1987 Johnson/Evinrude CU Outboards 35A thru 55 Service Manual, Page 269Get this manual
Rectifier Ohmmeter Tests
INote Disconnect
battery cables before proceeding with this
1Disconnect all rectifier leads from terminal board
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
Repeat step using the rectifier yellow lead
Results should be similarRepeat step using the rectifier yellowblue lead