Circuit Testing Tools
You should know when to use and when NOT to use a 12-volt test light during diagnosis of electronic controls (Do NOT use this tester unless specifically instructed to do so by a test procedure). Instead of using a 12-volt test light, you should use a DVOM or Lab Scope with a breakout box whenever a diagnostic procedure calls for a measurement at a PCM connector or component wiring harness.
Electricity & Electrical Circuits
You should understand basic electricity and know the meaning of voltage (volts), current (amps), and resistance (ohms). You should understand what happens in an electrical circuit when it is open or shorted, and you should be able to identify an open circuit or shorted circuit using a DVOM. You should also be able to read and understand automotive electrical wiring diagrams and schematics.
You should have a basic knowledge of electronic controls when performing test procedures to keep from making an incorrect diagnosis or damaging components. Do NOT attempt to diagnose an electronic control problem without this basic knowledge!
Hand Tools & Meter Operation
To effectively use this or any diagnostic information, you should have a solid understanding of how to operate required tools and test equipment.
Malfunction Indicator Lamp
Emission regulations require that a Malfunction Indicator Lamp (MIL) be illuminated when an emissions related fault is detected and that a Diagnostic Trouble Code be stored in the vehicle controller (PCM) memory.
When the MIL is illuminated, it is an indication of a problem within one of the electronic components or circuits. When the scan tool is attached to the Data Link Connector (DLC) in the vehicle, it can access the DTCs. In some situations, without the use of a scan tool, the MIL can be activated to flash a series of long and short flashes, which correspond to the numbering of the DTC.
OBD II guidelines define when an emissions-related fault will cause the MIL to activate and set a Diagnostic Trouble Code (DTC). There are some DTCs that will not cause the MIL to illuminate. OBD II guidelines determine how quickly the onboard diagnostics must be able to identify a fault, set the trouble code in memory and activate the MIL (lamp).
Domestic vehicle manufacturers designed their computers to have an accessible data line where a diagnostic tester could retrieve data on sensors and the status of operation for components.
These testers became known in the automotive repair industry as Scan Tools because they scanned the data on the computers and provided information for the technician.
The Scan Tool is your basic tool link into the on-board electronic control system of the vehicle. Scan Tools are equipped with, or have separate software cards, for each OEM needed to be diagnosed. In this case, always secure a scan tool that has the latest OEM-specific diagnostic software included.
DaimlerChrysler specifies the use of a DRB-II or DRB-III scan tool with its diagnostic processes. However, there are aftermarket scan tools, that when equipped with the right software, can provide proper diagnosis as well.