Important: Perform the following steps in sequence BEFORE proceeding to the engine vibration analysis tables.

1. Perform the Vibration Analysis - Road Testing table in order to duplicate and effectively diagnose the customer's concern.
2. If powertrain related DTCs are present, there may be a powertrain performance condition present which could be a contributing cause to the customer's concern. Using a scan tool, determine if any DTCs are set. If DTCs have been set, refer to one of the following procedures:

•	Diagnostic Starting Point - Engine Controls in Engine Controls - 6.6L

•	Caterpillar® 3126 Unit repair Manual

•	Diagnostic Starting Point - Engine Controls in Engine Controls- 7.8L

•	Diagnostic Starting Point - Engine Controls in Engine Controls- 8.1L

3. After the Vibration Analysis -- Road Testing is complete and there are no DTCs which affect the customer's concern, go to Vibration Analysis - Engine/Accessory Isolation .