeasurably more important and more complex than just a few years ago. Today, the majority of vehicle functions are controlled by an invisible logic engraved on a silicon chip. Instead of linkage rods and cables, technicians are now dealing with a flow of electrons gliding silently through a copper wire. Given this scenario, it’s quite obvious why many technicians have become increasingly frustrated trying to understand how electronic control systems can be diagnosed and repaired.

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Know What You Need To Know

Obviously, the most important thing to learn in the diagnostic process is how things work. To illustrate in a context other than electronics, it’s practically impossible for a technician to diagnose an automatic transmission unless he understands how planetary gear trains, friction clutches, bands, roller clutches and electronically controlled shift valves combine to shift gears “automatically.”

The least important things to learn are the “silver bullets” and cookbook solutions that are often assumed to be a replacement for a sound understanding of how a vehicle’s electronics and processor operating strategies really work. It’s important to understand that, more often than not, the least knowledgeable technician can bedazzle listeners with a torrent of minutiae, factual distortions, silver bullets and just plain bad information. The most knowledgeable technician, on the other hand, often responds to a particular diagnostic situation with questions, not answers. The answers always come later, after the diagnostic procedure is completed.


Education is Key

What is the quickest way to learn how various electronic systems work? To begin with, the broadest range of knowledge is gained most quickly by enrolling in a good vocational auto mechanics program. Sure, some material is bound to be repetitive, but, because a formal adult-level automotive program exposes a student to all phases of the auto repair trade, it’s the quickest way for the beginner to become a journeyman technician or the veteran to expand his technical horizons.

In addition, by studying all eight basic ASE skill areas contained in a NATEF-certified program, the student becomes a much more rounded technician than if he were to learn the trade in an on-the-job situation. And that’s important because, as I said above, we’re now dealing with a vehicle in which most, if not all, conventional vehicle systems like engines, transmissions and brakes are now being integrated into mutually dependent operating systems.


Science Secrets

I’ve had high school students become disappointed when I emphasize the importance of science and math in the auto repair industry. Most youngsters visualize the auto repair trade as strictly a “hands-on, minds-off” trade in which academic skills aren’t required. Unfortunately, uninformed high-school counselors and teachers reinforce that misconception.

True, a technician doesn’t need science and math in order to change a muffler or a set of brake pads. But, when his work takes him into the realm of electronics diagnostics, having a good science background makes the learning process much easier. Without having had high-school level chemistry and physics, a student may find it difficult to understand how, for example, hydrogen ions can migrate through a seemingly solid substance – like a zirconium-oxide coated thimble – in order to produce an electrical current in an oxygen sensor. And that’s not to mention how more complex exhaust-sensing technology, like Honda’s Linear Air Fuel (LAF) or Toyota’s Air Fuel (AF) sensors, really work. Does technology like this boggle the mind? Yes it does, especially if the mind isn’t adequately prepared to deal with abstract scientific principles.


Nevertheless, without having had chemistry and physics in high school or college, it’s equally hard to understand how an EGR valve, for example, can control combustion temperature or how an engine with variable intake and exhaust camshaft timing can accomplish the same task by reducing the dynamic compression ratio in an engine. It sounds like black magic, but it’s only the laws of physics working in a mathematically precise manner.

When armed with a good science background, the technical learning curve also becomes much shorter for most technicians. By learning underlying scientific principles behind modern vehicle technology, those aspiring to become driveability technicians will also begin to develop a more intuitive knowledge of engine management systems.


It All Adds Up

Why do many post-secondary or college-level auto mechanics programs require college algebra? I’ve always looked at a math requirement as a screening process for auto mechanics students. In other words, if a student can’t learn to solve abstract math problems, he probably won’t learn how to solve abstract mechanical and electronics problems once he’s employed in the field.

Of course, that sounds like a harsh assessment, but I’ve found, during my years of teaching, that many students simply haven’t developed their logical and critical thinking skills to the point that they can understand, for example, how variable hydraulic fluid pressures can make an automatic transmission work or how electronic information engraved on a silicon chip affects vehicle performance.


Some students, for example, demonstrate an amazing ability to swap an engine or rebuild an automatic transmission in record time, but they never seem to be able to develop the knack of relating a specific performance symptom to a list of intangible causes.

To better illustrate, how does a student link a lean oxygen sensor code to a small particle of dirt stuck on a mass airflow sensor wire? Or how does he relate an ABS warning light to driving on a washboard road? There are lots of examples, but it doesn’t take long to understand that tactile or hands-on skills only do not make a good auto mechanic.


Problem-Solving Skills

On the other hand, when a student studies basic college algebra, he learns to analyze a problem and choose a mathematical process that will deliver a verifiable solution. Unless a technician has been exposed to a good math background, it’s really difficult to describe how the skills involved in solving a complex math problem transfers to solving a complex OBD II driveability complaint. But the logic in both cases is the same because the technician gathers as much factual data as possible, devises a mathematical or diagnostic strategy that will produce an identifiable result, and then proceeds to test the strategy in a real-world environment, such as in the examples below.


Keeping It Simple

Let’s illustrate “keeping it simple” by solving a P0304 DTC retrieved from a 1998 Ford Expedition with a 5.4L engine equipped with coil-on ignition. As most experienced techs know, component accessibility on these engines is poor, so testing mechanical issues such as cylinder compression, valve train action, vacuum leaks and fuel injector operation is difficult at best.

But, let’s say the customer interview indicates that the problem happened suddenly on the homeward leg of a long trip. The freeze frame data indicates that the problem happened at highway speeds at normal operating temperature. The engine has a detectable miss at idle and the misfire counter indicates that the #4 cylinder misfires on each combustion cycle. Would an ignition coil or spark plug fail that quickly? Not likely. But let’s look at the leaking heater hose just above the #4 cylinder. And let’s pull the coil off the spark plug to reveal a spark plug well full of coolant. From this point on, it’s easy to understand why problem-solving skills are so very important in keeping the diagnostic process simple.



Because electricity is an essentially invisible force, the beginning driveability tech often develops a technical blind spot because he will tend to visualize driveability complaints in terms of mechanical, rather than electrical failures. Let’s take an example of a cold-start complaint on a 1992 Chevy Blazer equipped with the 4.3L Vortec engine. Experienced driveability techs know that the fuel pump pressure must be perfect on a Vortec engine because a minimum of 55 psi is required to open the poppet valves on the “central” fuel injection system. The owner says he can’t hear the fuel pump activate on a cold morning. A fuel pump test reveals that the fuel pump volume is marginal.


But, let’s not assume that a worn pump is the sole cause of the complaint. The real diagnostic solution is to test the fuel pump relay during a cold start. By measuring the voltage level at the fuel pump test connector, it can be determined that the fuel pump is receiving inadequate voltage during cranking. So, while the fuel pump may need replacing, it’s obvious that a worn fuel pump relay, a bad starter, a bad battery or any combination of the three may aggravate the problem. Is this problem-solving strategy important? Yes, because the shop just avoided an embarrassing comeback on a new fuel pump.


By the Numbers

Last, let’s look at how a good driveability technician might solve the problem of diagnosing a shorted fuel injector on a four-cylinder engine with two sets or banks of fuel injectors hidden under an intake manifold plenum chamber. Clearly, it will require a lot of time to remove the plenum, disconnect the fuel injector harness, and measure the resistance of each individual injector. Furthermore, disconnecting the injectors may create more problems than it will solve, especially if the technician inadvertently repairs a bad connection in the process!

To use a hypothetical case, let’s say that the resistance specification for each injector is 12 ohms. The resistance for each pair of 12-ohm injectors operating in parallel can be calculated thus: R1 x R2 divided by R1 + R2 or: 12 x 12 (144) divided by 12 + 12 (24) which equals 6 ohms resistance for each pair of fuel injectors.


When testing at the injector harness leading under the intake manifold, if the resistance for one pair of injectors is less than 6 ohms, there’s obviously a shorted injector in the pair. If the resistance is 12 ohms, the tech is obviously looking for an open circuit in one fuel injector. Clearly, using the head is much easier than breaking the back when looking for open or shorted fuel injectors!

Understanding What You Know

Most of us know somebody (maybe ourselves) who “doesn’t understand everything they know.” Unfortunately, understanding isn’t always enhanced by experience alone. Instead, learning the underlying scientific and mathematical principles that a particular system creates is like turning on a light in a dark closet. Once true understanding is gained, it’s an experience that is rarely forgotten.

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