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Old 08-31-2018, 10:10 AM   #1
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Threw a fan belt?

I imagine many of us have had a fan belt break on one of our engines, land or sea.


Chugging along last Monday trying ever increasing rpm, now at 2000 rpm, keeping an eagle eye on the starboard's temperature gauge (due to that engine's [previous?] random overheating), all of a sudden the alternator idiot light came on. The ammeter continued to read the amount of charge that it had been. Shortly thereafter the temperature rose. My son diagnosed fan belt and I (thought I'd) shut it down.


He moved the furniture, opened up the hatches, and I shut down the port engine. Hmmm? The starboard engine was still running! It seems I was sloppy shutting it down; I presumably turned the key off before waiting sufficient time for the stop button to stop the engine. Turned the key back on and stopped the engine.


Figured out/realized later that the tach also went to zero without its signal from the alternator. He installed a new fan belt, and found the old one had merely popped off, unbroken. He noted that the crank pulley, the water pump pulley and the alternator pulley are all out of line with each other.


I started the starboard engine and all seems well. Ammeter came up with engine speed as I raised the rpm above idle; held its amps as the rpm came above 1000. So alternator is 'normal'. Tach works. Temp came down.



So! The question of the day is, why did the ammeter continue to read normal amps when the alternator was not turning?


(A few minutes later the port engine quit and required switching from the port tank to starboard, and bleeding, before restarting. That a mystery for another thread.)
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Old 08-31-2018, 04:55 PM   #2
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So! The question of the day is, why did the ammeter continue to read normal amps when the alternator was not turning?
)
Well the short answer is it couldn't. So the next question is what was the meter actually showing? Could the meters be reversed between the two engines? Is it possibly a voltmeter as opposed to an ammeter?

You probably want to see if you can get those pulleys into better alignment too.

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Old 08-31-2018, 06:26 PM   #3
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If your banks are tied together or you had the battery switch to BOTH, the other alternator could have been charging the offside battery bank.
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Old 08-31-2018, 07:23 PM   #4
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What is your current alternator/batteries/amp meter setup? This would be important info to help.

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Old 08-31-2018, 08:07 PM   #5
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What are the engines?
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Old 08-31-2018, 09:36 PM   #6
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My suspicions are being voiced. The engines are a pair of Perkins 6.3544 M s. I've wired the batteries through switches that cut off each engines' start batteries and can be combined to start either engine on either battery or start either engine from both start batteries, and...start either engine from the house bank, and start either engine from all three banks. As it turns out, the main house panel switch is rated for only 150 amps while everything else is rated for 300 amps (or nearly, after being de-rated by temperature or having been run in the engine space). Additionally, each engine's start battery is connected through ACRs to the house bank. The two ACRs disconnect the house bank from each start battery when the house is depleted, maintaining enough charge to start an engine.


So, there's plenty of interconnection possible. However, I set each engine's switch to start its engine from its own start battery. I set the house bank's switch to only use the house bank for the 12v panel. And, however, the ACRs really connect the three banks all the time except when the house bank is depleted.


Now, the two engine's panel ammeters should be independent, relying on each engine's alternator's output. I can't understand how the alternator's idiot light can give the correct (no charge) answer, while the ammeter was giving me contradictory information.


If the ammeter is really a voltage meter then all can be understood; there's plenty of charging voltage to be seen when everything is interconnected.
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Old 08-31-2018, 10:45 PM   #7
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"The two ACRs disconnect the house bank from each start battery when the house is depleted, maintaining enough charge to start an engine."

This doesn't sound right to me. ACRs combine when a charge voltage is present and disconnect when the charge voltage drops. This is on the charge side of the equation, not the loads.

Sounds like you're talking about load shedding which is a function of ACRs that I've not heard of before. Are you sure about your setup?
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Old 09-01-2018, 07:30 AM   #8
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To figure out the ammeter question, I think you need to find the shuts that are measuring the current and see what they are actually wired to. That should help solve that mystery.


As for the belt, it sounds like there was a pretty evident misalignment of the pulleys? That's an almost more important thing to resolve, especially if it's on both engines. The ammeter won't stop the boat, but the belt issue can.
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Old 09-01-2018, 07:52 AM   #9
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He noted that the crank pulley, the water pump pulley and the alternator pulley are all out of line with each other.
Seems like you will keep throw belts until that is corrected. Is this an OEM setup?
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Old 09-01-2018, 11:44 AM   #10
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FlyWright, you're right; that's the whole idea. The batts are connected during charging and disconnected when discharged below a certain voltage. See https://www.bluesea.com/articles/1366 .


Of course, you can certainly screw up and connect/combine the batteries via the battery switches. Starting each engine with its own battery using the switch to disconnect the other banks, and running the house on its bank using its switch to not draw from the engines' batts should make everything happy. I've enclosed the wiring diagram. It's mostly as Blue Seas diagram shows (I must never turn the engine's battery switches off while the engine's running).


With respect to misalignment, yes, that's OEM(!). There's some ability to mess with the alignment of the alternator, but the crankshaft and water pump are original and immutable.
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Old 09-04-2018, 03:34 PM   #11
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Failing bearings can cause belts to be thrown by allowing shaft wobble which produces greater pulley wobble.

Alternator, coolant pump, any idler pulley setup.

A loose pulley on any of the above also could throw a belt.

A failing bearing might allow a shaft to shift from normal position.

Check them all carefully.
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Old 09-06-2018, 04:12 PM   #12
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Why, oh why did I 'know' that the gauges were ammeters? They're not, my newly-met diesel guru says that ammeters have not been included since the days of generators; they're always voltmeters. In my own (weak) defense, I have a Perkins catalogue showing both ammeters and voltmeters as available and matching the style of the gauges in our boat.


Took the boat out today and it performed perfectly for my diesel guru. Ran it for a while at 2200 rpm (well more than we normally do).


He's gonna' replace the thermostats as a prophylactic measure.
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Old 09-06-2018, 04:22 PM   #13
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My Perkins had ammeters when I bought it but they were removed when I rewired the alternators directly to the batts with a Balmar alternator and installed a battery SOC meter with both amps and volts displayed. Using the ammeters as installed by the OEM required a long small wire run from the alternator to the helm and back to the engine with too much electrical loss.

Converting to a high output alternator and shore charger resolved my other charging issues.
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Old 09-06-2018, 04:26 PM   #14
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Well, that's one mystery solved.


Any word on the miss-aligned pulleys?
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