1991 Johnson/Evinrude Models "EI" 40 thru 55 Service Manual, Page 280Get this manual
Power Supply Tests 1Disconnect battery cables from batteryr"]2Disconnect yellowblue, yellowgray and yellow power supt!J ply leads from the terminal boardr) 3Disconnect the yellowred power supply lead from the stator
]4Disconnect the warning horn 3-pin Amphenol connectorr;] 5Calibrate ohmmeter on appropriate scaleConnect meter
black lead to terminal "8" (purple lead of engine's warning horn connector)f516Connect meter red lead to power supply yellowred leadNote meter readingReverse the ohmmeter connections, or press the "reverse polarity" button, and note the reading high reading in one direction and low reading in the other direction indicates the diode is OK Two high readings or two low readings indicates the diode is damaged