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Old 18-08-2013, 10:49   #1
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Can an Alternator Cause This?

First, it's an intermittent problem - and it goes like this:

Bosch 12v 55A alternator with 'W' output to drive engine rpm tacho.

Works OK when engine first started but these symptoms can occur as soon as half an hour after starting.

The charge voltage (measured with a Fluke digital meter) initially at the batteries is 13.8v or 14.2v which I imagine depends on the state of charge of the batts at first.

THEN the following symptoms occur very briefly, over maybe 2-3 seconds - the engine note changes as the rpm drops slightly; the rpm tacho drops to zero or almost zero.

THEN normal operation resumes but the charge voltage (again measured as above) is down to 13.5 0r 13.6v. If I load it with nav lights, deck lights etc to take around 20A it drops still further, maybe to 13.4v. I can't load it much more without running the anchor windlass.

Once these symptoms have occurred the charge voltage stays low and drag-downable but the rpm indicator is good and the engine revs are normal.... then it'll happen again - we notice an engine note change and the tacho flutters down but recovers almost right away.

The fanbelt is correctly tensioned and not slipping. The high current leads are good and secure all round. The alt has been tested locally by a Bosch dealer on his test bench- he showed me that it was giving 14.2v but I don't know if it was loaded and it was a short 30 sec test.

What could be happening here? I can't get a replacement alt locally (Spain) for under 250 euros (shiiped in).

Suggestions for further testing or possible causes are welcome! Or a cheaper diesel car type alt to fit?
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Old 18-08-2013, 10:59   #2
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Re: Can an alternator cause this?

Sounds like you have a smart regulator on your engine, like the Balmar unit?

My WAG, it could be the alternator temporarily puts a high drag on the engine which causes the drop in rpms. Then the high load stops as the alternator is no longer generating high output so the rpms go back to normal.

Have you checked the regulator? Can you monitor the system to see if there is a voltage spike when the rpms drop?
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Old 18-08-2013, 11:11   #3
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Re: Can an alternator cause this?

No smart reg, just the bare alternator...
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Old 18-08-2013, 11:33   #4
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Re: Can an Alternator Cause This?

That is a bit odd. I may be wrong but thought a standard, non-smart, internally regulated alternator put out a constant voltage.

That you are seeing a wide range of voltages sounds odd to me. If the batteries are way down I guess it could draw down the alternator voltage to some degree but once they are pretty well charged I would not expect to see that.

Are you measuring at the batteries? What if you measure the voltage right at the alternator? Does the output run through a switch? Are you sure there is no corrosion or bad connections between the alternator and the batteries?
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Old 18-08-2013, 11:55   #5
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Re: Can an Alternator Cause This?

Sounds like a two stage regulator. Hits absorption then drops to float. You wouldn't want to be motoring all day at 14.2.
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Old 18-08-2013, 12:13   #6
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Re: Can an Alternator Cause This?

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Originally Posted by familycruisers View Post
Sounds like a two stage regulator. Hits absorption then drops to float. You wouldn't want to be motoring all day at 14.2.
Actually 14.2 volts is pretty conservative, even for all day motoring. The risk of gassing and water loss is fairly small until you get over 14.4 V. A very interesting read on the subject here.

Musings Regarding External Regulation - SailboatOwners.com
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Old 18-08-2013, 12:29   #7
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Re: Can an Alternator Cause This?

Yes, my thoughts too.... the measurements were at the alt B+ terminal with a Fluke DMM, so right at the source...
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Old 18-08-2013, 12:33   #8
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Re: Can an Alternator Cause This?

Another possibility is the diode pack in the alternator. You could also have a voltage dependent short in your battery. To eliminate this second possibility, just substitute a different battery and test again. To test the diode pack I think you will need to open the alternator to remove it. Alternatively, just remove the alternator and take it to an auto parts store that will test it for free. Most of the major ones will do this.
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Old 18-08-2013, 12:56   #9
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Re: Can an Alternator Cause This?

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Yes, my thoughts too.... the measurements were at the alt B+ terminal with a Fluke DMM, so right at the source...
Well I see your problem right there. Of course you will get bad readings using a cheap piece of junk like a Fluke.

Only kidding of course but that does eliminate some of the possible sources of the problem. I would start focusing on an intermittent problem with the alternator/regulator. The fact that it seems to come on after half and hour or so may mean it's something heat related. That's about enough time for the alternator to heat up a bit.

It was mentioned that there might be a two stage regulator in your alternator. Not something I'm familiar with but that's not saying much. I mainly know about the multi-stage, external smart regulators. Do you have any details on how yours is regulated, alternator model or anything?
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Old 18-08-2013, 13:13   #10
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Re: Can an Alternator Cause This?

Do you have a solinoid that directs the charge between the two battery banks?

I find they dont last all that long. Maybe a year or two.


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Old 18-08-2013, 13:33   #11
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Re: Can an Alternator Cause This?

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That is a bit odd. I may be wrong but thought a standard, non-smart, internally regulated alternator put out a constant voltage.
they try to put out a constant voltage. however it's directly related to the state of the batteries. the lower the batterys, the lower the charge voltage, especially with small alts. they are simply maxed out and overloaded and can't get to 14.4 with depleted batteries. chargers work the same way.



have you tried turning off the engine, and restarting. too see if the voltage comes back normal? this may reset the interal reg, if that is the problem.

it could be a heat issue. cold alt works fine, then something happens when it heats up.
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Old 18-08-2013, 13:56   #12
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Re: Can an Alternator Cause This?

"9- FLOAT - Float voltage is a further reduced voltage LIMIT applied by a smart regulator. Dumb regulators do not do float. If you feel you need it, motor a LOT more than you actually sail, you'll also need a smart regulator. "

Thats a quote from the write up you posted. 14.2 will gas your batts if you're motoring down the icw for ten hours straight. 13.6 is average float I see. I use 13.7 from my solar controller.
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Old 18-08-2013, 14:21   #13
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Re: Can an Alternator Cause This?

I guess is not a Yanmar?
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Old 18-08-2013, 14:41   #14
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Re: Can an Alternator Cause This?

"I may be wrong but thought a standard, non-smart, internally regulated alternator put out a constant voltage. "
An old Delcotron would. An old Denso or Hitachi wouldn't, some of they vary all over the place. With hundreds of designs out there...you'd need to know what was normal for any one particular alternator.

It is probably worth the time to try asking Bosch directly. I have no idea if they will talk to consumers. Some companies do (like Delco and AC Delco and Leece-Neville) some won't. Like Denso and Hitachi.
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Old 18-08-2013, 14:48   #15
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Re: Can an Alternator Cause This?

Quote:
Originally Posted by familycruisers View Post
"9- FLOAT - Float voltage is a further reduced voltage LIMIT applied by a smart regulator. Dumb regulators do not do float. If you feel you need it, motor a LOT more than you actually sail, you'll also need a smart regulator. "

Thats a quote from the write up you posted. 14.2 will gas your batts if you're motoring down the icw for ten hours straight. 13.6 is average float I see. I use 13.7 from my solar controller.
This is the information I see from the article that seem to be very pertinent to charging voltages.

Regarding charging at higher voltages.

If you have an old 13.6V - 13.8V regulator, and wet cells, ditch it and get a replacement that is 14.2V - 14.4V or even 14.6V and you will charge a LOT faster. Battery gassing generally begins above 14.4 volts so a dumb reg that does 14.5V - 14.6V will cause your batteries to need water added more often. 14.4V would be better maintenance wise but 14.6V will charge even faster.

And,

A truly dumb regulator has but one voltage set point and that is it. If this set point is 14.4V you are going to do just fine.

And this regarding charging at lower voltages.

At 13.8V a battery can often plateau and hit absorption at about 65-70%+/- SOC, depending upon age and condition of the battery, and will charge quite slowly beyond the absorption voltage if it is held low.

Voltage in car charging systems is typically 13.8-14.2 volts, some as high as 14.7V. Plus car batteries are seldom deeply discharged so they are getting hit with this voltage at a high state of charge. So far I have never killed a car battery from overcharging at 14+ volts.
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