Uses and Limitations of Scan Tool Serial Data for Diagnosis

A scan tool is an exceptionally useful tool when diagnosing engine control system problems. It gives you access to vast quantities of information from a conveniently located diagnostic connector.

• A scan tool allows a "quick check" of sensors, actuators, and ECM calculated data. For example, when checking for sensor signals which may be shifted out of normal range, scan data allows you to quickly compare selected data to repair manual specifications or known good vehicle data.

• When checking for intermittent fault conditions, it provides an easy way to monitor input signals while wiring or components are manipulated, heated, and cooled.

There are, however, several important limitations you need to consider when attempting to diagnose certain types of problems using serial data.

• Serial data is processed information rather than a live signal. It represents what the ECM "thinks" it is seeing rather than the actual signal which would be measured at the ECM terminal. Serial data can also reflect a signal value the ECM has defaulted to, rather than the actual signal.

For example, with OBD, the Engine Coolant Temperature sensor data displayed with an open circuit is the failsafe value of 176'F. If the actual voltage was measured at the THW terminal of the ECM, it would be 5 volts, equivalent to -40'F.

In the case of output commands, serial data represents the calculated output, not necessarily what the circuit driver is doing. For example, when cranking an engine which is in fuel cut failsafe (due to an open IGf line), calculated injection pulse is displayed on serial data even though the injector driver is not being operated.

Knowing the Limitations of the Scan Tool

Sometimes data interpretation requires you to recognize subtle differences between "normal" and "abnormal" data. Can you pick the data stream displaying the open Engine Coolant Temperature sensor?

Scan Tool Serial Data

Using serial data to troubleshoot intermittent problems also has its limitations because of data transmission speed.

When the data refresh rate is slow, as it is with slower baud rate data streams, it is easy to miss changes which occur in a signal between display updates. As a result, intermittent signal problems are often not detected on a slow serial data stream.

For example, a Throttle Position Sensor signal wire that goes open circuit every time the vehicle drives over a bump. If the open condition does not last for at least 1.25 seconds, there is a good possibility that the change in signal value will go undetected by your scan tool.

When troubleshooting intermittent problems on vehicles without high speed serial data (like Enhanced OBD-II), it is much better to use serial data generated by V-BoB than to use the OBD serial data. It takes more time to connect V-BoB to the ECM, but if an intermittent problem occurs, the high speed serial data generated by V-BoB will catch the fault.

Given this information, it is clear that care must be exercised when interpreting serial data and using it to make diagnostic decisions. Once you are familiar with irregularities like these, the risk of diagnostic error is significantly reduced.

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