An apparatus which permits the automated diagnosis of automobile engine electrical system waveform patterns is disclosed. Employed in conjunction with an addressable random access memory which stores digitized information concerning the waveform are further means which logarithmically reduce input amplitude values of the waveform and coordinates sampling times for the waveform with engine RPM so that the memory capacity is fully utilized. Input conversion of the waveform is accomplished by a tracking analog-to-digital converter using comparator means which also provides a waveform peak detector. Upon an address command, the memory output is input into a read only memory which provides an anti-log conversion of the logarithmic value stored in the memory, thereby providing an output corresponding to an actual value signal.