1990 Johnson Evinrude "ES" 9.9 thru 30 Service Manual, P/N 507871, Page 301Get this manual
Rectifier Ohmmeter Tests
Disconnect battery cables before proceeding with this check
1Disconnect all rectifier leads from terminal board2Calibrate ohmmeter on high ohmms scale and connect one lead to clean engine groundConnect the other lead to the rectifier yellow gray leadNote the reading Reverse 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 damaged3Repeat step using the rectifier yellow lead Resultsshould be similar