Water Cooler Diagnostics
We’ve all heard the phrase, “codes don’t fix cars, good diagnostics does”. Codes are merely a direction or path, not the answer as some might think. Those “codes fix it all” believers are usually at the bottom of the diagnostic chain. You know the type; those Neanderthals with little wrenches and big cheater bars, or the ones that follow the old adage, “When in doubt-rip it out” method of diagnosing a problem.
It’s seems to me that car repair for a certain demographic of people has always been something related to hand-me-down repair information, not diagnostic skills. I believe it’s all because of the availability of cheaply made parts and bad information. Some of it is hearsay, but a lot of it comes from two guys chatting next to the water cooler at work, and neither one of them have any automotive diagnostics background at all.
This latest case study is a perfect example of why swapping parts and paying attention to those water cooler experts isn’t always a good idea. A trained technician with diagnostic background and less time at the water cooler may be what you need.
A 2007 Dodge 4.7L pickup came into the shop with a stalling problem. The owner had already stopped by the water cooler and made a trip to the code fairy. Since no codes were stored, there wasn’t much for him to do except follow the water cooler genius’ advice. He swapped out every sensor and computer part he was told about and a few more he could barely reach, just to be safe. All of which didn’t change a thing. Before writing up the work order, I had to listen to his story, which ended like most of them do, "I've already spent too much on this truck, and I don't want to spend a penny more." (I wonder what kind of commission the water cooler guy got from the part store for helping this guy spend all his cash.)
The stalling was pretty predictable, usually every 15 minutes. Just as it would stall, the check engine light would rapidly flash, then the truck would sit silent. If you turned the key off and back on, the truck would run perfectly as if nothing happened, right up to the very moment the whole scenario repeated itself.
Since the only odd thing was this momentary flashing of the MIL, I decided to hook up a scanner and wait to see if this odd failure would show up on the screen. Sure enough, code P0688 popped up momentarily, just as the truck stalled “ASD signal low”. Out of habit I reached up and cycled the key. Dang it, the code never stored and the truck is back to running correctly again. I’ll have to wait one more time and see if I actually had the right code number. Since it only occurred as it went through its death roll, catching this failure was going to be tricky.
It was the correct code alright, but no signs of dropped voltage or weak connections anywhere to be found. It’s time to pull out the big guns. Break out the scope boys! With the scope hooked up to two different injector leads and the remaining channels on a couple of coils, I spent the afternoon watching the ASD voltage like a nervous hen watching her chicks. As if on cue, the truck died. Not a bit of change on the scope. I’m definitely going at this the wrong way.
Something is dropping off, or at least I assumed it was. Instead of looking at the ASD signal, how about checking the injection signal and coil signals from the PCM? This time the scope did have a weird response. Just as it stalled there was a little extra squiggly line that didn’t belong in the pattern on the coil input leads. Very subtle difference, but enough of a difference that it needed closer attention. The voltage signal spiked a bit higher than normal just as the truck would stall, and then the voltage would drop to zero. It must be the PCM or a coil. Since the signal was only there for a brief blip on the scope, it wasn’t exactly something I could put my finger on just yet.
Time for some old school tricks. Since the PCM was new, I could at least (with some trepidation) rule it out for now. I could test further, or I could try to create a problem that might mimic what I was seeing on the scope pattern, or with luck, if it was a spike that was coming from a coil, disconnecting it could show the problem. I decided to give this truck a miss of my very own and see if I could increase that little squiggle into a bigger one.
I'll unplug one coil and watch the scope pattern. If I’m lucky, the truck will either stay running longer than it normally did, or it might show me a larger voltage spike. Sure enough, I found it on the third coil. As long as that particular coil was left unplugged, the truck ran well past the usual stall time. To verify it, I plugged the coil back in and watched the scope readings directly at that coil. A millisecond before the stall the coil spiked to the top of the screen as the truck shut off. Just as I suspected, if it was on the coil that was causing the problem the spiked voltage would show higher there than on the adjacent coils.
The big question for me was why did it not set a code? The reason was the coil lead led straight to the PCM. The extra high voltage going back into the circuit simply turned the PCM off as if the key was turned off. There’s no codes for shutting the truck off, only codes for failures that make it shut off. The solution...replace the coil.
Now and then there are problems that don’t follow the diagnostic steps laid out by the engineers. Even though you’d think every aspect and every type of condition has been tried and tested, or at least talked about around the water cooler. There are times when you’ve got to look past the “assumed” problem and dig a little deeper to find the cause. There's no doubt this repair is going to be another one of those conversations around the water cooler, but I seriously doubt anywhere in this story will the novice know-it-all admit that it took an experienced technician to locate his problem, not his water cooler buddy. Oh, and I don’t expect to hear him say as he leans on the cooler, “Codes don’t fix cars, mechanics do” even when there isn't a code.
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