As the on-site source for emergency AC power, emergency diesel generators at nuclear power plants receive a great deal of attention from plant staff as well as industry regulators. Peach Bottom Atomic Power Station recently undertook the task of reconstituting the design basis and improving the plant licensing requirements for the emergency diesel generators. Special testing was required to demonstrate that the engines, which have been in service for 20 years, could meet the new operating requirements.Special tests were written to coincide with other plant testing where possible, to minimize impact on emergency diesel generator availability. Test data was recorded on VHS tape to obtain a true analog representation of the selected signals and to allow collection of all dat from one test run. The recorded signals would be played back into a high speed Data Acquisition System (DAS) from which they could be analyzed and configured as necessary in a laboratory environment to reproduce the data. The recorded data can be kept indefinitely to allow reproduction and comparison at a later date as an aid in troubleshooting and monitoring system performance over time.Parameters monitored included generator voltage, watts, VARs, current, frequency, speed, exciter voltage and current, various relay actuations, and air start system pressures and temperatures.The test results show that the engine was not only able to meet the criteria to which it was licensed over 20 years ago, but that new requirements resulting from the enhancements to the plant Technical Specifications would also be met. The engine responses to the transients imposed were very similar to responses of a new diesel generator recently purchased and tested at the manufacturer's facility. The test results increased station confidence in its Emergency Diesel Generators and allowed it to make the licensing changes and commitments that increase the confidence of regulating agencies.This paper will discuss that testing, but more specifically, the special parameter monitoring equipment and techniques utilized for data gathering and analysis. The ability to record data for later conditioning and analysis has been the key to the success of the testing. The benefits of analog recording of data for future playback are still being realized by the station.