As driveability technicians, we've been taught to solve diagnostic problems by gathering and analyzing data. Data usually comes in three basic forms, the first of which is sensory data that we can see, hear, smell or feel. The second form is measured values gathered by using test equipment like lab scopes and multimeters. The third form comes from computer-generated data displayed on a scan tool.

Unfortunately, analyzing a data stream is a little tricky because what we see doesn't always represent reality. To illustrate, the injector pulse width displayed on the scan tool is often a calculated, rather than real, value. Differences can and do happen when the pulse width is measured with a multimeter or lab scope. Why do these differences between the data stream and measured values occur? At the most basic level, the answer is that the operating strategy programmed into the engine computer tells the PCM how to react to various operating modes. Unfortunately for the driveability technician, the reactions of the operating strategy to various operating modes may produce data streams that don't represent reality.

The Multiple Code Ranger

Let's begin by analyzing multiple codes and data streams on a 1996 4.0L, manual transmission Ford Ranger that came in during the middle of the winter with an illuminated MIL and an electronic transfer case that wouldn't engage. It also had several "Oh, by the way" complaints that inclu...