Mahe 36: LED replacements for Head and engine compartments

[rgary]

I am trying to install LED rail fixtures (Advanced LED) and am baffled with this issue. Fixtures have a RED and BLACK wire - boat wiring to the non-functioning factory fluorescents is RED and BLACK. Pretty simple install right?

Verified the fixtures work by direct connection to 12V.
Verified I have 12-13V on existing wiring by multi-meter read
Twisted wires together RED-RED & BLACK-BLACK
Try switches on fixture - NO illumination

FUSES are all OK

What to try next - ideas appreciated

[Joh.Ghurt]

Lets assume, it's +12 V between red and black. If not, reverse the polarity.

Check the voltage on between red and black when the fixture is turned on. If it drops (maybe to 2-5V) while the fixture is connected, the wire might not be connected properly at the other end or corroded inside. This is specially popular with badly connected ground wires (black)

Often you can measure 12V with no load, but as soon as you put something on it, those pesky volts decide otherwise.

[Spot]

I would check for polarity at the fixture wires to see red is indeed +12v VDC, in case the new LED's are polarity-sensitive.
Is there a wall switch that could be turned off?

Other than those I think you checked everything else that comes to mind.

PS-I would have not thought to test for voltage drop as Joh.Ghurt mentions above.

[rgary]

SPOT you were “spot” on the polarity issue
Apparently those circuits came from the factory wired this way
All is good - thanks for the quick replies

[Spot]

I am glad it worked. Even better when a person doesn't have to 'let the smoke out' to diagnose an electrical problem. One of my last goof-ups was wiring an accessory socket backwards and then falsely testing with a bulb (incandescent) rather than a meter. When I went to use a non-polarity protected USB charge plug I did indeed 'let the smoke out'...

[rgary]

Follow up on the install
Both the port engine bay light & head (Bluebird has starboard head only) light had the Polarity issues
The starboard engine bay was wired correctly.

[Moosemiester]

In upgrading the lights on Sea Moose to LED I purchased some nautical looking under counter fixtures. They had a round plug with a center pin, and one wire with a stripe.

The stripe was the ground. Whiskey Tango Foxtrot.

Either LED makers are on acid or we’ve stumbled on to something important here.

[rwidman]

When boat makers build a boat and install conventional incandescent lights, polarity doesn't matter and it is often reversed.

If red to red and black doesn't work, connect black to red and red to black and it will work. You may even find some of the fixtures on your boat have the polarity reversed and some don't.