Blue Sea Systems ML-ACR Automatic Charging Relay (300 A) — Operation, LED States & Troubleshooting

Overview

On a coach with separate chassis (start) and house battery banks, the ML-ACR is the device that lets one charging source top up both banks without permanently tying them together. It watches the voltage on both of its main studs; when either bank rises to charging voltage the relay closes and the banks charge as one, and when charging stops and voltage sags the relay opens so house loads can never drag down the start bank. The ML uses a magnetic-latch coil: it draws under 13 mA holding either state and only pulses current (briefly, up to 7 A on 12 V units) while changing state, so it costs essentially nothing in parked-coach battery drain.

Four part numbers: 7620 (12 V) and 7621 (24 V) without manual control; 7622 (12 V) and 7623 (24 V) add a manual override knob. All switch 300 A live at 12 V (150 A at 24 V), take 3/8"-16 studs torqued to a max of 140 in-lb, and are ignition protected (ISO 8846 / SAE J1171, IP66). Continuous capacity depends on cabling: 225 A with 2/0 AWG, 300 A with 4/0, 500 A with doubled 4/0.

Combine and open thresholds (12 V / 24 V)

• Combines above 13.5 V for 30 s, or above 13.0 V for 90 s (24 V: 27.0 V / 26.0 V).
• Opens below 12.35 V for 10 s, or below 12.75 V for 30 s (24 V: 24.7 V / 25.5 V).
• Undervoltage lockout: will not combine if either bank is below 9.6 V (24 V: 19.2 V).
• Overvoltage lockout: will not combine above 16.2 V (24 V: 32.4 V).

Because the relay senses voltage between its ground wire and the studs, both positive and negative runs must be short — voltage drop in long cables skews what the ACR thinks the banks are doing.

Remote switch and LED states

The included Contura remote switch is a sustained SPDT ON-OFF-ON: ON forces a combine (extra power for hard starts), OFF forces isolation (banks stay isolated, and remain so for at least 30 s after returning to AUTO), center is AUTO. Its two LEDs always act together:

• LEDs off: banks isolated.
• LEDs on solid: banks combined.
• Slow steady blink: Start or Engine Isolation is actively holding the banks apart.
• Quick steady blink: undervoltage or overvoltage lockout on one of the banks.
• Double blink (blink-blink-pause): manual override is engaged at the relay — or a mechanical failure; go look at the knob.

Any manual override from the remote switch suspends automatic operation for 10 minutes after the switch returns to AUTO.

Manual override knob (7622/7623)

The knob on top gives mechanical control with or without power: push the button until it latches to combine; rotate right to pop the latch and back left to REMOTE for normal automatic operation; rotate right to LOCK OFF to prevent any operation, and pass a cable tie through the hole to secure it for servicing — this is the lockout point when working the DC system. Knob operation also suspends automatic mode for 10 minutes, and the ACR waits 10 minutes if it tries to close while locked off.

Start Isolation and Engine Isolation

Three isolation wires (brown #1, green #2, orange #3, each fed through a 2 A fuse) configure one of two mutually exclusive behaviors. Start Isolation connects to the crank line (positive only while cranking) and opens the relay for the cranking event plus 3–5 minutes so starter transients never reach house electronics. Engine Isolation connects to the ignition-ON lines of two or three engines and holds the relay open whenever more than one engine is running. On a generator-equipped coach, Start Isolation is the common configuration. Unused control wires must be individually insulated — shorted control wires cause erratic behavior.

Wiring notes

Studs A and B are interchangeable. If the ACR also serves as an emergency cross-connect, ABYC does not require fuses in the A/B cables; if it is charging-only, fuse both cables close to the batteries. The black harness wire goes to DC ground at the main negative bus; remote-switch circuits use 16 AWG minimum with 2 A protection.

Diagnosing 'will not combine'

Blue Sea's flowchart runs two checks. Manual test: remote switch OFF to ON — no click means check the ground wire first (common ground or battery negative); click but no LED means check the yellow LED wire on switch pin 7 and +12 V on pins 3 and 8; wiring good but still no click means the relay has failed. Auto test: switch to AUTO, start engine or charger, wait 90 s. No combine with a terminal below 10 V is undervoltage lockout; above 16 V is overvoltage lockout; neither terminal above 13 V means no charging source is actually reaching the ACR — fix the charging problem, not the relay. If a terminal is above 13 V and it still will not close, disconnect the brown/green/orange isolation wires: if it then combines, a mis-wired Start Isolation input was holding it open.