Newbie fixing running lights

[wgerstmyer]

Hi, pretty lame question I’m sure.
I tested the voltage at the breaker switch and my two, suddenly non-working forward running lights. The switch says 12.8v and the two lights each are 3.7v.
Shouldn’t all 3 running lights be getting 12.8 volts ? Does this mean that somewhere between the two a connection has soured? Not looking forward to cleaning out the V-berth and crawling into the chain locker to where the connection likely is between the two forward lights but I’m guessing that’s what you’d recommend? Thanks, Will

[5BTM]

12.8V at the switch and 3.7 after means a bad connection or a bad switch.

[Mickeyrouse]

Running lights are notorious for having corroded contacts. Even if they’re above the so-called “splash zone” the marine air will corrode supposedly sealed lights. That corrosion acts as anything from a poor conductor to an outright insulator. As corrosion increases, resistance increases, voltage goes down ( and various permutations of same.) for anything other than brand new, I’d be surprised to find exactly the same voltages on two different sockets on the same circuit. And add in the variable of quality of connections from breaker(s) to sockets…well, you get the picture.

[Kettlewell]

My guess would be a corroded connection nearish the lights, or where the wires connect to the lights. Usually the wire run to the lights will be inside the boat and the middle of wires are unlikely to corrode or break. Look for a splice near the light, and likely right where the wires connect to the light fixture.

[donradcliffe]

Depends whether the lights are incandescent or LED.

[Spot]

Originally Posted by wgerstmyer

Hi, pretty lame question I’m sure.
I tested the voltage at the breaker switch and my two, suddenly non-working forward running lights. The switch says 12.8v and the two lights each are 3.7v.
Shouldn’t all 3 running lights be getting 12.8 volts ? Does this mean that somewhere between the two a connection has soured? Not looking forward to cleaning out the V-berth and crawling into the chain locker to where the connection likely is between the two forward lights but I’m guessing that’s what you’d recommend? Thanks, Will

Good question actually.
So, to be clear, you probed before and after the breaker/switch and removed the bulbs and probed the contacts to get the readings of 3.7v on the two forward bulbs? Was the third bulb 12.8 like the breaker/switch? Bulbs are incandescent?

I am leaning towards Kettlewell's suggestion that there is corroded splice somewhere forward.

[wgerstmyer]

I can probe more places if recommended. The 12.8 v reading was right off the terminals of my breaker switch and ground. The 3.7 v reading was of the wires immediately before they enter the light fixtures. I did not do a reading of the stern running light yet, it is old and incandescent and has not worked for a few weeks. All my other lights in the boat are LED. Does this help?

[Jammer]

Originally Posted by wgerstmyer

Hi, pretty lame question I’m sure.
I tested the voltage at the breaker switch and my two, suddenly non-working forward running lights. The switch says 12.8v and the two lights each are 3.7v.
Shouldn’t all 3 running lights be getting 12.8 volts ? Does this mean that somewhere between the two a connection has soured? Not looking forward to cleaning out the V-berth and crawling into the chain locker to where the connection likely is between the two forward lights but I’m guessing that’s what you’d recommend? Thanks, Will

Probably a bad crimp terminal at the switch end of the run, check that before digging.


If your meter or test light has a needle probe, use that, otherwise you can carefully strip a little of the wire and tape it when you're done.

[Spot]

Originally Posted by wgerstmyer

I can probe more places if recommended. The 12.8 v reading was right off the terminals of my breaker switch and ground. The 3.7 v reading was of the wires immediately before they enter the light fixtures. I did not do a reading of the stern running light yet, it is old and incandescent and has not worked for a few weeks. All my other lights in the boat are LED. Does this help?

Yes, nice clarification and good procedure so far, thanks.

I would check the rear light as well. If it is also 3.7v then there may be a electrically weak connection to the wires running both fore and aft.

Other lights on other circuits being LED should not matter.

If you replace the bulbs, make sure they are the correct kind, esp. if they have a bayonet base. The automotive types with the shell being ground can be 'false friends' in fixtures meant to take dual contact un-grounded shell bulbs.

[5BTM]

Disconnect the breaker and test the resistance, should be 0Ω when on and infinite off.

I make my own probes using WAGO 221 connectors and whatever I need at the time. This ones are for back probing the oxygen sensor on my son's car. Just a couple of paper clips.

[Americat3014]

Find another loose wire long enough to run to the furthest light fixture and the breaker
that you can use for testing continuity (Ohms on the tester). The problem could be in either the positive or the negative wire. First connect the tester to both ends of the loose wire and make sure the resistance is zero or next to zero. Setting the tester so that it buzzes is easiest. Test the positive wire and the negative wire using the loose wire to bring the circuit back to the breaker and use the tester as part of the circuit. Hopefully, that will tell you whether the positive or the negative circuit is the problem. If the wiring runs up to the port or starboard light and then over to the other light, you want to test each segment of the wiring to try and find the problem. It is likely to be a connector, but I just replace a section of wiring that was the problem so wire that looks OK can still have broken copper inside. Replace one connector at a time and measure the voltage or see if the lights work.