Ninety-Nine Times Out of a Hundred… – UnderhoodService

Ninety-Nine Times Out of a Hundred…

Let’s face it: all of us occasionally get that haunting feeling of inadequacy when a simple pattern failure turns into “rocket science.” Like most technicians, my rocket science feeling happens most often when the problem at hand doesn’t fit within the confines of my previous experience. Ninety-nine times out of 100, my experience works to my advantage. As far as the 100th instance goes, well that’s what this month’s “Diagnostic Dilemma” is all about!

Speedometer Dilemma
Case in point, a Ford truck with a failed speedometer and an illuminated orange ABS warning light is, in my previous experience, indicative of a failed rear axle ABS sensor. Perhaps it isn’t responsible for 99 speedometer failures out of 100, but the sensor is nevertheless a relatively inexpensive and a high failure-rate item. Because it’s a high-failure item, I usually remove the sensor, inspect for metal buildup on the sensor tip, and test the sensor’s resistance to confirm a failure before installing a new sensor.

Sketchy Information
Intermittent body control or bus communications problems cause a lot of head-scratching because they’re a relatively new concept in vehicle operation. Sure, most of us have dealt with the old Chrysler CCD bus systems, but when it’s “new stuff” like a 2002 Ford F-250 diesel, it may be that hundredth case that tosses my previous experience right out of the diagnostic window. In this case, my information system was very sketchy about the layout and general specifications of the F-250’s ABS system. Their resistance specification was incorrect because the resistance of the replacement sensor duplicated that of the original. Nevertheless, because the axle speed sensor at least “used to be” a high failure item, I replaced it and drove the truck to see if the speedometer problem was still present.

Of course it was. An anomaly that I had discounted for the moment was that the scan tool I was using wouldn’t communicate with the ABS module. This isn’t unusual with aftermarket scan tools, so I tried a scan tool with dedicated ABS software only to discover that it wouldn’t communicate either. In addition, two “U” communications codes were stored in the diagnostic memory, a U1027 (SCP data, tachometer rpm signal) and a U1262 (SCP J1850).

Not So Quick
At this point, a quick search to “test H” revealed that a 17-step procedure might be required to diagnose the U1262 body control communications issue. At that point, it also became clear that the failure of the ABS module to communicate with the scan tool indicated that, for whatever reason, the ABS control module wasn’t operational.

So, what I had expected to be a quick speed sensor replacement had turned into a major computer communications diagnostics issue that could require minutes, hours or days to resolve. The “U” codes and failure of the ABS module to communicate could involve a power or ground, bus communications or module failure issue. Given the customer’s fast-paced schedule and the fact that I had to be in Denver the following day for an association committee meeting, I was forced to refer the owner to our local Ford dealer for further diagnosis. Oh, well!

The Rocket Scientist
As I’ve said in this column a number of times, our previous experience can often prove to be our worst enemy in the diagnostic process. Many years ago, I employed a sharp young high school kid who is now, in the most literal sense, a rocket scientist. Too many times I would rely on my experience, whereas my young employee would start from scratch by reading the manual. More than once this 16-year-old kid made me look like the one who should be going back to school because his approach wasn’t tainted by previous experience!

Not that experience is bad or isn’t necessary. Experience is an accumulation of information about diagnostic and repair processes that we can’t find in print. Experience can be that visceral, intuitive sense that tells us when the facts we have at hand don’t add up. Experience is also that vital ingredient that separates the veteran technician from the sharp trade school graduate who knows all of the “book” answers, but struggles to diagnose a shorted brake light bulb.

Blue-Car Diagnostics
On the other hand, experience can pave the way for complacency, which we often call the “blue car” syndrome. In other words, if a particular diagnosis worked on the last blue car that came in the door, it should work on the next blue car that comes in the door. So it was with my 2002 Ford F-250. A few model years before, the PCM and ABS modules were essentially configured as stand-alone units. Today, the same units are networked with at least several other computers or modules. The net effect is that these formerly independent modules are now mutually dependent because they need to share data in order to operate.

With any multiplexed electronics system, it’s important to make sure that scan tool you’re using can communicate with all the modules present in the system. This is equally important because some modules may contain diagnostic trouble codes that affect the operation of other modules. To prevent this type of oversight, DaimlerChrysler usually inserts a DTC in the PCM’s diagnostic memory that indicates DTCs are stored in other system modules. In other cases, a technician must poll each module to verify communications and retrieve trouble codes. In any case, body control diagnostics can become a can of worms simply because many on-board diagnostic systems lack the sophistication to pin-point a module-to-module communications problem.

New Problems on Old Technology
Let’s take a look at how previous experience can cause a misdiagnosis on more conventional technology like Ford’s infamous Thick Film or TFI ignition. Brought into popular use around 1983 and ’84, the TFI ignition quickly developed a bad reputation for module failure. The TFI failures became so pronounced that, a few years ago, they actually became the focus of a successful class-action lawsuit brought against Ford Motor Company by several people who had become involved in serious traffic accidents because a TFI module failed in the middle of a busy intersection.

Like thousands of other veteran technicians, I’ve replaced perhaps hundreds of defective TFI modules. The symptom is usually a “hard” cranking, no-spark failure or a short series of “soft” intermittent failures followed by a single hard failure. In addition, Ford also manufactured distributor pickups that became badly deteriorated through exposure to heat and oil. Before lab scopes came into common use, I bought a set of LED testers that tested the pick up coil. Even at that, my confidence in testing heat-sensitive pick up coils was such that I adopted the practice of installing a new distributor pick up along with the new replacement module as preventive maintenance.

So, wouldn’t you know it, a 1994 Ford F-150 with the 5.0L V8 engine was towed unannounced to my shop with the owner reporting a sudden ignition failure at highway speeds! Although I knew that 99 times out of 100 that the TFI module had failed, I went through my standard testing procedure of using a lab scope to look for a PIP signal coming from the distributor pick up. Sure enough, the distributor sent a good PIP signal to the ECM, but I also noticed that the ECM wasn’t returning the prerequisite SPOUT or spark-out signal to the ignition module. At this point, I would have still bet on the module being defective and, indeed, I did hang a known-good module on the distributor just to satisfy myself that the module was good.

The Missing SPOUT
Of course, by now I’m beginning to suspect that the PIP signal is becoming lost in the ECM. A good way to eliminate a suspected ECM issue on Ford EEC-IV vehicles is to remove the ignition timing or SPOUT connector. On early EEC-IV Fords, the SPOUT is a round, single-wire connector located near the distributor. On later EEC-IV systems, the SPOUT is a square “plug” located near the distributor or module. With the SPOUT removed, the ignition system suddenly began producing a good, blue spark. Unfortunately, the engine still wouldn’t start. Inserting a “node” light into a fuel injector connector indicated that the fuel injectors weren’t being triggered by the ECM. Okay. Now what?

Some research into the International Automotive Technician’s Network archives revealed that a faulty PIP signal could cause such a problem. Refusing to believe that the ECM could be at fault, I again tested the PIP with a lab scope and carefully examined the stored pattern. At cranking speeds with the SPOUT disconnected, the narrow “signature” square wave indicating number-one cylinder at TDC was followed by an equally spaced series of wider square waves indicating the timing pattern for the remaining cylinders.

The evidence thus far indicated a failed ECM. The problem I have with such a diagnosis is my experience indicated that EEC-IV ECM failures were extremely rare. Of these few failures, most were caused on early EEC-IV systems by a shorted actuator or by a voltage spike originating from a non-suppressed starter relay. But, as much as experience indicated otherwise, I ordered and installed a reman ECM and the engine immediately started and ran as it should. But the “rocket science” question remained: why had the normally reliable EEC-IV ECM failed in the first place?

Yikes, Spikes
At this point, I asked the owner if the electrical system had recently been repaired or modified. The answer was no. After explaining my concern about protecting the new ECM, the owner suddenly recalled that the truck had stalled a few days after a remanufactured alternator had been installed. I asked if he had jump-started the engine after the alternator had failed. Again, the answer was no.

Before beginning a diagnostic process, I usually test all starting and charging systems with a compact tester, which tests and records battery condition and dynamic starting/charging results. If you do this as a matter of routine, you’ll be amazed at the number of driveability problems you’ll discover that are caused by bad batteries, corroded connections, or by alternating voltage leaking into the DC system from the alternator itself.

Immediately, the battery tester indicated excessive alternating or “ripple” current. A scope test indicated that the alternator was producing a huge AC voltage spike far in excess of the industry standard of 250 millivolts of AC voltage. Could the faulty alternator have caused the normally reliable EEC-IV computer to have failed? I really can’t say for sure, but I disconnected the alternator and recommended that the young owner return it to his supplier for warranty replacement.

Lessons Learned
Although I’m sure that Underhood Service readers can cite more relevant examples, each of the above had become a first-time experience for me because the actual failure had far exceeded what my experience had initially indicated. The lesson learned is that, regardless of whether the system is old or new, it can fail in ways not included in our experiences. With the popularization of new systems like body control technology, any diagnosis becomes more complicated because we may be forced to discover, through trial and error, the operating strategy employed to make the system function. Yes, sometimes pattern failure problems do become rocket science, but it’s all in a day’s work for the modern diagnostic technician.

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