Testing for short circuits

[rayharrison]

Found that my 90 amp fuse on Mercruiser I/O 5.0 liter was blown which led me to suspect a short somewhere. Found a great troubleshooting process on this website. Disconnect positive battery lead, turn off all load at switches, connect voltmeter to positive cable then to positive post of battery. Read voltage, if any voltage your have a short. Move on by isolating each circuit to see if any voltage is present. if you have voltage move on to next circuit, if no voltage you have isolated the problems circuit.

Now we come to my question. Did all of the above, had voltage thus a short somewhere. Disconnected individual circuits did not get voltage drop, disconnected all grounds to battery, repeated measurement still show voltage even though ground is disconnected. How can this happen and what does this indicate?

 

[dingbat]

Your test method wouldn't be my choice. As you have found, there can be grounds not necessarily referenced to battery ground. Your outdrive for one.

I would shut off the battery and check the positive side of each circuit for continuity to ground. Find continuity, you found the fault

 

[leftyfrazel]

Ray Harrison, the website has mislead you, the short your looking for is not necessarily a short, it's known as cross battery... Dingbat explained it best your going to be looking for a continuance to ground, there is where your blown fuse is coming from...

 

[Chris1956]

When you put a voltmeter in series (between the battery post and the boat wiring, you are checking for amperage. If you are not careful, you will blow the meter.

Secondly, if you blew a 90Amp fuse, the short could be very dangerous, as it can cause a fire. The good news is that only thick cables (like battery cables) can withstand a 90A short, w/o melting.

Look for the battery cables to be touching somewhere. Secondly is to check for a short in the starter solenoid or starter. PTT pump/solenoid is another possible area to check.

Checking for resistance to ground with the voltmeter connected to the positive battery cable (battery disconnected) and ground can indicate a short. Ohm's law will tell you the approximate current of the short.