Diagnostic Dilemmas: Making the Case with Scan Tool Graphing – UnderhoodService

Diagnostic Dilemmas: Making the Case with Scan Tool Graphing

I’ve recently updated to test equipment that’s able to save graphed data on a compact flash or CF card. In this case, while the equipment happens to be a Snap-on SOLUS scan tool and a Snap-on Vantage Pro lab scope and graphing multimeter, keep in mind that other equipment on the market is equally capable of performing this same task.

Cool Runnings
Whatever the brand, the advantages of graphing and screen storage can be well illustrated on a recent case study involving a very well-maintained ’94 Plymouth Voyager equipped with the 3.3L engine, automatic transmission and air conditioning. The customer’s complaint was poor cold-engine performance and a cooling fan that would run intermittently, even when the engine was cold. A scan tool test revealed a DTC 22, which indicated that the engine coolant temperature (ECT) circuit had some type of defect. A quick data stream analysis revealed no other faults at that time.

Because I’d had bad batteries cause similar poor cold-engine performance complaints, I began by routinely testing the battery and alternator. Testing revealed that the Voyager’s battery had a bad cell, which, at least according to my experience with Chrysler products, could certainly be a factor in causing a cold driveability complaint.

The battery test indicated that, while the battery would alternately pass and fail an electronic conductance test, it would consistently fail a carbon pile load test. In addition, the battery wouldn’t accept a charge, so it became obvious that the battery should be replaced before proceeding with the diagnosis.

The DTC 22
Another shop had earlier tried to remedy the DTC 22 by replacing the ECT sensor. Of course, even with the battery and the ECT sensor replaced, the cooling fan and DTC 22 issues remained. According to available literature, if the voltage signals from the coolant temperature sensor are too high or too low, a DTC 22 will be stored in the diagnostic memory.

Fahrenheit +261
After cold-starting the engine several consecutive mornings, I was convinced that replacing the battery had remedied the cold driveability complaint. On the other hand, the cooling fan continued to randomly activate, regardless of ambient or coolant temperature. Because I’ve had several instances of rodent damage lately, I carefully inspected and performed a wiggle test of the coolant temperature sensor connector and the wiring harness located under the intake plenum. The inspection didn’t indicate an intermittent open, short or short-to-ground condition.

But, a chance observation of the data stream on my scan tool indicated that the battery temperature would intermittently pop up to 261° F. temperature (see Photo 1). Clearly, it’s highly improbable that the battery temperature sensor could have actually sensed an ambient air temperature of 261°! So, obviously, it’s time to do some research before proceeding with the diagnosis.

Photo 1, 2

At this point, let me explain that Chrysler has, in the past, installed a battery temperature sensor on the battery case to measure battery or ambient air temperature. This input would provide a temperature reading by which the ECM would adjust alternator charging voltage and various engine performance parameters.

On this vintage of Voyager, Chrysler builds the intake air duct onto the ECM. Because the intake air duct is located next to the battery on this application, the “battery temperature” sensor is integrated into the ECM itself, which allows it to measure inlet air temperature (IAT) or “battery temperature.”

Fahrenheit -198
With these facts in mind, let’s proceed with graphing both coolant and ambient temperature readings. The following screens illustrate the usefulness of graphing because randomly recorded data lines won’t disclose the full range of temperatures indicated by the battery temperature parameter indicator data (PID). At various points on various screens, the graph would range between ambient and -198° F temperature (see Photo 2). Since +261° F and -198° F are far beyond any conceivable operating environment, the battery temperature sensor appears defective. As illustrated in Photo 3, the Voyager’s battery and engine coolant temperature failures at first seem to be related. In this particular instance, the cooling fan activated even though the coolant temperature hadn’t reached a full 190° operating temperature.

Photo 3, 4

Several failure scenarios may cause an ECM to activate the engine’s cooling fan. First, it’s important to understand that the ECM may activate the cooling fan regardless of coolant temperature if it detects a failure in the ECT sensor, ECT wiring or the ECM’s printed circuits. Second, let’s remember that if the ECM suffers catastrophic failure it places all of the actuators such as fuel injectors, spark advance, idle speed, cooling fan and other such functions in a base operation or “limp-in” mode. This limp-in mode ensures that the driver can reach a service facility without damaging the engine. Could the ECM be experiencing an intermittent limp-in condition? At this point, anything is possible.

Default Values
Photo 4 presents an interesting graph because it captures an erratic fluctuation in both the battery and coolant temperatures. This would be impossible to capture in a data stream. In addition, this screen indicates that the battery and coolant temperatures fluctuate independently of each other.

Data stream analysis can be tricky on early Chrysler products because, if an open or short occurs in a particular circuit, the PCM may indicate a default (rather than actual) value in the data stream. In other words, 115° appears to be a default value because it would occur when the ECT was disconnected or, similarly, when the ECM senses an open condition in the ECT sensor, wiring, wiring connectors or the ECM’s printed circuits.

Since the battery temperature sensor is integrated into the ECM, the wiring or connector issues seemed to be ruled out. After physically disconnecting the ECM connector and its connections to the engine, I applied some Stabilant-22 to the exposed pins to enhance conductivity. After reassembling the connectors, the erratic voltage values persisted. This particular bit of evidence seemed to indicate a faulty ECM.

The Process of Elimination
Following the old principle of garbage in, garbage out, diagnosing any engine control module is usually done by the process of eliminating the external parts, such as sensors, actuators and wiring, as a cause of failure. According to the database in my Vantage Pro, the ECM sends a pulsed reference signal to the ECT sensor. As Photo 5 indicates, the real-time signal was missing pulses when compared to the above “ideal” signal drawn from the Vantage Pro’s database. I might also add that the Vantage Pro database noted that the ECT sensor input is fed into two separate circuits within the ECM — a high-resistance and low-resistance circuit. The ECM shifts from the high resistance to the low resistance circuit at about 120° F. This shift from high to low resistance occurred at about 125° F on this application. This strategy allows the ECM to achieve much higher accuracy in measuring the increase in engine coolant temperature.

Of course, I had recorded the various erratic battery temperature and ECT PIDs (see Photo 4), so I had good evidence that some type of internal fault was the root cause of the cooling fan complaint. At this point, I replaced the ECM and, after performing several consecutive cold starts, the problem seemed to be solved.

Screen Graphics

Photo 5, 6

The advantages of being able to store and recover screens of scan tool data are obvious. The voltage values in the stored graphs, for example, are completely erratic and seem to share no common thread other than the fact that both are measuring temperature. Second, the graphs do illustrate a relationship between the battery and coolant temperature sensors. This is valuable information because the ECM depends upon both inputs for engine management and they tend to indicate that the failure is most likely caused by a cracked circuit board or solder joint in the ECM itself.

On the Snap-on SOLUS screens can be continuously saved by simply pressing the “S” button on the front of the scan tool. The number of saved graphs is limited only by the size of the optional CF card. Keep in mind that this button has several functions and must be set to perform the Save function. The screens are saved in a database memory and are logged according to time and date (see Photo 6). This system allows a technician to time the sequence and interval of each saved screen.

Could I have diagnosed this particular problem without saving graphed data? I could probably have caught some of these values in a snapshot “movie,” but the amount of evidence would have been remarkably slim, given that most of these screen captures happened in a few thousandths of a second. Since the conventional data lines are limited by their display rates, such rapid-fire failures would have likely gone unnoticed. Here again, the ability of a scan tool to graph various lines of data is but yet another tool in the arsenal of the professional diagnostic technician who is confronted daily by intermittent failure issues.

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