MF 925 2xd3160 12V installation

[lupino]

Hi, can anyone help? I have 2 x VP D3 160 and I have the same thing on both of them.  I don't know is it normal but when I disconnect "+" cable from the alternator I can measure 12V between the cable and the place where the "+" cable was connected and the cable is on "+" potential.  I can't measure any current with an amperemeter. The same thing happens on "-" pole. when "+" is connected and I disconnect "-" cable I can measure 12V between cable and ground and ground become "+".Both alternators normally charge batteries. I have changed one regulator but everything remains the same. Is this normal? Can someone explain this situation? What happens when I disconnect the main switch off on one motor and the other motor main switch is on?  Actually, I am investigating this because my echosounder has no normal sensitivity.