Electrical short

[Davncong]

Went out last weekend and immediately noticed my fish finder/chart plotter wouldn't come on. Everything was fine a week prior. Started troubleshooting when I got home and found power to my master switch was 12.6v but immediately dropped to 9.5 when switch toggled on. Radio and horn work, bilge and livewell pumps do not. Lighting works but seems to flicker some.

Disconnected everything at the battery (engine and power steering pump) and only left the power that runs to the helm. No changes, 12.6v with master switch off, 9-10v with it on. Power appears to go battery->master breaker->master switch->circuit breakers->circuit switches->circuits

Battery load tested - good
Disconnected each breaker one at a time - no change
Disconnected each switch one at a time - no change
Disconnected bilge - no change
Cannot find anywhere to disconnect livewell pump except at the switch.

Any ideas how to isolate the issue other than what I've already done? Battery terminals also cleaned. I'm thinking possibly a grounding issue but not sure how else to isolate where.

Any ideas appreciated. Thanks

 

[Mc Tool]

Might not be a short , could be a bad connection . You need to do a volt drop test .
If you have a bad connection you will loose voltage over that connection and high current drain stuff like pumps wont run fast enough to pump properly and electronic stuff might not power up at all ,and the more stuff you turn on the worse it will get.....like the radio might work untill you turn on the pumps .
When your meter is set to dc volts be thinking that the meter will show you the difference in potential between the two probes ....stick them on the battery and it will show you the difference ....being battery voltage . If you have a bad connection ( might be a plug ,fuse or corroded contacts ) and you put your probes one on either side of that connection it will show the difference in potential ....might be only .5v . You would think that if your putting both probes on what should be the same power feed there would be no difference ,specially if they are only an inch apart . Ideally it should show 0v but if you get a reading its telling you that connection is bad . Your looking for 2.6v to 3.6v ( going by the voltage figures you stated ) and it might not all be in only one bad connection.
So ,with the power on place one probe on the battery + (and leave it there for the test )and use the other probe to touch the power feed wire from the battery ...working away from the battery ,when you get a significant reading ....like the couple of volts your missing you will know that the last connection your probe passed over will be the crook one .
The meter might show 0.1-0.2v and this shows the loss due the the resistance of the wire itself and that is normal and will grow as the length of wire between the probes increases but should not get more than 0.2 -0.3v, so ya gotta allow for that but anything that adds extra on to that should be viewed with suspicion .
Once you have found something suss like a circuit breaker for example put one meter probe on each side of the breaker , ideally it should show 0v but if it shows something like your missing 2.6-3.6v ....thats where the fault is .
Sometimes if the fault is a plug you can squirt a bit of wd40 in there and plug -unplug a few times to scrape corrosion of the terminals will fix the fault.
You may be able to find a vid showing how to do a volt drop test which may help to see what its all about ( monkey see monkey do sorta thing ). Hope this helps