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First principles

Every now and then a vehicle from the seventies or early eighties comes through the door that has a running problem. I always have to stop in my tracks and reevaluate my diagnostic approach. This age of a vehicle may or may not have a computer that is in control of the fuel injection system or the carburetor. What it does not have though is a so called Onboard Diagnostic (OBD) system.

No OBD system means I cannot use my computer (scan tool) to query the vehicle’s control system for clues (codes). Nor can I look at any data indicative of how the vehicle is running or not running. Inputs like the coolant temperature, air flow rate or oxygen sensor value can quickly guide a diagnosis. On these old vehicles these inputs must be directly measured.

It quickly becomes apparent that newer vehicles actually present their problems through their OBD system. Many times the check engine light is on and there is a diagnostic code that will assist with the diagnosis.

When the code description (e.g. P0301 misfire on cylinder number 1) coincides with the what the problem feels like (like it is idling rough) I can go straight to my analysis of the problem (cylinder number one). If it were a cylinder one misfire issue I can test cylinder number one ignition system, fuel injection system or for mechanical issues.

When the old car comes in idling rough I might decide that it is a misfire on one of the cylinders but I have to find out what cylinder is at fault and why. If this vehicle has only a four cylinder engine with easily accessible cylinders I can get to the culprit quickly but six, eight, or more cylinders can add significantly to the identification.

New age OBD systems give a lot of information with regards to what might be the cause of the misfire on cylinder one and also what isn’t the cause. If there are any open or shorted circuits to cylinder one there will be codes related to that cylinder’s electrical circuits (the primary ignition circuit or the fuel injector circuit).

The old vehicle will require actually testing the primary ignition circuit and the fuel injection circuit to make sure they are not shorted or grounded.

So maybe this new technology has its benefits. It better have some benefits because when I open the hood on the new vehicle I am lucky if I can even find number one cylinder spark plug, primary ignition circuit, or fuel injector circuit.

Working on older vehicles reminds me the way my grade 12 physics teacher taught me to approach all problems: from “First Principles”. The internal combustion engine has not fundamentally changed very much in thirty years. I know the systems that are involved in the problem. I know how to test them. Without OBD I hope I can access them.

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