Functional Test

With ignition switch off, connect a 12-volt test light between glow plug module test terminal and ground. See Figure. Position test light so that it can be seen from driver's seat. Turn ignition switch to ON position and monitor test light. Compare test light times to chart. See Fig 1. Total test light ON time includes time from the beginning of the initial ON cycle to the end of the last ON-OFF cycle measured in seconds.

Fig 1: 7.3L Glow Plug Module Functional Test Chart
Courtesy of FORD MOTOR CO.

NOTE: The "WAIT-TO-START" indicator and/or test light may not illuminate if engine temperature is at or near operating temperature.
If test light times are within specifications, system is functioning correctly. End of test. If test light times are not within specifications, disconnect both batteries and replace glow plug module. Repeat FUNCTIONAL TEST.