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Finding the voltage drop culprit

Monday, June 5, 2017 - 07:00
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I always expect the unexpected, the unusual or the bizarre solution will be what it takes to fix the car when the owner of Ken Davis Auto Repair calls me in to look at a customer’s vehicle in his shop. I was pleasantly disappointed this time!

I have a great deal of respect for Ken, and all others in automotive repair, who attend training on a regular basis. At an early point in my career, I recognized that in order to make my life easier during my work day and prove my claim of wanting to work smarter and not harder, I had to accept the fact my job isn’t just 9 to 5. It’s one that requires frequent afterhours (and before-hours, weekends, etc.) investments in training.

I mean after all, when you think about it, where is the MOST expensive training facility?  It’s that service bay where a car sits, and you don’t yet know how it’s supposed to operate (and therefore, don’t yet know how to fix)! I’d much rather the cars come in, get fixed, leave and it’s on to the next one than to be stuck learning a new system, holding my bay hostage and impacting an entire shop’s scheduled work load. Ken is an owner/tech and is very much like me in his desire for peak efficiency. So when he calls on my diagnostic services, I kind of expect there’s something REALLY weird going on. In the case of this 2014 Chevrolet Impala LTZ, this was so true!

GDS2 report screen

During his initial phone call he mentioned a few of the strange things the car started doing a couple of months ago that have happened on a more and more frequent basis.  Ken told me, “The vehicle had been trouble free in the 20,000 miles since the current owners had purchased this used car. Recently, an intermittent no-crank, no-start condition has worsened and now numerous systems not performing as designed.”

More specifically, the problems identified by the vehicle owner are that the starter would not make any sound when the push-button to start the engine was depressed; while driving, the instrument cluster would suddenly and without any common reason, just go dark and all gauges would seem to lose power; the headlights illuminated normally but at times would go out and then come back on. There were more symptoms, but it isn’t necessary to list them here. This description of symptoms sounds fairly complicated, doesn’t it? What are your initial thoughts?

Then he went on to say the owners had first taken the car to the Chevrolet dealership where it was noted that there had been some body repairs made to the front of the vehicle, which the owners were unaware of at the time they purchased it. In part, due to the repairs including aftermarket parts, the dealership politely refused to accept the vehicle into their service bays and suggested Ken’s shop might be better suited to “handle it.”

I’d made some suggestions that he and his tech could check, things that are typical causes of the intermittent phenomena. The next day, in his second call to me, he elaborated on the test results for the suggestions I had made. I’d asked them if a complete electrical system test had been performed. I had also asked them to perform a vehicle-wide Diagnostic Trouble Code (DTC) check and to record the findings. In addition, I was curious if any aftermarket electrical devices were installed on or in the vehicle.

Their aftermarket scanner was able to access a majority of the modules on the network in this well-equipped Impala. Almost every module had stored codes, some relating to “Low Voltage” and most had DTCs that I like to call “Tattle-Tale” codes. Those are codes about module “State Of Health” messages.

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Each module is supposed to identify itself on the network at some point. All modules are told to “keep a look out” for the other modules’ report that they are on the network and that they are working properly. When a module does not receive an expected notification that another module was supposed to send, then this module stores a DTC that implies the other module is not healthy. It matters not for whatever reason that message didn’t arrive; when a module expects the announcement on the network that the other module is “healthy” and it is not received, the code is stored.

When Ken received the test results from his tech Jerry and saw how many DTCs were stored in every module, he asked Jerry to hold off on doing any more testing and made an executive decision. Jerry was well-versed in network diagnostics, but felt a bit intimidated by the number of codes set and also had some big jobs in the shop that needed to get done that week. So Ken asked me to take the job. It didn’t hurt Jerry’s feelings in the least when I showed up to relieve him!

General Motors’ Global Diagnostic System version Two (GDS2) is the diagnostic software used on this car by the dealership technicians and is also available to the aftermarket repair shops. It is the diagnostic tool I chose to use in this case. I have numerous aftermarket scanners and PC-based diagnostic tools I could have selected from but had I done so, I may not have gotten all the information the car had to offer.

I started my diagnosis by reviewing with Ken and Jerry what I thought I’d heard them say. We all have been guilty of multitasking while conversing and not hearing everything correctly. I was driving at the time Ken made his first call to me, so distraction from my call could have occurred. Our chat confirmed we were all on the same page, so I proceeded to perform my diagnosis.

I began working on the car with a verification of the electrical system integrity. This is where every (electrical fault-related) diagnostic sequence should begin. Since the battery is the starting and ending point of every circuit in the car, shouldn’t we make sure there is nothing wrong with it? I tested the vehicle’s battery with a conductance tester and a carbon pile load tester, and then I tested the alternator and the starter. All passed with flying colors! Jerry’s test results had been the same.

GDS2 low voltage

Then I performed a vehicle-wide DTC & ID Information scan, which indicated numerous instances of "Low Voltage" problems and network-wide communication faults. During this time I was closely monitoring the Data Link Communication (DLC) voltage displayed on the GDS2 screen. I saw it drop a few times to below 10 volts as modules were queried for their information one at a time.

It was during a visual inspection that ground wire terminals were found to be attached to painted surfaces rear of the left headlight and on the left strut tower. Here is where I say something’s wrong. Remember what I said about the car’s battery being the beginning and the ending point for all circuits? I haven’t had the privilege (in very many years) of inspecting brand-new Impalas as they were prepared for delivery, but it seems as if attaching ground wires to painted surfaces could potentially cause problems right from the start. So, I don’t know if that’s how they come from the factory — but I do know that once I cleaned the paint off one of the studs, I recorded a reduced amount of voltage drop — by a large amount! Looking closer I found evidence indicating this vehicle had been repaired after a frontal collision.

Ground on a painted surface

The voltage drop tests performed while cranking the engine showed no less than 216 mVDC (0.216 VDC) differences on any of the first four ground terminal eyelets tested (those were the easiest locations to test). Could this amount of ground circuit “loss” cause all the modules to act as if they’ve lost their voltage supply (lost their minds)? Maybe.

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