Electronics That Reset or Shut Off When You Start the Engine: Finding and Fixing Voltage Sag

Does it reset only during cranking, or also when a big load starts (trim, windlass, thruster, livewell pump)? Does it happen with a cold engine only, or after the electronics have run a while with the engine off? Is it one device or all of them? One device usually points to that device's own wiring or fuse; everything at once points to the battery, main cables or the shared power feed.

With everything off for at least a few hours (overnight is best), measure across the battery posts. As a rough guide, a 12-volt lead-acid battery near 12.6–12.7 volts is fully charged, and around 12.2 volts is about half charged (typical figures; check your battery maker's chart, and lithium batteries read differently). A low battery is the most common cause of resets. Charge it fully and see if the problem goes away. See marine battery care.

Put the meter on the battery that feeds your electronics, then have a helper crank the engine for a few seconds (clear of the prop and belts). Note the lowest number you see.

• It dips well below 10 volts: the battery is weak or undersized for the engine, or the starting cables have a bad connection. Have the battery load-tested before buying anything else.
• It stays reasonably high at the battery, but the electronics still reset: the loss is between the battery and the device. Move on to the wiring steps.

Repeat the same test with the meter at the electronics' power terminals or fuse block if you can reach them safely. A big difference between "at the battery" and "at the device" means resistance in the path.

Switch the battery off. Look for white or green crust, loose wing nuts, stacked terminals that wobble, or a cable that turns on its post. Remove, clean to bright metal, reinstall snugly and coat with a corrosion inhibitor. Do the negative cable too: a bad ground causes exactly the same symptoms as a bad positive. See electrical corrosion and connections.

If a cable or terminal feels hot after a few starts, that is resistance turning into heat. Stop cranking, switch the battery off and find that connection before you go on.

Trace the negative wire from your electronics back to the negative bus or battery. Look for corroded ring terminals, a ground screwed into a random bolt, or several grounds piled under one loose nut. Electronics should share a clean negative bus that runs directly back to the battery negative. A ground that wanders through the engine block or a steering cable can drop voltage when the starter runs.

Pull each electronics fuse and look at the blades and the holder. Cheap in-line fuse holders corrode inside and create resistance even when the fuse is good. A holder that is green inside, loose, or warm is a likely culprit. Replace it with a sealed marine holder of the same fuse rating. See fuses and breaker panels.

Thin wire run a long way loses voltage. Blue Sea Systems, working from ABYC E-11, lists electronics among the "critical" circuits allowed only about 3% voltage drop, versus 10% for things like general lighting. Remember that circuit length is the round trip from the battery to the device and back. If your electronics feed is a thin wire that snakes from the bow to a console, it may simply be too small. Wire sizing charts (Blue Sea has a free one) show the gauge for your amps and length. Re-running a feed is moderate DIY for some owners; if you're unsure, have a marine electrician do it.

If engine data or a network sensor drops out, look at where the network gets its power. Garmin's design rule is no more than 1.67 volts of drop between the power T-connector and the farthest device; long backbones with many devices may need the power fed into the middle instead of one end. Garmin also recommends powering the network through the ignition or a dedicated switch. See NMEA 2000 network care.

On boats with two batteries, the most effective fix is to run the electronics from a house battery that is not connected to the starter while you crank. With a manual 1-2-Both switch, start on the engine battery alone, then switch as your manual says. Starting with the switch on "Both" ties the electronics straight into the cranking dip. See battery switches and dual batteries.

Many boats use an automatic charging relay (ACR) that joins the batteries when charging and separates them when not. Blue Sea's start isolation feature goes one step further: a signal wire from the key switch tells the relay to open before the starter engages, so the house battery and electronics are separated during cranking. Blue Sea notes that this wire is optional for normal relay operation, but it is what protects sensitive electronics from the cranking dip. The signal must come from the wire that is live only in the START position. If your boat already has an ACR, check whether that wire was ever connected. Installing or rewiring an ACR is best left to a pro if you are not comfortable with the starting circuit.

Single-battery boats, or boats where the electronics must stay on during cranking, can add a DC-DC converter (sometimes sold as a voltage stabilizer or "power conditioner") between the battery and the electronics. These units hold a steady output even when the input dips, within their rated input range. Victron, for example, makes isolated Orion-Tr converters with a remote on/off that can be triggered by a low-current switch such as the engine run/stop circuit. Size the converter for the total current of everything it feeds, with margin, and follow the maker's fusing and mounting instructions. A converter does not fix a dead battery or a corroded terminal; do Steps 2 through 7 first.

Repeat the cranking test from Step 3 with your helper, watching both the meter and the electronics. Record the lowest voltage at the battery and at the device, and what you changed. BoatUS also suggests measuring at the device's plug with the engine running: a reading much higher than normal while running points to the alternator's regulator rather than the wiring. If the problem started after you added new equipment, note that too, as it points to an overloaded feed or a new ground problem.