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The Modern Diagnostic Process


11/1/2002
By Gary Goms

 

Just before the beginning of the 20th century, Sir Arthur Conan Doyle's famous fictional detective, Sherlock Holmes, revolutionized criminal investigation by using the scientific process to arrive at a factual, rather speculative, outcome.

For the past decade, a similar revolution has been taking place in the automotive service industry. Today, a new type of technician is changing how the aftermarket industry does business by applying the scientific process to the everyday world of automotive diagnosis.

MODERN VEHICLE TECHNOLOGY
The genesis of the modern diagnostic process began in 1968, when Volkswagen introduced the first computer-controlled, electronic fuel injection. General repair technicians found the VW system difficult to repair because the system's microelectronics required a conceptual, rather than a visual or tactile, diagnostic process.

By 1990, electronic fuel injection became standard equipment in every mass-produced vehicle in the United States. By 2000, automatic transmission, anti-lock brake, airbag and air-conditioning systems were controlled by microprocessors and microelectronics. By 2002, multiplexed electrical systems, which allow a single wire to do the work of many, are being introduced into mundane automotive products like SUVs and light pickup trucks. To gain a better understanding of how the modern diagnostic process can and should change how we think about repairing vehicles, let's begin by looking at how a modern scientific diagnostic process begins.

MASTERING MONITORS
Modern OBD II self-diagnostic systems have become so complex that not even many factory engineers have a complete understanding of how a computer's operating strategy affects the overall performance of the vehicle. Basically speaking, the OBD II systems built into vehicles manufactured since 1996 are designed to monitor or test every part of the vehicle operating system that affects exhaust emissions. Each monitor is designed to occur during the presence of predetermined operating conditions. The powertrain control module (PCM) or computer, for example, may begin monitoring for cylinder misfires shortly after the engine is started. The PCM, on the other hand, may wait a full month before a set of driving conditions occurs that are suitable for monitoring the efficiency of the catalytic converter. If a component failure doesn't endanger the catalytic converter, the PCM may store a diagnostic trouble code (DTC) as a pending DTC. If the failure continues through three warm-up cycles, the PCM may turn on the Malfunction Indicator Light (MIL). This light is more commonly known as the "Check Engine Light." If a component such as a faulty spark plug causes a misfire that might endanger the expensive catalytic converter, the PCM will immediately illuminate the MIL. If the misfire exceeds a specified frequency, the PCM will flash the MIL, indicating an emergency failure situation.

THE SERVICE WRITER'S ART
Although the duties of service writers are generally presumed to revolve around scheduling work and ordering parts, good service writers do much more than that. Good service writers must, for example, know how to interview customers and accurately determine the nature of a driveability complaint. Good service writers must then relate the complaint (let's say: MIL on, rough cold idle) to the driveability technician who is to diagnose and repair the vehicle. Then the service writer must translate the technician's technical findings into language that the lay customer can understand.

Many progressive shops have devised diagnostic checklists to help with the customer interview. The checklists might detail the vehicle's maintenance history and the history of the driveability complaint. Both types of information are vital to the quick solution of the complaint. If, for example, the vehicle has exhibited a MIL on, rough cold idle complaint since new, the problem may lie with the original programming or operating strategy built into the engine computer. If, on the other hand, the MIL cold-idle complaint developed after a tune-up, the technician may look for something as simple as a disconnected vacuum hose or a cracked spark plug.

THE DIAGNOSTIC PROCESS
Driveability technicians usually begin the diagnostic process by retrieving diagnostic trouble codes (DTCs) from the diagnostic memory of the on-board, OBD II engine computer. Since the number of potential DTCs in today's OBD II systems literally number in the thousands, the tech follows a hierarchy of codes. The "P" series followed by a "0" indicates, for example, an emissions-related failure in the powertrain. The "0" indicates that it is a generic DTC, which is the aftermarket version of the factory code system. If the P0 is followed by a "3" the DTC indicates a cylinder misfire. If "01" follows the 3 (P0301), the DTC indicates a misfire on cylinder number one.

A good driveability technician will then analyze the "freeze-frame" (FF) data that accompanies the P0301 DTC. The FF data may tell the tech that the misfire occurred during the initial warm-up, with coolant temperature about 100 degrees F, at an engine speed of 1,350 rpm and at a vehicle speed of 15 mph.

With the FF data collected, the tech will then access a technical service bulletin (TSB) library or Internet database to determine if the misfire is a common pattern failure or a unique occurrence. This is an important step, since some vehicles have been manufactured with plastic intake manifolds that are prone to temperature-sensitive vacuum leaks. In other cases, fuel injectors may stick closed when cold or a dirty airflow sensor may cause the PCM to inaccurately calculate the air-fuel mixture going into the engine on cold start-up.

THE DIAGNOSTIC TREE
Automotive engineers have long designed "diagnostic trees" that theoretically enable even a marginally trained technician to diagnose complex electronic failures. Diagnostic trees are by their very nature designed to create hard data upon which to base diagnostic decisions. Each diagnostic step therefore becomes a pass, no-pass situation.

Unfortunately, many "cook book" trouble-shooting processes rarely address the real world of driveability diagnostics while others may simply be in error. Top-drawer diagnostic technicians, while they may consult OEM diagnostic trees, often address unique situations by designing their own trees. In our example of the hypothetical P0301 DTC in which the engine has developed a cold misfire, several possibilities exist, including a vacuum leak, a stuck fuel injector nozzle, incorrect valve lash or a high-voltage leak in the spark plug or plug wire.

In the following scenario, the technician establishes his own diagnostic tree by duplicating the FF data, cold-start condition in the morning. The next morning, the technician hooks up an ignition scope to the engine. He also has an aerosol can of carburetor cleaner handy. The engine immediately starts missing after start-up. The ignition scope pattern indicates that the cylinder is running lean on air/fuel mixture, so the tech sprays carb cleaner around the No. 1 intake port.

The carb spray doesn't make a difference, so the tech's next step is to test the fuel injector. An injector can be tested acoustically by listening for a signature pintle valve click, by measuring amperage flow demanded by the injector, by observing the injector waveform on a digital lab scope or by observing the reduction in engine speed when the injector is disconnected with the engine running.

How the tech tests the injector is largely determined by accessibility to the injector wiring harness or the injector itself. In this case, the injector is inaccessible because it is located inside the intake manifold. The tech devises an alternate method that increases fuel line pressure by temporarily pinching the return fuel line closed. If the engine immediately smoothes out after the fuel return line is restricted, the technician may recommend a fuel injection and intake cleaning procedure to clean the injector nozzle and cure the cold-start driveability complaint.

MYTH VERSUS REALITY
The most common myth in our industry is one that portrays the technician as an "equipment" operator. According to the myth, the "equipment" tells the technician which parts to replace. To extend the myth a bit further, many uneducated technicians claim that, "If they only had the equipment, they could solve the problem."

The reality is that to be effective, a driveability technician must understand the basic operating strategies built into the vehicle computer. Equipment and databases are simply tools that the educated and disciplined technician uses to solve unique diagnostic problems. While tools and information are necessary components of any diagnostic strategy, the key component of the modern diagnostic process is the educated, and often gifted, technician who, like Sherlock Holmes, applies these and many other resources to arrive at a truthful and factual outcome.













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