Boat Wiring
Written by Todd S. Go2marine/LFS Tech Support
A boat’s wiring can be a nightmare; I will go over a few simple tools and steps to help you diagnose some on the most common problems on a boat. I will cover boat charging systems, battery testing, fuel sending units, and freshwater pumps. This will be basic general wiring on the 12-24VDC side of the electrical system on a boat. Most testing can be done with a simple multimeter.
Operation of a multimeter and symbols are complex. I want to cover just the basics needed to operate the multimeter for these specific tests. The first one is going to be VDC which stands for Voltage Direct Current and the next one will be ohm which stands for ohms or resistance. I will put the symbols below to show which is the correct symbol on the multimeter to select when testing.
There are a few common tools that a boater should always have onboard to help troubleshoot issues with their boat. From a small trailer boat to a large cruiser these simple tools can save you a lot of downtime and money by diagnosing problems yourself. One of the main tools to have is a Multimeter, this tool will help with about every electrical issue that would arise. Also, a Tool Kit, consisting of wrenches, sockets, pliers, and screwdrivers. A good idea is to build yourself a small electrical kit, my perference is aTacke Box, Enough storage in the top for bigger items and smaller compartments to sort out terminals. It will consist of, Fuses, Wire Cutters/Strippers, Electrical Tape, Electrical Terminals electrical terminals, and Terminal Crimper.
Starting with the charging system for example, if you notice your alternator or voltmeter below 12VDC. It will either have a bad gauge, or the alternator has failed. With the engine running take a multimeter set it to VDC and measure across the battery terminals. You should have greater than 12.4 volts, keep in mind some engines must be above idle in order for the alternator to start charging. An acceptable range is 14.4VDC-14.2VDC, usually it will be around 13VDC is optimal depending on battery load at time of testing.
Checking a battery can also be accomplished with a basic multimeter, needs to be set on VDC for this test. If your boat is starting slowly or not starting at all. Take the multimeter and measure across the terminals on the battery, note the reading. Have another person try and start the boat while observing the multimeter. Any reading below about 9.7VDC while starting indicates a bad battery and needs replacing. Once a new battery is installed always go back and check the alternator for output. I will recommend that you also keep a Jump Box on board at all time.
Fuel Sending Units are another simple check with a meter. There are two ways to check the sending unit along with a way to check the Fuel Gauge . When checking the gauge, remove the ground wire from the tank or sending unit, then touch it to the signal wire usually attached to the sending unit via a stud in the center of the unit. Observe the movement of the gauge, the gauge should sweep "needle movement from empty to full". If the gauge does sweep, then it is time to check the functionality of the sending unit. Remove the sending unit from the boat. That usually consist of 6-8 8mm screws, once removed, set the multimeter to the ohms setting and attach it to the signal and ground point of the unit. When the float is all the way in the full position you should see a reading of 232-252 ohms any other reading than that it is a bad sending unit. In the empty position or float all the way at the bottom you should get a reading of 28-36 ohms, again anything different than that the unit is bad and needs replaced.
Diagnosing a faulty Fresh Water Pump is an easy task also. For the first test we need to check for voltage coming into the pump. Set the multimeter on VDC and measure across the positive and negative power wires coming into the pump. If power is present move on to the next test, if not then we need to back track down the line to the breaker or fuse to see what the issue is. If it is fused, with the multimeter set on VDC find a good common ground from the boats 12-24VDC power source and test each end of the fuse for voltage. It can be tested that way or to test a fuse in general, remove the fuse and set the meter to ohms and measure across the fuse. If the reading is open or OL that means the fuse if blown, if it has resistance which is what ohms is measuring then it is good. Once the power source is corrected if the pump is still nonoperational proceed to the next test. For this test set the multimeter to ohms and disconnect the power and ground wires from the pump. Measure across the power and ground wires for the pump, the measurement should be less than 1 ohm with the pump off.
