1988 Johnson/Evinrude "CC" 40 thru 55 Models Service Manual, Page 273Get this manual
Rectifier Ohmmeter Tests
Disconnect battery cables before proceeding with this check
1Disconnect all rectifier leads from terminal board2Calibrate on high ohmms scale and connect one lead to clean engine groundConnect the other lead to the rectifier yellow gray 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 damaged3 Repeat step using the rectifier vellow lead Results should be similar]