Mechanics,
When performing continuity checks on the S39 circuit continuity switch-box, use the Fluke® 27-II multimeter. Here’s why.
TM 1-2840-248-23&P (Oct 19) has a caution concerning the S39 that says not to exceed 1.5 volts direct current (VDC) because going beyond that can damage the digital electronic control unit (DECU) and the enhanced digital engine control unit (EDECU).
However, in aviation maintenance action message (AMAM) H-64-20-AMAM-03, the voltage warning for the S39 connector check was raised to 10.0 VDC.
Here’s the rub. The current Fluke® 1587 multimeter doesn’t have settings to the voltage at or below the 1.5 VDC. Although the DECU/EDECU can handle up to 10.0 VDC, exceeding the 1.5 VDC for continuity checks can energize some internal DECU/EDECU components and cause false readings while performing the S39 circuit check.
Based on this information and to maintain the correct voltage, it’s better to use the Fluke® 27-II multimeter, NSN 6625-01-581-4058, because it maintains less than 1.5 VDC to perform the S39 circuit check with the most accuracy. You can find the multimeter in the aircraft maintenance hydraulic repairer tool kit (HYTK).