1990 Johnson Evinrude "ES" 40 thru 55, P/N 507872, Page 259Get this manual
Rectifier Ohmmeter Tests
INote Disconnect
battery cables before proceeding with this
2Calibrate ohmmeter on high ohmms scale and connect one lead to clean engine groundConnect the other lead to the rectifier yellowgray leadNote the readingReverse the ohmmeter connections or press the "reverse polarity" button and note the reading very high reading in one direction and low reading in the other direction indicates that diode is OK Two high readings or two low readings indicate that diode is damaged
1Disconnect all rectifier leads from terminal board