Chapter 34 Advanced Diagnostics 663 Copyright by Goodheart-Willcox Co., Inc. Advanced Scan Tool Tests In addition to retrieving trouble codes, modern scan tools can be used for advanced diagnostic procedures. For example, the scan tool can take a “picture” of operating parameters at the moment a problem occurs, display “live” operating values as an engine is running, and check actuators for proper operation. Scan Tool Snapshot A scan tool snapshot is an instantaneous reading of operating parameters that are present when a problem occurs. Th is feature is oft en used when a problem is hard to fi nd or when intermittent troubles are present. Most scan tools can be programmed to automatically take a snapshot of operating parameters whenever a diagnostic trouble code is set. Th e snapshot feature also can be triggered manually. A manual capture is oft en performed when the vehicle does not generate a trouble code. For example, the operating parameter may not be tripping a trouble code, but it may be aff ecting vehicle operation. Th e manual capture feature requires you to monitor oper- ating conditions and to press a button on the scan tool when the problem occurs. For example, if a car only acts up when driving at a specifi c highway speed, you can drive the vehicle at the trouble-causing speed and scan under these conditions. When the symptom occurs (engine misses), press the appropri- ate scan tool button to capture the operating values. Aft er returning to the shop, use the information pro- vided by the snapshot feature to determine what is caus- ing the problem. Look for any operating parameter that is almost out-of-specs. You might have two sensors ready to fail or a poor electrical connection in combination with a mechanical failure. Scan Tool Datastream Values Scan tool datastream values are “live” electrical val- ues measured with the vehicle running and, in some cases, being driven. You can read the scan tool screen to see weak values or values that are almost out of specs. In most cases, you can choose which datastream values you want the scan tool to display. For example, you may want to look only at the inputs to the ECM. If you have a performance problem but no trouble codes have been set, study the datastream values. Values that are almost out-of-spec may signal a problem area. Datastream values give added information for fi nding troublesome problems. Figure 34-11 gives a few datastream values that can be read by a scan tool. Scan Tool Actuator Tests Most scan tools can switch computer-controlled actuators on and off . Th is allows the technician to verify the operation of these components. For example, a scan tool can be used to fi re an ignition coil, control the idle speed motor, or disable a fuel injector. If a vehicle engine is overheating, you might want to turn on the radiator fans to check their operation. Th e scan tool can also be used to perform a power bal- ance test. Th is test involves disabling a fuel injector or spark plug in a specifi c cylinder while monitoring the correspond- ing rpm drop. Power Balance Test A power balance test, also called a cylinder balance test, measures the power output from each of the engine’s cylinders. As each cylinder is disabled, the scan tool should indicate an rpm drop. During a power balance test, all cylinders should have the same percentage of rpm drop (within 5%). If a disabled cylinder does not produce an adequate amount of rpm drop, the cylinder is not firing properly. You may have to disable the idle control solenoid or motor for an rpm drop to show up. Caution: Never short a cylinder in a vehicle with a catalytic converter for more than 15 seconds converter damage could result. If the rpm drop in one or more cylinders is below nor- mal, a problem common to those cylinders is indicated. Th e cylinders could have low compression (burned valve, blown head gasket, worn piston rings), a lean mixture (vacuum leak, faulty fuel injector, computer malfunction), or other problems. OBD II Drive Cycle Th e OBD II drive cycle is normally performed when- ever the battery or ECM has been disconnected, or aft er diagnostic trouble codes have been erased. Additionally, some states require the drive cycle to be performed before an emissions test. Th e drive cycle is designed to tell the technician whether the OBD II system is operating properly. It involves attaching a scan tool to the vehicle and driving the vehicle for a specifi ed amount of time. Th e drive cycle includes peri- ods of acceleration, cruising, and deceleration. Performing the OBD II Drive Cycle Note: Carefully study the drive cycle procedure and scan tool operation before starting the drive cycle. You should be reasonably sure that you can complete the drive cycle from beginning to end. If the drive cycle has to be aborted for any reason, the engine must be allowed to cool, which can cause a considerable delay.