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Discussion Starter #1
This last weekend after a non eventfull motor of about 3hr's to an anchorage for the night, we started to get underway the following morning. When I got the motor running I noticed that I didn't have any voltage output from my alternator
on the control pannel also, at this time the motor's RPM's started to surge up and down all over the place. I thought I was going to have a run away on my hand's but after about 3 min. the alternators pannel showed the correct voltage and the motor ran fine. This is the first time this has ever happened and it took me by surprise to say the least. Could the lack of 12 volts coming from the alternator have caused this? On the way back, 2hrs to our marina everything was alright. Any thought's out there? I know the first thing I need to do is check the connections on the alternator and other places, but it sure was funny (not at the time) how the motor surged and all.

Take Care,

Mike & Glinda
s/v Blue Bayou
 

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Perhaps your alternator brushes are nearing their end... if they made intermittent contact your alternator could load and unload - that would also explain your lack of alternator output/voltage. We had similar symptoms (plus our alternator-driven tach stopped indicating) - had the alternator rebuilt and found one brush down to a nub.
 

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Another possibility is slippage of the drive belt. Your alternator has two outputs - one 12-13 VDC to charge the battery and another which is unrectified AC and drives the tach. The tach does not measure the voltage, but measures the frequency, which is directly proportionate to the speed of the engine. If the belt slips, obviously the alternator slows. Belts do not usually slip smoothly, but slip and grab, slip and grab. Drives the tach nuts! I had a Balmar 105 amp alternator on my Perkins 4-108 which did that for a bit then blew a 130 amp fuse.
 

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Discussion Starter #4
I always try to keep a positive though and use the KISS (keep it simple stupid) method. I plan to go down and check the wiring attachments and also the belt. It just sceems odd that everything ran smooth just the day before. Ahhhh boats don't you love them, especially when there giving you problems 15 miles from your marina and the weather prediction is for 25 to 32 mph winds and rain.

Keep the ideas coming,

Mike & Glinda
 

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If its not the belt or a loose connection then go with a brush hanging in the alt.Eather from dirt or getting to the end of its life.
Problem with a brush hanging is there is not a real"TEST" you can do on it once it unhangs.Besides taking it apart.Thats not real hard to do eather just a pain.Cheak your belt and do the wire wiggle test on your connections.
The easyest way i know to test for a hung brush is the hammer test....I know..Can hear the oh god dont do this crowd allready.
Keep a small hammer at hand.And your engine cover open when starting.IF the problem happens again tap the alt housing on a MEATY part of the alt casting a few times.If you have good ears you will be able to hear when the brush makes proper contace.Otherwise have the mate watch the gauges as you tap.Has worked for me more than once.
My Dumbass Solution:laugher
Mark
 

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intermittent short circuit?

Any of the suggestions here doesn’t take into account the fact that « the motor's RPM's started to surge up and down all over the place.”

Neither an "alternator brushes" nor a "slippage of the drive belt" would produce a surge up and down of the engine RPM ?

Although it will be good to check the belt and the brushes, I will suggest one other possibility such as an intermittent short circuit.

During an intermittent short circuit. the voltage will drop and the charge on the alternator will cause it to slow down the engine, when the short circuit resume, the motor surge up !..
 

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If the RPM increase/decrease was an actual change in idle speed (not just the tachometer); it could be a fuel supply problem. But that would not explain the zero output from the alternator.

Another possibility is a frozen alternator due to bad bearings. That would explain a surging engine due to the load on the alternator belt and an alternator with zero output. Check to see that the belt is not slipping and the alternator spins freely when the engine is running or if this condition returns.
 

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So when you left on you weekend the boat was docked and hooked to shore power had you batteries topped off.

After a night on the hook on the batteries when you started the motor you had no output from your alternator and the motor surged.

Did your volt meter surge also ?

Voltage low from overnight use. You started the motor and the altenator tried to put out 40 amps? but the belt is old and said no way, I'll give you 2 and sliped unloading the motor and allowing it to rev.

After awhile the belt said ok I am warm now from slipping and can handle a little more and the altenator agreed because even while slipping the belt still brought the voltage up enuff so that the altenator only need to put out 20 amps.

All was well and you went on your merry way.

New belts need a break in time and then an adjustment after break in. After that if they need another adjustment they need to be replaced if they slip.
 

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The revs would change a wee bit if the alternator was loading and unloading, but I don't know how much "all over the place" really means. If the load comes off the alternator, the motor revs will vary a wee bit untill the governor regains control of the revs. It's normally prompt.
An easy way to check would be to switch the ignition on and off a few times with the engine running (or pull the field wire off the alternator and make/break the connection there) to see if it gives the same symptoms.
 

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Hey guys, a 4-108 can drive any reasonable alternator from no to full load with barely any effort. The question is ?Is the surge a tach only indication or did the engine revs really change? If so, by how much?
 

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I was presupposing that the engine was at an idle in neutral so a change of 80 rpm might well be 10% or more.

Hey guys, a 4-108 can drive any reasonable alternator from no to full load with barely any effort. The question is ?Is the surge a tach only indication or did the engine revs really change? If so, by how much?
 

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Discussion Starter #12
As soon as I noticed I had good water flowing my next spot to check is the oil pressure guage, it takes about 20 sec to regester a pressure reading, but after about 30 sec I got worried and thats when I went below to check things out. The engine was at idle in neutral and when I saw that the alternator volt meter showed zero output the engine started to surge faster than idle and then it started to go surge slower than idle. All & all I've got to get down to the boat and trouble shoot all of the great ideas everyone has given.

Thanks,

Mike & Glinda
 

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If the engine is not "racing", but alternately speeding and slowing slightly (+/- 100 rpm or less, say) it could well be the alternator loading/unloading, esp if it's a fairly powerful one.

Good luck.
 

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Take a new belt with you to replace the spare belt you are going to replace and throw the old one away.
 
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