How Do I Test a DC Volt Meter?

[Andrew Shadow]

nothing wrong with 14.5. check for a/c voltage

From what I understand a consistent 14.5 volts is high for a stabilized voltage for a vehicle having a charging system controlled by the ECM. The old standards of when the alternator was controlled solely by the voltage regulator and returned consistent readings within a very narrow window no longer apply.

Regardless, I don't know what voltage I have, which is part of the problem. I am reading anywhere from 13.0 VDC all the way to 15.15 VDC depending on when I check it and what meter I use. I first need to know which meter reading is correct before I can begin to figure out if the reading is good or not.

 

[Andrew Shadow]

The best way to make 3 voltmeters to agree is attach all three to one power source and with the back of 2 removed turn the potentiometers so they 3 all agree.

I am not interested in having them agree. I am interested in knowing which one is correct, or at least the closest to correct.

Without a known and accurate power source, how do I know which two meters need to be adjusted and which one is the control meter? This can just as easily result in three meters having the incorrect result. This is why I asked if there is a common household DC voltage source that is known to be fairly and cosistently accurate to compare against.

 

[spiderman302]

USB chargers are not consistent and maybe between 5.0 and 5.2 volts.
What you need is a simple precision voltage reference.

Post 15, with the TL431 is a very good low cost choice.
It is a precision voltage reference that will be accurate over a wide temperature range.
It can be set for 2.500, 5.000, 10.000 volts or any custom voltage.

or you could just get a LM340T-12 (LM7812) which is a 12.0 volt regulator that you could attach to your bike battery.
as long as your battery is 13 or more volts it will work.

 

[Obo]

Trying to figure out a possible vehicle charging issue. I don't even know if there is a charging issue. The first step was to check voltage. After getting what seemed to me like strange readings I can't tell if they are normal due to what I described in post #12 above or due to my meters being inaccurate.

If it's a newer vehicle that might explain your issues. Many have these new smart alternators that vary the output based on the load and the battery. The premise is the less the alternator is loaded and outputting, the less resistance on the drive belt and thus a better fuel economy. It's the same way they use pulse width modulation on electronics (bulbs etc) turning the LED's on/off vs varying the voltage thru resistance.

What is a smart alternator and have I got one?

What is a smart alternator and do I have one? This article is an updated version of our hugely popular 4 step simple smart alternator test. I hope you find it useful. Smart alternators are something that’s coming up more and more frequen...

rayneautomotivehelp.freshdesk.com

 

[Obo]

From what I understand a consistent 14.5 volts is high for a stabilized voltage for a vehicle having a charging system controlled by the ECM. The old standards of when the alternator was controlled solely by the voltage regulator and returned consistent readings within a very narrow window no longer apply.

Regardless, I don't know what voltage I have, which is part of the problem. I am reading anywhere from 13.0 VDC all the way to 15.15 VDC depending on when I check it and what meter I use. I first need to know which meter reading is correct before I can begin to figure out if the reading is good or not.

From my post above:

"So how do I know if I have a smart altenator and what’s the solution?
The test, mercifully, is much simpler than the science behind the smart alternators’ use. You will need a multimeter capable of reading 12V DC.

1. Locate your main vehicle battery
2. Start the engine, ensuring any fans, radio, lights etc. are turned off.
3. Take a voltage reading across the main vehicle battery
4. Leave running for around 5/10 minutes, then repeat step 3

If your readings are around 14.4V DC then you most likely have the traditional, non-smart alternator.
If your reading is closer to 12.5-13.5V then it’s likely that you have a smart alternator."

 

[Nashcat]

If you had a bridge rectifier that converts AC to DC that would give you an accurate DC voltage, but it would be at 3 digits (120v DC)

Actually the voltage would be 170 VDC coming from the rectifier. The DC voltage output is 1.41 times the AC input voltage.

The problem with having 3 meters is that 2 of them are usually wrong. Deciding which one is right, is the problem. I have 3 Fluke meters and a Snap-On. They all read exactly the same…..when they’re turned off. That’s the only time they agree. I almost forgot that I have a Simpson 260 meter, that has a calibration sticker on it. Calibration was required, back in the day, when I still worked.

John

 

[Obo]

Actually the voltage would be 170 VDC coming from the rectifier. The DC voltage output is 1.41 times the AC input voltage.

Thanks for that. I learned something new today

 

[Nashcat]

I believe that desktop computers have 12V and +/- 5V outputs that are pretty much dead on. There’s usually an extra pigtail from the power supply, meant for additional drives, that would be available to check.

John

Edit: There’s also a 3.3 VDC from the supply. If the power supply was built to spec, the accuracy is +/- 5%.

 

[Uncle Phil]

Actually the voltage would be 170 VDC coming from the rectifier. The DC voltage output is 1.41 times the AC input voltage.

The problem with having 3 meters is that 2 of them are usually wrong. Deciding which one is right, is the problem. I have 3 Fluke meters and a Snap-On. They all read exactly the same…..when they’re turned off. That’s the only time they agree. I almost forgot that I have a Simpson 260 meter, that has a calibration sticker on it. Calibration was required, back in the day, when I still worked.

John

It's kinda of like the feller with three watches - he never knows what time it is ...

 

[Al st1100]

From my post above:

"So how do I know if I have a smart altenator and what’s the solution?
The test, mercifully, is much simpler than the science behind the smart alternators’ use. You will need a multimeter capable of reading 12V DC.

1. Locate your main vehicle battery
2. Start the engine, ensuring any fans, radio, lights etc. are turned off.
3. Take a voltage reading across the main vehicle battery
4. Leave running for around 5/10 minutes, then repeat step 3

If your readings are around 14.4V DC then you most likely have the traditional, non-smart alternator.
If your reading is closer to 12.5-13.5V then it’s likely that you have a smart alternator."

I would not chase any charging problems if voltace was between 12.5 and 14.7 unless I had a charging system DTC. Some Jaguars had a charging voltage chart and on start up 15 volts for 45 seconds before it goes down and that one not at specs would throw a code.

 

[Larry Fine]

If you want to check for charging voltage, the voltage increase between not running and running is possibly more important to know than the absolute voltage, so any meter will display that accurately.

 

[drrod]

Some Jaguars had a charging voltage chart an

Do you think it is a good idea to use Jaguar electrics to solve an electrical issue?
(I know, I know, old, but deserved, reputations take a long time to die!!

 

[SMSW]

@Andrew Shadow, you stated a fallacy in your first post. Line voltage, provided by the utility, varies depending on location and demand of other users. We speak of nominal 120, 240 volts, but what is supplied can vary from around 110v up to 130. It will probably be fairly constant over a short period of time, but again, demand by other users screws that up. Back in the 70's (maybe 60's, too) Pratt and Whitney had a huge wind tunnel in Hartford, Conn. When they fired this thing up, the lights all over West Hartford dimmed. Your own voltage might dip when a large electric appliance starts.

@Uncle Phil, old Chinese proverb...man with two clocks......

 

[Andrew Shadow]

you stated a fallacy in your first post. Line voltage, provided by the utility, varies depending on location and demand of other users. We speak of nominal 120, 240 volts, but what is supplied can vary from around 110v up to 130.

I have checked the line voltage many times over the years and I have never seen it vary more than a few tenths of a volt. I am not disputing what you wrote because I have no idea what tolerance the utility operates within, I'm just saying that I have never seen any significant variance. I don't know if it makes a difference but I am on a grid that is overwhelmingly residential.

 

[Andrew Shadow]

If you're really worried grab a cheap 10v reference board off of evil bay like this TL431 board.

I didn't take the time to look at that yesterday. Interesting, I had no idea such a thing could be had for so cheaply.

 

[Andrew Shadow]

If you want to check for charging voltage, the voltage increase between not running and running is possibly more important to know than the absolute voltage, so any meter will display that accurately.

I thought that as well until I started checking the voltage and the readings made no sense to me. They made no sense because I am not familiar with these computer controlled charging systems. Those types of tests that used to be the norm to diagnose a charging system on older stand-alone alternators no longer work with the newer alternators that are integrated in to the vehicle's computer system and are computer controlled. With alternators that are controlled by the BCM and the ECM the difference between those two values can be close to nothing. Depending on when in the discharge/recharge/maintenance cycle that it is checked you could get a reading of battery voltage. That does not mean that the alternator is not working, it might only mean that nothing more is required at that instant. The negative battery cable has an inductive ammeter built in to it that monitors current demand and flow the same as the clamp meter that you probably use in your business. These systems are on-board smart chargers but probably even smarter. They vary the charge constantly and maintain the battery in a very similar manner to what smart chargers do, only providing the the battery with what is needed and when it is needed to maintain the battery at optimal levels depending on demand, state of charge, even battery temperature. Much more complicated than it used to be, which is a double edged sword.

 

[Andrew Shadow]

I would not chase any charging problems if voltace was between 12.5 and 14.7 unless I had a charging system DTC. Some Jaguars had a charging voltage chart and on start up 15 volts for 45 seconds before it goes down and that one not at specs would throw a code.

I was wondering the same thing Al. From things that you written it seems clear to me that you a much higher familiarity with newer vehicles than I do, so you may well be right and there is no charging system problem. The charging system idiot light does not illuminate. I put a scanner on it and there are no trouble codes current or historical. While I had the scanner on I checked the ECM voltage. It was a different value again from my three meters.

 

[Al st1100]

I was wondering the same thing Al. From things that you written it seems clear to me that you a much higher familiarity with newer vehicles than I do, so you may well be right and there is no charging system problem. The charging system idiot light does not illuminate. I put a scanner on it and there are no trouble codes current or historical. While I had the scanner on I checked the ECM voltage. It was a different value again from my three meters.

Some (quite a few) cars 2015 about and newer have 2 batteries. And some do a real good job of hiding the second one.

 

[DannoXYZ]

Fully charged, the bike battery should be 12.6V. I think this chart is accurate for a sealed lead acid battery......

That's "resting" voltage of battery which can take anywhere from 24-hrs to 2-weeks, depending upon health of battery. Right after you stop bike, it can be 13-13,8v, depending upon battery and charging system condition. So unpredictable of what actual voltage would be at any given time after stopping bike.

 

[DannoXYZ]

If you had a bridge rectifier that converts AC to DC that would give you an accurate DC voltage, but it would be at 3 digits (120v DC)

You could try and measure the output from a USB charger device. They are usually quite accurate at 5V DC output.

You must better hydro power than us. Power lines vary from 117-125 VAC here. And rectifiers drop anywhere from 0,7-2,5v going across diode.

That was one of my thoughts as I have a couple hanging around like most people do, I just didn't know how close to 5 volts they usually are.

They vary greatly as well depending upon actual load. Depending upon model, you may see no-load voltage in +5,5v range. Connect phone or big pad and it can drop to 4,5v.