1998 Johnson Evinrude "EC" 150, 175 FFI Service Manual, P/N 520211, Page 141Get this manual
Auxiliary Battery 1Check battery ground cable for continuity
2With key OFF, check battery voltage at battery, then check voltage at auxiliary wire post connection If they are not the same, replace wire to auxiliary battery
3With engine running, check for voltage increase at battery should be approximately 14.5 volts4No increase, turn key to OFF and check redblack wire for continuity Continuity indicates rectifierregulator problem No continuity indicates wire problem26-Volt Circuit