Navigation wiring issue

I have a 1996 Shamrock 22' WA with an aluminum top. I noticed the anchor light on top of the AL top was not working. This light has 3 wires going to it (Black, solid gray, gray with with wite stripe). The light is not an "all around white", it has 2 different bulbs (one in the front one in the back). I assume with the light switch in "NAV" the front side only should be lit up along with the stern white light and the port and starbord red/green lights. I assume at anchor (switch down in "ANCHOR" just this light on top of the AL top should be lit up but both sides should be lit so it gives light all the way around.

I replaced the bulbs but only one side would lite up when the switch is in anchor position. Upon looking at it I decided to replace the whole receptacle, it was pretty cracked up. I cut the wires and decided to test the wire ends with my meter while the light was out.

No mater what position the switch is in (NAV or anc) I never get and voltage between the black and gray with white stripe.

With the switch on NAV I only get 6 volts between black and solid gray.

With the switch on "anc" I get 12 volts between the black and gray.

I didnt pull the switch panal yet but tomorrow I will do that and see if I can see anythin obviuos.

Man I am stumped, how am I getting a voltage loss?

All other light work perfect even the deck light which is right under the anchor light.


Any helpfull hints would be appreciated.
Thanks
KMD

 

Assuming the wires are properly sized for the run, the most common reason for voltage drop is to many or bad connections. I would probably start by confirming that it isn't the switch that is bad (just because it is the easiest to check), then start tracing the wire making sure that there aren't any other things added to the run (extra lights, bilge pump take off, ect), then probably replace all of the connections.

 

I'd bet the 'ground' wire is disconnected, or 'connected' to the aluminum top.

Aluminum is notorious for corroding where any electrical connection is made to it.. Separate ground wire back to the boats common ground is needed.

6 Volts anywhere sounds like an aluminum "lossy diode" is somewhere in the actual circuit (not necessarily the circuit you think you have!)...

OR: The two bulbs are effectively in series because of the 'open' ground, and acting as a voltage divider. Ever seen a car with both high and low beams on dimly?? Usually a bad ground to 'chassis'..