Start Isolation for Your ACR: Stop the Chartplotter and Stereo from Rebooting at Engine Start

Write down exactly when the reset happens. It points to start isolation if:

• It happens only during cranking, not when the trim, windlass or bilge pump runs.
• It's worse right after a run or with a charger on (when the ACR is likely closed) than after the boat has sat overnight.
• Electronics on the house battery reset, even though you start on the start battery.

If the electronics are wired to the start battery or the engine side of the switch, start isolation won't help; they need to be moved (Step 7).

Why "right after a run" matters: Blue Sea's own explanation of ACR operation shows the relay does not open the instant the engine stops. Voltage has to fall a set amount below the combine point and stay there for a delay (about a minute in their example) before the batteries separate. A quick stop and restart, or a charger holding the voltage up, can leave the banks joined when you turn the key.

Find the ACR or combiner near the batteries or battery switch. Note the brand and model from the label and download its installation manual. Look for a small terminal or wire labeled something like "start isolation," "SI," "start" or "ignition," separate from the ground wire. Not every combiner has the feature; if yours doesn't, the fix is a relay that does, or a dual-circuit switch layout (Step 8).

Many boats have an ACR with the start-isolation terminal left empty, either because the installer skipped it or because it was "optional." Follow the small wire (if there is one) to see where it goes. If it's taped off, coiled up, or connected to an accessory or ignition wire, it isn't doing its job. An ignition or accessory feed is live the whole time the key is on, which is the wrong signal: the relay needs a wire that is live only while cranking.

The signal you want is the wire from the key switch (or start button) to the starter solenoid, the "S" or "start" wire. Blue Sea's application note says exactly that: connect the start-isolation input to the starting circuit between the start key switch and the starter solenoid, so the same signal that engages the solenoid opens the relay just before the starter turns. ABYC's recommended color for that wire is yellow with a red stripe (NOAA's published wiring color chart lists the same), but builders and engine makers don't always follow it, and many modern engines run the key switch through a harness or a digital control module. Use the engine rigging manual or the builder's wiring diagram to find where that wire is reachable: behind the key switch, at a harness plug, or at the starter solenoid. Many owners find it hard to reach on outboards because the key-switch wires run in a harness straight to the engine; if you can't find an accessible point, that's a dealer or electrician job (see wiring diagram DIY for drawing your own map).

This is the step that prevents the most common mistake. With the battery switch on and a helper at the key:

1. Set the meter to DC volts, black lead on a good negative.
2. Probe the candidate wire (back-probe a connector or use a piercing probe only where your manual allows; seal any hole afterwards).
3. Key OFF: it should read about zero.
4. Key ON (RUN): it should still read about zero.
5. Key to START: it should jump to battery voltage while cranking, and drop back to zero the moment the key returns to RUN.

If it shows voltage in RUN, it's an ignition wire, not the start wire. Keep looking. If your engine has a separate start button, test the button's output the same way. PBO's installer made a point of using the terminal that is live only when cranking.

With the batteries switched off:

• Run the signal wire from the start-only point to the relay's start-isolation terminal, in the wire size the relay's manual specifies, with a fuse near the tap if the manual calls for one.
• Use a heat-shrink butt splice or a proper marine tap; avoid household "vampire" taps that cut strands and corrode.
• Support and label the wire at both ends ("ACR start isolation") and keep it clear of hot or moving parts.
• Check the relay's ground wire too: Boating Magazine's install puts a fuse in the ACR ground lead, and a loose ground stops the relay from working at all.

Anything you add in a gasoline engine compartment must be ignition protected; see ignition-protected parts.

Start isolation only protects loads that are fed from the house side. Walk through the boat and note which battery each load uses:

• Electronics, stereo and VHF: house side.
• Engine, engine gauges and the engine's own electronics: start side (as the engine maker specifies).
• Bilge pumps and alarms: their own always-on fused circuit, never cut by a switch or relay.

PBO's installer found a cabin heater wired to the engine's supply; after the change it was running off the isolated start battery, so he moved it to the battery switch's accessory terminal. Look for surprises like that, and keep a list in your BoatMend log.

Many ACR kits come with a dual-circuit battery switch. Instead of a 1-2-BOTH selector, it has two separate circuits: the start battery feeds only the engine, and the house battery feeds only the house loads. Boating Magazine's ACR install wires the start battery to the switch's engine circuit and the house battery to the house circuit, and uses "combine" only when you need the house battery to start the engine in an emergency. With this layout plus start isolation, the house electronics never see the cranking current in normal use. Blue Sea also points out the protection works both ways: keeping the circuits apart stops house-side electrical noise from reaching the engine's electronic controls, and it lowers the chance of flattening both batteries at once.

If your boat has the older 1-2-BOTH switch, starting on BOTH ties everything together and defeats start isolation. See battery switches and dual batteries.

Switch the batteries on and turn the plotter and stereo on. Put the meter across the house battery. Have the helper crank the engine (outboards in the water or on external flush muffs, never dry; inboards in the water):

• The house voltage should barely move during cranking.
• The ACR's status light (if it has one) should show it open while cranking, then close again after its delay once the engine is charging.
• The electronics should stay on.

Repeat right after a run, when the relay would normally be closed. PBO's installer used his battery monitor to confirm that starting no longer affected the house bank. Write down the readings.

Know how to start from the house battery if the start battery dies: the switch's combine position, or the relay's manual override or remote button if it has one. Try it once at the dock so you aren't learning it at sea. Then return the switch to normal, because leaving it combined defeats both the isolation and the protection of the start battery.

If the electronics still reset with the relay proven open during cranking:

• The house battery itself is weak or deeply discharged. Load-test it.
• The electronics feed is undersized or has a bad connection or ground (see the dropouts guide).
• A device is actually fed from the start side.
• The device needs a steadier supply than any battery gives at a low state of charge. A DC-to-DC converter or stabilizer for that device is the next step.