Searching for the Diagnostic Path

Searching for the Diagnostic Path

Most recently, I’ve found myself dealing with an increasing number of random, multiple-code complaints that involve computer operating strategies and networking communications issues in which the multiple trouble codes or DTCs tend to be symptomatic, rather than indicative, of a component failure. A basic illustration of what I’m talking about is a simple DTC 44, which is a trouble code for OBD I General Motors products indicating a lean air/fuel ratio. In reality, the DTC 44 may not be indicating a lean air/fuel ratio as much as it’s indicating that the oxygen sensor itself is out of calibration. This process of making sense out of multiple and sometimes contradictory data often becomes what I call “.”

Time, Money and Probability
Searching for the diagnostic path recognizes that any diagnostic procedure is limited by time, money and the probability of success. Time, for example, is always limited by the need to make a profit at a reasonable cost to the customer. As for money, it’s pretty easy to let expensive learning curves, tech hotline fees, non-returnable parts and special, one-time use tools eat up the anticipated profits. And it doesn’t take a rocket scientist to understand that random complaints and predictably low success rates go hand-in-hand. If we had an engineering lab at our disposal where we could subject a vehicle to varying degrees of heat, cold, vibration and humidity, we could practically guarantee that we could diagnose and repair the random performance complaint during the first visit. But, working in the field, the limitations of time, money and probability mean that we can’t simply diagnose and repair a random complaint on the first visit.

Instead, we might find ourselves taking a more conservative approach by perhaps recording and clearing the DTCs from the PCM’s diagnostic memory and waiting until a second or even third visit to see how the problem develops. Many times, the second visit yields information not found during the first visit. Following a protracted diagnostic strategy also becomes part of “.”

The Diagnostic Path
Of late, I’ve become increasingly wary of how I approach random complaints because, first of all, pattern failures are becoming more the exception than the rule. Second, most random performance complaints automatically become open-ended repairs, which is something a price-conscious consumer may be unable or unwilling to understand. And third, the learning-curve time is increasing dramatically because of the proliferation and complexity of vehicle operating systems. Needless to say, the concept of “fixing it right the first time” is becoming less achievable by the day.

Consequently, whenever a random complaint comes into the shop, I begin searching for a diagnostic path that will bring me as close to the solution with as few twists and turns as possible. Case in point is a 2001 K1500 Chevrolet pickup equipped with the 5.3L engine and automatic transmission. The complaint is that the engine will occasionally start, but fails to keep running.

I’ll tell you up front that, as of this writing, the owner is still deciding whether to spend money on further diagnosis of the complaint. Because this random failure occurs only once each week or two, I wasn’t able to duplicate the complaint. The fact that I couldn’t duplicate the complaint significantly decreases the probability that I can identify the failure or verify the repair on the first visit.

Nevertheless, during the initial interview, the owner did tell me the fuel gauge wasn’t working properly. Fuel gauge and fuel pump issues go hand-in-hand on Chevy trucks equipped with the modular fuel pump system. According to my experience, an erratic fuel gauge goes hand-in-hand with a worn fuel pump. This is an experience that may not be shared by other technicians, but it was enough to begin my search for a diagnostic path that would lead me closer to the primary failure.

Code Search
My search for the diagnostic path really began when I retrieved the following DTCs from the vehicle’s history codes: P0463 (erratic fuel level), P1631 (the Passkey system didn’t recognize the ignition key), U1026 (loss of class 2 communications) and U1041 (loss of EBCM communications).

Okay, given the owner’s statement that the fuel gauge wasn’t working, the P0463 I could expect. But the two “U” networking history codes intrigued me because both DTCs seemed to indicate that the truck had some wiring issues at some time or another.

Nevertheless, I had to deal with the fuel gauge and pump issue by taking a pressure and volume test that indicated excellent operating pressure, but barely enough volume to keep the engine running. A quick inspection revealed that, at 80,810 miles, the original fuel filter was still in place. Replacing the filter remedied the fuel volume issue.

Was this a good place to stop in searching for the Diagnostic Path? My initial reaction when the customer described the complaint was that the fuel pump was going bad. But, on one hand, I’ve had dirty fuel filters cause intermittent fuel delivery issues. On the other, I’ve had worn fuel pumps on some Chevy trucks burn the pin connectors on the fuel pump module itself, causing the same type of intermittent fuel delivery issues.

The Half-Way Point
Nevertheless, at this point, I felt I had traveled only half-way down the diagnostic path. Sure, based on my past experience, I could recommend an expensive modular fuel pump replacement. Or I could reduce the risk factor by looking up the OEM diagnostic procedures for the P0463, P1631, U1026 and U1041 DTCs just to see how they might explain the random “failure to keep running” complaint.

Interestingly enough, the P1631 Passkey diagnostic procedure indicated that, if the ignition switch failed to identify the correct ignition key, it would cause the fuel pump to shut down immediately after starting.

At this point, it was time to continue traveling down the diagnostic path by having another interview with the owner. Some customers think that visiting a diagnostic shop is a lot like visiting the dentist’s office: the less you say, the less you have to pay! Of course, following such tactics often leads to getting the wrong tooth pulled. To avoid “pulling the wrong tooth,” I asked the owner point-blank if any aftermarket electronics had been installed on the truck. The answer was, “No, but the truck does have a remote starting device, which I never use.” Bingo! Now I can continue following the diagnostic path!

Of course, the owner didn’t have a manual to go with the remote starting system and I had run out of time for that day for inspecting the installation. My feeling, however, is that the P1631 DTC is being caused by the remote starter randomly failing to defeat the Passkey anti-theft system. The “U” networking DTCs might have been caused by a sloppy installation of the remote starting system or by the system interfering with networking communications.

To gather more information, I posted an inquiry on the International Automotive Technician’s Network (iATN) Technical Discussion Forum about remote starting system failures. At least six experienced technicians said that remote starter systems could cause the fuel pump to shut down immediately after engine start-up due to a failure in an aftermarket remote starting system or to a failure caused by a sloppy installation of the system.

In review, the diagnostic path revealed a faulty fuel gauge, a potentially faulty fuel pump, a clogged fuel filter and the possibility of an aftermarket remote starting system shutting down the fuel pump immediately after start-up by interfering with the Chevy’s Passkey anti-theft system.

So here I am at the end of my diagnostic path, waiting for the owner to decide whether to proceed with further diagnosis. I can confirm the diagnosis only by getting the truck back for an intense inspection of the under-dash wiring harness. In the meantime, I’m not sure I’ve convinced the owner of the connection between the remote starter and the stalling symptoms. Not a great position, I’ll admit. But, with the initial diagnostic fee in hand, it’s a much less vulnerable position than if I had failed to solve the complaint by installing an expensive new fuel pump.

Second Gear Only
Let’s look at how the diagnostic path meandered through a whole week on another Chevy truck, which was a 1995 K-2500 equipped with the OBD I powertrain management system and the 480LE electronically controlled automatic transmission.

The truck came in late on a Friday afternoon with the transmission stuck in second gear. The owner revealed that the transmission had just been serviced by a reputable out-of-town shop a few days before.

Reverse, neutral and park worked okay. I retrieved two current trouble codes, a DTC 83 (torque converter quad driver failure) and 81 (shift solenoid “B” circuit failure). The code history also contained the following DTCs: 68 (transmission slipping), 83 (torque converter quad driver failure), 82 (shift solenoid “A” circuit), 81 (shift solenoid “B” circuit failure) and 85 (gear ratio selection failure).

Searching for a Path
The diagnostic path was pretty clear. First, since the fluid was clear, I didn’t suspect a clutch failure in the transmission. Second, I checked all fuses just to make sure that one wasn’t loose or burned. Third, I inspected the electrical connector at the transmission for signs of moisture and corrosion. Last, I pointed out a second problem, which was that the range selector indicator on the transmission wasn’t reporting the correct gear range selection to the PCM.

In order to relieve worry that the transmission itself was failing, I explained to the owner that the numerous DTCs contained in the current and history codes were likely caused by a power or ground failure in the transmission electronics. When the PCM detects any failures or inconsistencies in the transmission electronics, it will cause the transmission to default to second gear in order to prevent damage to the transmission clutches and gear train.

Repairing the Obvious
After recording the DTCs, I cleared the on-board memory and scheduled the truck for continued diagnosis on the following Tuesday. When Tuesday arrived, the only DTCs contained in current and history codes were the code 81 (solenoid “B” failure) and code 85 (gear range indicator failure). At that point, I decided to remove the transmission oil pan and inspect the internal wiring for possible damage. Since the “B” or 2-3 shift solenoid on this transmission is very inexpensive and prone to failure, I replaced it along with the gear range indicator. I cleared the DTCs and successfully road-tested the truck.

Beginning, Middle or End?
Okay, am I at the beginning, middle or end of the diagnostic path? Going back to the beginning, I knew I had a power or ground problem because the PCM was detecting extreme voltage variations in all the transmission solenoid control circuits. Despite the fact that battery voltage was present at all solenoids and that the ground had load-tested perfectly with a test light during the first two visits, I knew I had an intermittent voltage supply problem to the transmission solenoid control circuits.

Fortunately, the owner was receptive to my suggestion that he drive the truck for a few more days. On the third day, the transmission once again locked in second gear and, as requested, he dropped it off so I could continue the diagnosis.

Between the second and third visits, I did some extensive research on the wiring schematics and troubleshooting procedures on the Chevy’s 480L-E automatic transmission. First, the troubleshooting information was very emphatic about making sure the battery and charging system had correct voltage. Next, I read at least one bulletin describing the ignition switch as the source of an intermittent power supply issue to the transmission. Last, I traced all the fuse feeds on a wiring schematic and had decided that number 20 “trans” fuse contained in the instrument panel fuse box was the prime candidate for testing.

I drove the truck into the service bay, being careful not to shut off the ignition, slam the door or otherwise shock or vibrate the electrical system. Very carefully, I removed the fuse panel cover and discovered that I had only 8.5 volts at either pin on fuse 20. Because I had followed a planned diagnostic path, I knew that fuse 20 was supplied by a dedicated wire coming directly from the ignition switch. Grabbing a medium-sized soft-face hammer, I rapped the steering column at its base. Immediately, fuse 20 jumped to battery voltage.

Quite clearly, since fuse 20 was wired directly to the ignition switch, I had a voltage drop inside the ignition switch itself. A new and very expensive ignition switch replacement solved the complaint of the transmission intermittently locking in second gear.

The Diagnostic Path
Here again, the diagnostic path was very clear. Multiple DTCs suggested a strong possibility of a power or ground failure. A second failure was indicated when the scan tool data was indicating an incorrect gear selection. During the first two visits, the powers and grounds tested perfectly. During the third and final visit, the voltage at fuse 20 tested low. As the failure became more frequent, the probability of finding the problem also increased. So, keeping in mind that new ignition switch installations cost in the neighborhood of about $400 for this model, it paid to take the extra time to pin-point the failure before suggesting an expensive repair.

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