VP D3 160 alternator behavior
 

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.