electrical gremlin

[gettinthere]

2 cabin lights with G4 LED bulbs started going dim after 2 years of good performance.
Ultimately I ran into this while trying to track down the issue...
On the red/black feed I can get 12 volt nominal. But when I switch the bulb on it goes to zero. I switched to the old quartz bulbs with the same issue. Figured it would be the switch so I connected the G4 pins directly to the known hot red/black. No light from the bulb. Continuity checked OK. Tried another good bulb anyway and no light at all.
How can I get 12 volts out of a pair of wires and not a flicker from a bulb when touching the leads direct? Yea not a perfect connection but I should get some flicker from the bulb.

 

[Maine Sail]

2 cabin lights with G4 LED bulbs started going dim after 2 years of good performance.
Ultimately I ran into this while trying to track down the issue...
On the red/black feed I can get 12 volt nominal. But when I switch the bulb on it goes to zero. I switched to the old quartz bulbs with the same issue. Figured it would be the switch so I connected the G4 pins directly to the known hot red/black. No light from the bulb. Continuity checked OK. Tried another good bulb anyway and no light at all.
How can I get 12 volts out of a pair of wires and not a flicker from a bulb when touching the leads direct? Yea not a perfect connection but I should get some flicker from the bulb.

You have a bad connection or switch. A bad connection can show voltage because there is no current being used to measure voltage, but it can not pass current. This can be a dangerous situation so do search it out and find where the wire is broken, chafed or the connection failing. A bad switch can also show volts but not be able to pass current I run across this frequently..

 

[ISABELLA5]

voltage but no light. Your symptom indicates a high resistance (corrosion) in either the ground or "hot" lead. A high resistance will indicate the proper voltage but will no allow enough current (amps) to operate the lamp. With all power removed check continuity from ground buss to lamp ground then from main switch for lighting to lamp supply terminal. I would not think it should be more than 1 to 2 max. A continuity test is done with an ohm meter (harbor freight $8.00). Never use an ohm meter with power applied. If you symptom affects all cabin lighting then look from switch to batt pos buss.

 

[Daydreamer]

On a DC circuit I use a voltmeter to test for voltage drop. With the circuit switched on I use the voltmeter as a parallel to test from the battery to various components along the circuit. Anywhere the reading of the voltmeter increases above 1 volt indicates resistance in the circuit. This method will show where the circuit goes open when loaded.

 

[gettinthere]

Well yea, I get that a weak connection could lead to minimal amperage flow. But this G4 bulb is a very minimal draw and ot doesnt flicker or anything. I bipassed the switch and hot wired the bulb direct to the hot wires with no result at all.

Pulling new wire will be a MAJOR headache

 

[mitiempo]

You should not have to pull a new wire unless the issue is chafe along the route it takes. As posted it is likely that it is either the switch or a connection at either the positive or ground at the panel.

 

[gettinthere]

Finally found the source end of this wire. Placed my G4 bulb hot on the wires and it lights up big. At the fixture end it will not light up the bulb yet I get 12 volts. There is NO switch in this test, it was removed. Touching direct to wires.
From source to fixture, I get continuity on the red side but not on the black side...even tho I do get voltage across those red/black wires
So there is some kind of chafe or break or corrosion in the black wire...hidden in the headliner which is held in by trim pieces that are bunged in. Great

 

[Maine Sail]

On a DC circuit I use a voltmeter to test for voltage drop. With the circuit switched on I use the voltmeter as a parallel to test from the battery to various components along the circuit. Anywhere the reading of the voltmeter increases above 1 volt indicates resistance in the circuit. This method will show where the circuit goes open when loaded.

Remember that 1 volt is an 8% voltage drop and pretty much unacceptable for any critical circuits..... For cabin lighting 8% is okay, with every light turned on in the circuit, but not for something like nav lights etc.....

 

[Maine Sail]

Finally found the source end of this wire. Placed my G4 bulb hot on the wires and it lights up big. At the fixture end it will not light up the bulb yet I get 12 volts. There is NO switch in this test, it was removed. Touching direct to wires.
From source to fixture, I get continuity on the red side but not on the black side...even tho I do get voltage across those red/black wires
So there is some kind of chafe or break or corrosion in the black wire...hidden in the headliner which is held in by trim pieces that are bunged in. Great

This is why I continually preach "home running" wires to lighting fixtures that will be buried or hidden The only connections n should be accessible at the fixture and then back at an accessible busbar.. Hidden connections are verboten in land based wiring but boat builders think nothing of it. I have seen "wire hats" with six wires in each and one fractured off, Scotch-Loc failures, four wires into a butt splice etc... etc..... Customers hate the 16 hour labor bill of tearing the entire boat apart to effect a 2 minute repair....

TIP: Your overhead lights are likely daisy chained in parallel. Pull them down one by one and redo every termination. Often times that can solve it. if not you have a hidden treasure... Oh how I love hidden treasure.... NOT......

 

[Rick D]

Bingo

Finally found the source end of this wire. Placed my G4 bulb hot on the wires and it lights up big. At the fixture end it will not light up the bulb yet I get 12 volts. There is NO switch in this test, it was removed. Touching direct to wires.
From source to fixture, I get continuity on the red side but not on the black side...even tho I do get voltage across those red/black wires
So there is some kind of chafe or break or corrosion in the black wire...hidden in the headliner which is held in by trim pieces that are bunged in. Great

Had this happen in my bow nav light wiring. Made me nuts. Tore half the boat apart to find nice tinned wire up to the pulpit. Then, it tied into junk wire which was obviously installed by the pulpit jobber. Now I have to pull the pulpit and try all the methods guys here suggested to get new wiring through the many twists and turns of the tubing. Good luck!