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Author Topic:   Compass Illumination Wiring
waterguy posted 06-14-2006 01:17 PM ET (US)   Profile for waterguy   Send Email to waterguy  
I have an in-dash compass whose illumination circuit I'd like to wire up so that it lights up with the rest of the gauges when the ignition key is on. Can I simply run the positive wire from the compass to another gauge and then find a ground for the compass? What would be the best connection for the ground? Some of the electrical I see here behind the dash is more than I am familiar with. This would be for our 2001 Montauk.

swist posted 06-14-2006 04:11 PM ET (US)     Profile for swist  Send Email to swist     
Any other gauge that lights up will have a black ground wire (negative) and a blue wire (positive) for the light, you can simply connect the compass lights to those terminals.

On some boats the instrument panel lights are always on when the ignition is on (i.e. they do not have a separate switch, nor are they wired to the nav/anchor light switch). In this case you may find the ignition circuit (purple wire) connected to the lamp terminals of the gauges instead of a blue wire. You can connect the compass light to the purple wire in this case.

jimh posted 06-14-2006 06:04 PM ET (US)     Profile for jimh  Send Email to jimh     
The best connection for the battery negative return (not exactly the "ground" on a vessel) from a load is to connect the lead to the nearest battery negative distribution bus. In your Boston Whaler 2001 MONTAUK this should be right in the console interior below the compass.

The normal practice on instrument lighting is to wire the instrument lighting circuit loads in parallel and connect them to a switched feed of the battery voltage to control the illumination. The ABYC standard for this circuit is to use a dark blue wire for the positive branch of the circuit and to wire the negative branch using the normal black color or in some cases (if the vessel has an AC power distribution system wherein the HOT conductor is wired with a black colored cable) a yellow color conductor.

Larry posted 06-15-2006 10:31 AM ET (US)     Profile for Larry  Send Email to Larry     
I seem to remember that the two wires should be twisted around each other so the DC current would not affect the magnetic field.
swist posted 06-15-2006 12:24 PM ET (US)     Profile for swist  Send Email to swist     
Good point. With that and other electrical stuff in the vicinity, make such you calibrate your compass AFTER everything is hooked up and wires are in their final resting places.
jimh posted 06-15-2006 09:24 PM ET (US)     Profile for jimh  Send Email to jimh     
Most wiring is non-ferrous. The concern for wiring affecting a compass is when the wire is carrying a direct current. A wire carrying a direct current generates a magnetic field around it. The strength of the field is proportional to the current flowing in the wire and the distance from the wire.

Much of the wiring on Boston Whaler boats is done with paired cables, often called "zip cord." (On my boat this is how the factory-installed compass lighting circuit is wired.) One conductor carries current in one direction, and the paired conductor usually carries the same current flow in the opposite direction. The two wires will tend to cancel out each other's magnetic field.

To enhance the cancellation effect, paired wires are often twisted together. This makes the cancellation more uniform. The current flow in the wires leading to the compass lamp is quite small. I suspect the chances of it influencing the compass indication are slight. You can test this by observing the compass and turning on the illumination. If you see any deflection of the compass, you should investigate the wiring to see if there is something awry.

On the other hand, a single conductor carrying a large current will probably affect the compass. You should avoid running non-paired conductors near the compass, and particularly those which carry high current.

waterguy posted 06-16-2006 08:07 PM ET (US)     Profile for waterguy  Send Email to waterguy     
Thank you for the advise guys. I will be hooking up the connections this weekend.

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