A probe containing both an ionization sensor and a luminosity sensor was installed in place of a glow plug in both an Oldsmobile and a Volkswagen diesel engine in order to evaluate these two devices as “start-of-ignition” markers. Information on ignition delay relative to injection timing might well be the key item in fuel scheduling to effect maximum fuel efficiency and reduced exhaust emissions. The results indicated that at low engine RPM both sensors responded simultaneously, but at high RPM, independent of load, there was a noticeable and repeatable delay of the ionization gage signal. Also, the ionization sensor signal showed larger cycle-to-cycle variations in both shape and peak amplitude than the luminosity sensor making application of that signal to produce a reliable marker more difficult. The luminosity sensor showed consistent behavior under all test conditions.