Malfunction indicator lights are the coolest things to hit instrument panels since the speedometer and fuel level gauge. Yes, I love those lights that make an instrument cluster look like a Christmas tree each time you turn on the ignition switch! What is even better is when those lights come on and stay on and even better, when a warning light is on and a chime is going off telling the driver something is wrong and needs to go see the doctor.
These kinds of lights amuse me and get my adrenalin pumping, how about you? Do they excite you, or make you want to put on your running shoes and put some distance between the vehicle and your tool box?
|2011 GMC Terrain|
This story is based on a 2011 GMC Terrain. The odometer shows a little over 77,000 miles have been traveled. The vehicle is powered with the 2.4 Ecotech engine and uses a six-speed automatic transmission to provide power to an all-wheel drive drivetrain.
The owner’s complaint is the A/C doesn’t work, along with a radio that will not turn on. There are several warning lights that come on intermittently, as well as a warning chime that will start sounding from time to time. The headlights will not come on, although the daytime running lights are on all the time and the turn signals and four-way flashers do not work. At times all the gauges in the instrument cluster also will not function. Does this sound like a problem you would want to tackle?
Time to get some diagnostic information.
With the vehicle in the shop, I connected a scan tool and polled all modules for communication and diagnostic trouble codes. All modules reported and many modules had communication codes. In fact, all modules on the low-speed GMLAN network had communication codes, as did the Body Control Module (BCM).
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Electrical system basics
Before we embark on our quest to find the problem, I would like to take a few minutes to discuss the diagnostic process with electronic modules.
Different vehicle manufacturers do things a little different, but on all electrical components the most basic need is to test for proper power and ground. In the case of an electronic module (computer module) there is always one added need: the need for the proper network communication signal. Many General Motors products add one more needed item; the need for the proper wake up signal. This wake-up signal will be a light green wire and will be labeled “wake-up signal.” If this signal is required and is not present, the module will become a no communication issue.
When embarking on any diagnostic journey, it is very important to start with a plan and stick to that plan. By using a plan, you will be more accurate with your problem analysis and put your time to a better use. When I start on problems like this, I always like to do a vehicle wide DTC scan. This is a great way to gather a lot of good diagnostic information in a short period of time.
With my scan tool hooked to the DLC, I was able to find many different DTCs in many different modules (Fig. 1). The modules shown on the scan tool screen include the ECM, EBCM, SRS, BCM, FSHM, ICP, MPIM and PAM. There were so many modules the one scan tool screen could not display them all at the same time. Let’s stop here a minute and analyze the captured data.
Another important thing in any diagnostic process is to put things in order. At this point, the scan tool should be used to browse through some of the different modules to gather information on the DTC’s that are stored. In Fig. 2, the SRS module shows it has lost communication with several other modules. Fig. 3 shows communication problems with several other modules and Fig. 4 also shows the BCM is having communication problems with these same modules. If we were to start researching each DTC on each module, we could spend the rest of the day in front of a computer screen. It might lead us to the problem but in all reality it would give most people a headache and cause mass confusion. In a case like this, the best thing to do is to go to your service information and print out a wiring diagram of the computer network system and use it as a roadmap to the next step in the problem analysis.