12V Voltage fluctuation in my 12V wiring

[BFDfirefighter16]

Hello everyone,

I recently bought a 67' 44TC and at random I have come across a situation where I use my voltmeter and the wire I was testing would read just over 12 volts and then in a matter of seconds later if i retest the wire it reads about 8.2 volts, and if retested a third time will be back up to over 12 volts.. This has happened when I was in the process of reinstalling my refinished masthead with a new steaming/anchor light and weeks earlier when i was installing my Garmin chartplotter. Is there something that could be causing this problem that I am overlooking? I am open to all suggestions. I am not sure if it would be related but I do know that I have at least one bad battery because I am able to start my port engine but my stbd engine never has enough battery power to start without paralleling.

Thank you everyone for your help it is very much appreciated.

Alex Sherard

 

[MikeP]

You probably already checked this but the first thing that occurs to me is simply a bad connection between the meter probe and the wire/connector in the circuit you are testing. But it sounds too consistent for that. If you leave the voltmeter connected to the circuit does the voltage reading vary as opposed to connecting it/checking/disconnecting/rechecking ? Is there ANYTHING in that circuit that draws a good bit of power that can cycle on/off...maybe a bilge pump, some other type of motor, etc that would draw down the power when it is on? Insufficient wire gauge for the load or poor connections/corroded wiring could aggravate that voltage reduction under load.

 

[BFDfirefighter16]

I have left the probes connected and could watch the fluctuation and the same thing would happen. The wiring is mostly all original from 1967 and as far as I can tell there appears to be nothing on because I keep most of the breakers turned off. My bilge pumps hardly ever have to kick on.

 

[MikeP]

Hmmm

Does the same voltage variation happen if you check a different 12v circuit (from a different breaker) or does it only occur on that one? If it occurs on both do you have another multimeter that you could use to make the same check? That would rule out some problem in the meter itself.

Disconnect the batteries from the system and do the voltage check on the battery bank itself.

Bad connections could easily cause the voltage differences but I would not expect them to regularly come and go...unless heating/cooling of the circuit/connections is involved.

 

[racclarkson@gmail.com]

Battery in tester can cause it to bounce around.

 

[BFDfirefighter16]

I just replaced the batteries in my voltmeter and also borrowed another voltmeter and both are still showing the fluctuation at different 12V sources. Also they are on different breakers, so my next step is to isolate and test each battery and connections I guess.

 

[oscarvan]

Many electrical problems can be traced to a spotty ground.

 

[osprey]

I have had similar problem a couple times. Replacement of fuse worked for me.

P

 

[bigbill]

check the connections on the back of the breakers, i had a bunch of loose wires that i ran across by accident

 

[SEVEN]

I know you said no other motors were running but I thought I would throw in a AC/DC refrigerator. Do you have one? If not sorry for asking.