A question for you multimeter gurus out there....


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bohdan
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A question for you multimeter gurus out there....

Postby bohdan » Sun Mar 13, 2016 6:30 pm



Okay, trying to check my alternator output with my multimeter. The book says to remove the main fuse and the amps and volts. The volts are okay, albeit 12.13, but my amps were jumping way out there. 9 to 3 range. Where I am looking for about 2. My backup did the same. I do t use the harbor freight ones, but Innova from auto zone. Any ideas? Mighty obliged.....



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Re: A question for you multimeter gurus out there....

Postby ct1500 » Sun Mar 13, 2016 7:30 pm

A DMM by itself without other devices is incapable of measuring alternator amperage output as the best will top out at 15A before blowing meter fuses or cooking it. Voltage readings alone will tell you if the charging system is working or not and if all you get is 12.13V while running it is not.
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Re: A question for you multimeter gurus out there....

Postby bohdan » Sun Mar 13, 2016 8:32 pm

Thanks ct1500. Yeah the max is 10 amp on these dmm.s. If the voltage alone would give me the adequacy of the charging system. , I am game. I put the charger on the battery. I u destined that the voltage should be 12.8 ish without running and in the 14's while it is if I don't get these readings I guess a new alternator. Mighty obliged

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Re: A question for you multimeter gurus out there....

Postby WingAdmin » Mon Mar 14, 2016 10:31 am

You can read amperage coming from an alternator, but you need a shunt or a shunt meter to do it. This contains a high-power resistor that most of the current shunts through. The remainder of the current goes through the meter, which reads this small amount of current. For a current shunt, you then apply a multiple to the value on the meter to get the true amount, or for a shunt meter, it does this math for you and shows the actual current on its screen.

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Re: A question for you multimeter gurus out there....

Postby bohdan » Mon Mar 14, 2016 8:29 pm

Thanks WA . Given that it appears that it is a bit more complicated with a shunt meter, would reading only the volts be adequate for alternator adequacy like you ct1500 suggested? If I do t get 13.5, is it toast? Mighty obliged......

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Re: A question for you multimeter gurus out there....

Postby Bluewaterhooker0 » Mon Mar 14, 2016 11:38 pm

A lot depends on what alternator you have in the bike. I'll assume you have the stock OEM alternator in there. I've upgraded mine, and get 13.8 -14.0 V at all times (rpms). With the stock alternator you won't get those type readings. It's been a while since I changed mine, so I'm at a loss as to the exact voltage readings I used to get. But, they will vary by a lot, depending on the rpms of the engine. At idle, you will probably be somewhere near the battery voltage, say 12.9-13.1 V. At around 1500 - 2000 rpms you should be significantly higher, maybe 13.6-13.9 V. The stock alternators on these things were a bit light on capacity at 40A. I could tell, because my headlights would dim or get brighter depending on rpms, when I revved the engine, they would brighten up. My old alternator would keep the battery charged, but only because I usually made some fairly long runs after it was started. It died with a bad rotor in January of 2015, with about 50K miles on it. I put in an 85A replacement, and have been loving it ever since. When I used to put the battery (Yuasa AGM) on a tender, it would run for hours to get a peak charge. With the new alternator, it peaks out in about 1/2 hour, indicating the battery is much better charged by the new alternator.

I assume there is a reason you are asking this question. What causes you to be testing your alternator ?

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Re: A question for you multimeter gurus out there....

Postby bohdan » Tue Mar 15, 2016 7:46 am

HEY Bluewater Hooker, Yeah I have the original OEM alternator on my bike. It has been about 11-12 years since the brushes were replaced. So now, I have a battery that reads about 12.41 volts right after a full charge. However, if I take a voltage reading at starting I get 10.6ish. Which makes me wonder if it is the battery and not the alternator. Even though, Auto Zone said the battery was good. So maybe now its the battery and not the alternator as originally suspected. I remember reading the ignition needs about 10.6 to fire? I also have added that I did get a backfire during this time. Thinking atleast that maybe atleast I am getting fuel? Thanks for your info.

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Re: A question for you multimeter gurus out there....

Postby Bluewaterhooker0 » Tue Mar 15, 2016 11:47 pm

Well, your battery is probably weak, but it's not toast. I believe you should still be getting readings similar to the ones described above from the alternator. I gather you are looking at the alternator because of hard starting issues, since that is what you are talking about. If you don't already have an AGM battery, I would suggest replacing yours with one. They are very reliable, and have the rear end to start these engines. I was fortunate to have bought the bike with an almost new AGM installed. So, I have no reference point on a lead-acid non-AGM. But from everything I've read, the liquid lead-acid batteries just don't cut it with these bikes. Mine is coming up on 4 years old, and starts virtually instantaneously when cold with a good 'choke' (enrichment), or when warmed up and stopped for a while. But, if you are buying a new battery, make sure your alternator is working properly to take care of that new battery.

BTW.
"So now, I have a battery that reads about 12.41 volts right after a full charge."
If that is the reading right after charging, maybe your battery is toast. You're supposed to let the battery settle down after charging, I think like several hours, before checking its voltage. It is always peaked out right after charging, and drops to its true charge state after it has sat a while. That will give you a more accurate reading of its condition. If 12.4 right after charging, you may find it's in the 11V range after it sits. And that's no good.

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Re: A question for you multimeter gurus out there....

Postby bohdan » Wed Mar 16, 2016 4:25 pm

Thanks Blue water. yeah, I guess I will take a look at the charge again. Thanks

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Re: A question for you multimeter gurus out there....

Postby hugger-4641 » Sat Mar 26, 2016 6:40 am

Ditto what has been said. I have a fairly new AGM battery and fully charged is about 13.2 volts, during starting will drop to 11.5. At idle, I'm seeing about 12.5 to 12.8 until all the electrical stuff starts draining the battery. As soon as voltage drains down to about 12.3 my regulator kicks in and alternator starts putting out about 13 volts. If I bring the rpms up to 3k, the voltage will come up to about 14 and then the regulator starts dropping back off. If you have a bad regulator, sometimes they will still charge a little at idle, but will usually shut off before you get to 14 volts and/or 3000rpm and voltage will drop below 12v if you keep the rpms up.

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Re: A question for you multimeter gurus out there....

Postby hugger-4641 » Tue Mar 29, 2016 2:20 am

After reading another post, I checked my little cheap-o travel meter against my Fluke 87V and it is way off. My Fluke says I am getting 13.8v at idle and 15.2 at 3000rpm until the regulator starts backing off and levels out at 13.5v. This is with 90amp alternator and all my extra lights and radio running.

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Re: A question for you multimeter gurus out there....

Postby WingAdmin » Fri Apr 08, 2016 12:05 pm

hugger-4641 wrote:After reading another post, I checked my little cheap-o travel meter against my Fluke 87V and it is way off. My Fluke says I am getting 13.8v at idle and 15.2 at 3000rpm until the regulator starts backing off and levels out at 13.5v. This is with 90amp alternator and all my extra lights and radio running.


The cheapo meters are not good at measuring voltage with ripple in it. Put one of the Harbor Freight meters on a 13.8 VDC bus that has a little bit of ripple, and they tend to read 1-2 (sometimes more) volts high.

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Re: A question for you multimeter gurus out there....

Postby hugger-4641 » Sun Apr 10, 2016 2:15 pm

Yep, you are right. I knew it probably wasn't as precise as my 87V, especially on a rectified wave, but it was off a little more than I expected. It's one of those little Greenlee pocket sized meters, works good enough for roadside emergencies and takes up a lot less room in the saddle bag than my Fluke. I guess if I wanted to split hairs I could hook up my Fluke 123 scope meter and see what it says, lol, but I sure wouldn't want to pack that around with me.
Since we're on the subject of meters, I should share that I did buy one of those little digital volt meters and mounted it in front of my ignition switch in a home made waterproof box. I noticed something curious about it. It is the three wire kind, and I was in a hurry when I hooked it up, so I just connected the power wire and the signal wire together with a circuit I already had available under the left storage pocket. It worked ok that way, good enough that I could tell if my alternator was working or not, but I knew it was reading a little lower than what my Fluke said. Later on when I added some more LED lights, I took time to rewire the voltmeter properly and attached the signal wire directly to the battery. Now it reads dead on with my Fluke.




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