Wiring a sonar/gps combo

I am wiring a sonar/gps combo and encountered something strange. I wired it to what I believed to be a bus (correct term?) that (appears to be) connected directly to the battery. It also appears to be have a lead to a stereo coming out of it and another lead, which I think goes the main switch panel. I pulled power directly from this bus and the locator reads 10 v. When I instead draw power from the main switch on the panel, it reads 12 v. Any ideas what is going on? I am running a 3 amp in line fuse as well. Is it problematic to split the wire coming off the main switch from the panel to two different sonar/gps devices? Also, I need to wire a marine radio. Where should I draw power for this? The bus has several empty spots to connect to (but only seems to have 10v) and I’m having trouble finding any spots to wire into the panel (other than splitting an existing wire). Thanks in advance for any thoughts, and sorry for the 20 questions.

My guess is there is a bad connection or component between the batt abd the fuse block. I would find the problem sooner than later.

I would also recommend pulling power for the unit straight off the battery. Some units are surge and voltage sensitive. Just put and inline fuse in and remember to turn it off. Also if you are running an external gps antenna it is recommended that you hve that on a switch and fused ast it will stay on even though sonar head is off. (On lowrance anyway, might be diff for HB). -Mark

Chucker, so I see in a Lowrance manual that I should put a switch in the power to the unit and in the line that powers the GPS module. Right now they are both run through the same switch, do you think that matters. Am I understanding you correctly that you would power the unit without a switch, but leave the switch on the gps power? I played around with it tonight and only became more frustrated. It would read 12v when wired to the main switch, but 10 v when wired to an accessory switch. I’m going to continue unplugging power to things one-by-one & see if it makes a difference.

Any H-bird guys know if the gps module on a bird is powered in the same way where a switch needs to be installed. I couldn’t find anything telling me to do so in those instructions.

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Any H-bird guys know if the gps module on a bird is powered in the same way where a switch needs to be installed. I couldn’t find anything telling me to do so in those instructions.

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Humminbird GPS receivers are powered by the head unit and do not require a switch.