Fusable Link on Boat Engines?

[SweetSurrender]

Hello friends,

I am helping a friend troubleshoot an electrical issue on a 2002 320DA w/6.2MPI's.

Issue: When leaving the dock; the port battery gauge reads a strong 14volts. After underway (approx. 30mins or so) - the port battery gauge drastically drops to ~11-10 volts. If you shut off the port engine; you must use emergency start to re-crank.

What load was on when this occurs? Standard cabin items: Head System; Cabin Lighting; Refrig; Power Vents

Instant thought: Bad Alternator and/or Battery issue. Right? So...

Several weekends ago, all 4 batteries were removed and taken to a local battery dealer. They were able to do a thorough load test as well as a cell-by-cell test -- All 4 batteries tested equal to brand new ones. In fact; these 4 batteries only have 2 seasons on them.

Next steps - Replace alternator - believe it or not; my friend had a brand new Mercruiser (Quicksilver) alternator on hand. We replaced the alternator this past Saturday with no change. The same issue occurs.

My thought - The port engine is running strictly off the batteries - the alternator charge isn't "making it" to the batteries. After applying AC Converter; the batteries recharge.

Is there a "fusable link" on a marine engine/alternator like an automobile? If so - and it's bad; this would prevent alternator charge from reaching the batteries, correct?

Any other thoughts?

Thanks all,
-Capt. Ryan

 

[fc3]

There's a fusible link, but I don't think there is one for the alternator's output. You can check this with a simple voltmeter.

Take all reading on the terminals of the alternator.
Ignition off, engine off:
+12 on the orange output wire
+12 on the purple excitation wire
0 on the red sense wire

Ignition on, engine off
as above except
+12 on the red sense wire

Ignition on engine running
At least +13.6 on the orange wire and at the battery. 13.9 to 14.2 is nominal charging voltage with charged batteries.

Note that some twin engine Sea Ray's have a battery isolator installed. This allows either alternator to charge both batteries. The isolator is mounted on the bulkhead and has large heat sink. If the isolator is bad, the batteries won't charge. The isolator is just a series of rectifiers. Test using any ohmmeter that can do a diode test.

Best regards,
Frank C

 

[SweetSurrender]

Frank,

Thanks a million. I knew you would have some helpful troubleshooting ideas. I do have the orange wire connected to the back of alternator. There is also a ground wire. Then I have a small two-wire plug that snaps into a "socket". Is this the "Sensor" wire you are referring to? What purpose does this serve? Sensor information back to the ECM?

I appreciate your help,
-Ryan

 

[fc3]

The wires that plug into the alternator on that connector are the sense and excitation wires that I describe above. Both should have +12 to them with the ignition on. The ground wire should be well bonded to the engine with zero (or nearly so) ohms to ground.

These alternators are self exciting. The regulator's output supplies the power to run the field coils. But until the alternator starts to produce power, you have to supply it with power for it to start working. That's the purpose of the excitation wire. It provides the initial current for the alternator to start operating. Once it is generating power, the regulator runs off the output stud. Since the regulator is running off the output supply, the regulator needs some other way of sensing the battery's state of charge. The sense wire carries very little current, but the voltage present on that wire allows the regulator to measure the battery voltage in order to control the alternator's output. All sense does it to provide a reference voltage that represents the battery voltage.

Best regards,
Frank C

 

[SweetSurrender]

Thanks again, Frank. I'll perform these checks when I am at the boat; hopefully this weekend.

Assuming your reccommended tests pass; what other reasons could cause the port battery gauge to drop below 12? The port engine will continue to run until I shut it off - Then I have to (a) use emergency start or (b) crank genny to charge batteries.

Again, thanks a million.
-Ryan

 

[fc3]

Alternator failure. Intermittent operation could be brushes or regulator.
Bad battery isolator. Note: if you have a battery isolator, there may not be +12VDC at the output stud (orange wire) when the engine is not running.
Wiring problems.
Excessive load.

Best regards,
Frank