Alternator Diagnosis Suggestions - MVAgusta.net
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post #1 of 18 (permalink) Old 06-30-2013, 11:41 PM Thread Starter
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Alternator Diagnosis Suggestions

Uuugh,

Went on the worlds shortest ride the other day. Started my F4 and got a few miles from the house only to have the Battery light illuminate. It flickers at idle but goes on solid when you rev the bike. Stuck my multi meter on the battery (Shorai Lithium) and it gives out 13V when the bike is off but drops to 12 once the bike is started and drops voltage as the RPM's go up.

Where would one start diagnosing this? Is a dead alternator assumed right away or are there other things to look at first?

Any suggestions appreciated.
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post #2 of 18 (permalink) Old 07-02-2013, 07:37 AM
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check all the grounds, but I'd say the alternator (or the regulator which is attached to it) is giving up
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post #3 of 18 (permalink) Old 07-02-2013, 08:10 AM
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Or, you could get rid of the lithium and go back to traditional AGM for longevity.

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post #4 of 18 (permalink) Old 07-02-2013, 08:11 AM
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BTW...how many volts on the system when running around 5000 rpm?

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post #5 of 18 (permalink) Old 07-02-2013, 01:29 PM
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Regulator...but also could be just a poor connection....voltage drops in the ground circuit will cause the regulator to back down the charge rate as output amperage rises with rpm.

13 open volts on battery is good. Voltage drops when electrical system is turned on due to system draws (fuel pump, lights etc). Charge system should immediately overcome the draw and voltage should return to near battery open when engine has started, and rise slightly with rpm increase to regulated max.
The voltage should not go down with rpm increase. If voltage initially returned to near battery open when engine started and then drops with rpm increase the regulator is malfunctioning. Ampere test will confirm negative or positive charge rate better than just voltage.

Alternator can be repaired (it is an automotive component after all). See your friendly auto electrical repair shop.

You are dealing with an excited field alternator, so brushes are at play, and the regulator manages output by varying the strength of the field. There is a way to full strength the field to confirm the base alternator (rotor, brushes and stator) is good, There is also a way to test the diode array of the rectifier...thereby eliminating all but the regulator from suspicion....but this is not stuff easily taught in a forum post.

Some older Japanese bikes had a cush drive for their excited field alternators and the drive (bonded rubber coupling) could occasionally shear, so the charge system worked at idle when the rotor would spin properly, but drop with rpm when the coupler began to slip and rotor speed didn't keep up. I don't think MVs have this problem, though, as cush drive is not a bonded rubber affair.

Jason has some great tips on alternator R&R and repair...unfortunately.

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post #6 of 18 (permalink) Old 07-02-2013, 02:43 PM Thread Starter
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Hey. Thanks for all the good suggestions. I'm starting to suspect a bad ground. I have issues with my turn signals too and I recently did a track day and may have vibrated something loose. Lets hope at least cheap fixes are always more fun than ones with more than 3-4 decimal points in them.

Been busy getting ready for the 4th so have not had a chance to pull the panels off and have a look in depths yet. I'll let you guys know what I find.
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post #7 of 18 (permalink) Old 07-02-2013, 03:51 PM
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Wink voltages?

battery 12.63V
key on 12.21V
idle 14.84V
@4,000 14.84V

measured 5 minutes ago with a Fluke 87 lll


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post #8 of 18 (permalink) Old 07-02-2013, 07:48 PM
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Wink red light district?

my vote: bad grounds

start at the battery with an Ohm meter, neg terminal
touch the engine, starter and alternator, i see 0.01 Ohms or more its a bad connection

the ECU is 'seeing' the not charging, thus the red light

if the regulator isn't 'seeing' battery/system voltage correctly, it won't charge
if the field windings aren't getting the correct voltage from the regulator, it won't charge

the manual has a 7 page explanation if you want more detail
there is a good electrical trouble shooting guide in the back of the service manual

if you suspect the alternator (which i doubt)

call Rick's;
http://ricksmotorsportelectrics.com/

the young lady knows the electrical value across the slip rings if the alternator is good.....they have regulators and diode sets

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post #9 of 18 (permalink) Old 07-10-2013, 02:22 PM Thread Starter
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Sorry I never came back to this thread. Interestingly I got an e-mail notification of a new message and now it's gone.

In my case the 40A fuse in the charge circuit blew. Since my amp meter only goes up to 10A I could not measure the current draw so I replaced the fuse and all is well now I ran it for quite some time the other day and the fuse never blew.

In my case the fuse was the original the bike was delivered from the factory with. I did a track day that shook up a lot of the electrical components and I was doing a lot of diagnostics with the key on for a turn signal switch that stopped working (reseating the flasher fixed it instantly). I think when the battery needed to charge a massive amount it was too much for the 11 year old fuse to handle. That's what I hope at least.

Noel. Some really good ideas there. I'm planning on doing some cleaning and other service work soon so I may actually go over your diagnostics just to be sure once I strip all the bodywork off. Finding out that I was wrong being a long distance away would suck.
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post #10 of 18 (permalink) Old 07-10-2013, 02:26 PM Thread Starter
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Some photos in case the original poster returns.

Blown Fuse.



Here's where the fuse lives on the left side of the bike.



Here's the assembly pulled out. The fuses are on the back and under white translucent plastic covers. One is a spare. The other is the actual fuse.

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