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Leon Sisson
Master Marine Consultant

Members Avatar

USA
1889 Posts

Response Posted - 08/01/2018 : 11:12:22

Lots of good advice already. Here's my contribution to muddying the situation.

I've seen a lot of boat electrical problems caused by installing non-marine components and wire in a marine environment. For example, the original wiring, switches, and cabin lights in older Catalina boats. (Not trying to pick on Catalina. Way back when, crappy electrical systems were the industry standard.)

Here's another troubleshooting technique I didn't notice mentioned above. With a malfunctioning circuit turned on, use your volt meter to locate voltage drops.

I seem to remember that ABYC standards allow up to 3% voltage drop on safety related circuits (nav. lights, nav. equip, comm. systems, bilge blowers, including main feeds to same), and up to 10% on non-critical circuits (i.e. comfort, convenience, entertainment, etc.). When sizing wire I try not to go much above 3% volt drop, except for electric starters, spotlights, and other high current intermittent loads. Three percent of 12.V is about 1/3 volt, obviously 10% is 1.2V. (I hope my side trip into ABYC wiring specs isn't too far off topic. On boats as old as ours, I've found that selecting and installing new marine grade wiring can be part of fixing electrical gremlins.)

Applying the volt meter probes straight across the battery terminals, note the measured battery voltage. If chasing voltage drops takes a while, then periodically recheck voltage directly at the battery, and update any affected calculations accordingly.

Next, apply the volt meter probes across the load (bulb, motor, etc.) as close to the actual load contacts as you can get to. (In the case of lights on the mast, you may need to settle for the backside of the mast side of the deck connector.) The voltage reading across the load should be within a few percent the reading at the battery.

If you find more than a few tenths of a volt difference, then use your volt meter to search for it. With the circuit still on, apply the volt meter probes across sections of the circuit which should have low resistance. Start with the most suspect items, such as switches, connectors, screw terminals, fuse, etc. As emphasized by others, don't neglect to check the zero volt return portion of the circuit as well (a.k.a. 'ground' wiring).

At some point (or points) you should find your missing volts. Repair or replace the component with the voltage drop across it. If you find a significant volt drop across a wire run, then either it's too small for the current it's carrying, or it's damaged, perhaps inside the insulation somewhere, maybe from a drill bit or screw cutting through the insulation, allowing salt air to get at the copper. You can double check the suspect wire by temporarily connecting a good wire in parallel, and seeing if the voltage drop goes away.

By the way, the reason you might find an odd colored return (ground) wire in mast wiring, is the builder shopping for 4- or 5-conductor cable got the best price on bulk spools of wiring bundled for some other application. When rewiring boats, I find that the least expensive multi-conductor cables tend to have color codes for the most likely application of that particular gauge and number of wires.
3-conductor: green+black+=120vac,
4-conductor: green+yellow+brown+white=trailer lights,
5-conductor: red+blue+white+black+green(maybe with yellow stripe)=multi-phase ac motor wiring,
and so on.

-- Leon Sisson

� Leon Sisson

Voyager
Master Marine Consultant

Members Avatar

USA
5231 Posts

Response Posted - 08/01/2018 : 18:04:09

Leon, your voltage drop concept is right on the money for an old boat.
That�s where my 20 ft long jumper with alligator clips comes in.
You can use a piece of 16 GA lamp cord (use both wires tied together) and solder or screw on the alligator clips.
Put a piece of shrink tubing over the clips to prevent shorting out to nearby components.
This can be used to bypass a suspicious cable or switch.

Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain � Milford, CT