The ground wire to the alternator is extremely important for protecting the alternator. The alternator's connection to the engine block is through some less than perfect electrical
conducting bolts and brackets which frequently results in blown out alternators. The ground wire direct to the alternator is a very common solution.
- - However the main "fat" ground wire to the engine starter is probably connected to one of the starter to engine block main bolts. It all depends upon the physical location of the electrical panel's ground buss bars. From there the "fat" ground wire is routed to the nearest major attachment point on the engine - normally an engine mount bracket or other large bolt on the engine. Then another "fat" ground wire is lead back to the battery's negative terminal.
- - IMPORTANT -If you have a flexible coupling or "drivesaver" in your engine/prop shaft system you may have an "electrically isolated" prop shaft which means there is NO earth ground in your boat's system anymore. Some installations will have a wire from one side of the flexible couple/drivesaver to the other side to restore the electrical pathway. For the DC system this is not a problem as the batteries and DC loads are a closed circuit. But for the AC system you may have a serious problem. I have a separate grounding plate on the hull specifically for the AC earth ground so that the AC and DC never shall meet.
- - Normally the only thing interrupting this DC ground wire back to the battery would be a "shunt" used by amp-hour meters to calculate the total power used. Anything else would be highly suspect. Ampere meters are normally in the main positive feed wire between the battery switch and the load c/b's.
- - It is common to connect the AC system earth ground to the engine block and subsequently also to the DC system ground. A leaky AC appliance or receptacle or improper wiring
connection can "leak" AC voltage to the earth ground and impress it also on the DC system which is not good for your delicate instruments. Of course, if you do not have any leaky AC components this is not a factor/problem.
- - Additionally, alternators are AC generating devices which are rectified to output DC. Failing diodes can allow AC to get into the DC system - but if your tests are done with the engine shut down, this would not be a factor.
- - Try disconnecting the AC system earth ground (and if you have a genset - also the neutral) from the engine or DC system ground buss and see what your readings are or if they change.
- - Remember there will be some milli-volts of readings in any bi-metallic system where sea-water is involved. The Corrosion Meter gauge is painted with the acceptable amount of voltage for each type of metal in the system. Also the chart in my post #27 shows the normal voltages encountered. Only if you are getting voltage outside these normal ranges do you have a "under/over" zinc'd problem.