Tackling the wiring portion of check engine light code diagnosis can be overwhelming without the right skills. I am always amazed at how many different ways automotive professionals rationalize and test automotive circuitry.
Electrical concerns are actually my favorite repairs to make because you really get to think outside of the box. And when dealing with check engine light concerns you get to do a lot of electrical testing! It's pretty cool to examine a schematic and narrow a concern down to very specific probable causes or locations. You can then go in to isolate and repair the problem in a very short (no pun intended) period of time. Today I would like to share my ideas on the most effective and efficient ways to approach circuit testing in today's cars.
Anyone up for some check engine light circuit testing? Let's start with the necessary tools required for most tests. These are very basic and consist of: a DVOM (Digital Volt Ohm Meter), test light and some test leads/jumper wires. The DVOM should have an ohm scale of at least 0-20 K Ohms, a voltage scale of at least 0-20 Volts and an Amperage Scale of at least 0-10 Amps. You will also need the required hand tools to gain access to the various connectors and components to be tested. There are a lot of tools designed to ease the testing process and speed things up, but these are not absolutely necessary. I recommend checking out some of my favorites through the "Tools" link if you do anything more than an occasional circuit test, as some of these testers can save you considerable time and frustration on those tough ones! Let's get this check engine light taken care of!
Automotive electrical testing as a whole is a very large topic, and often requires very specific test steps for things such as module and component testing. This being said about 90% of the testing required to diagnose check engine light concerns is very basic, and nearly all circuit concerns can be isolated using the same simple basic techniques. There are many different methods used for circuit testing. I am going to share the methods I have developed and use to pinpoint wiring issues in the least amount of time when specific testing instructions are not available (or needed for that matter). Keep in mind-you will run into some weird issues such as frequency interference along circuits, but these are beyond the scope of this article. We will cover all the basic testing needed to find 90% (or better), of all wiring issues in today's cars. So let's get started!
In a nutshell, wiring problems come down to only a few possible causes, and they are:
1. Open circuit (or partially open such as a chaffed line which causes high resistance due to heat when circuit load is on)
2. Short to power
3. Short to ground
4. High resistance (usually caused by corrosion or poor connections)
Testing for all cases is fairly straight forward once you have a schematic to deal with so you know what wires you need to test and where they run. We are going to use a simple lighting circuit to demonstrate how to test an individual wire for each of the above, and then we will troubleshoot a concern. I know it's not really a check engine light concern, but we're trying to get the basics down for now, and then we'll get into the good stuff! First take a look at the schematic: