which way to hook up voltmeter

[dacicci]

i have read many of the threads on here about which is better volt or amp meter. i know their are more opions on that than -holes. my question is i bought a voltmeter, on the directions to hook it up it said to run the wire from the alt that charges the bat to voltmeter then to bat. it also said not to hook it to ground. i read on internet that you can just run a wire from bat to + post on voltmeter, than another on- post to ground. i would like to do it this way but will it cause problems. will it always be putting a draw on my batt. or will it cause meltdown. just for gen onfo i am running a gm 94 amp one wire alt. any help would be great.

 

[Yoda]

I have mine from ign to ground. Goes on/off with key. Also have the 94 Amp GM alt.

 

[dacicci]

sweet that was suggested to me by one of my buddies. always looking for another opion thow. when were talking about wire fires. i never trust myself when it comes to wiring when i am not sure i always ask. thanks for the info

 

[cheap 40]

Alternator to gauge to battery would be for an ampmeter, no?

My voltmeter is connected to + on one side, - on the other.

Tony

 

[theo]

A voltmeter IS an ammeter in the sense that they both measure current. The difference is the voltmeter uses a resistor to shunt a tiny fraction of the current through its coil and lets the majority of the current flow unimpeded. An ammeter runs all of the current through its coil. So I would follow the manufacturers advice and run the Bat wire from the Alternator to the gauge then to the battery.

ps - Electronic voltmeters uses a different principle.

 

[Rice]

i have read many of the threads on here about which is better volt or amp meter. i know their are more opions on that than -holes. my question is i bought a voltmeter, on the directions to hook it up it said to run the wire from the alt that charges the bat to voltmeter then to bat. it also said not to hook it to ground. i read on internet that you can just run a wire from bat to + post on voltmeter, than another on- post to ground. i would like to do it this way but will it cause problems. will it always be putting a draw on my batt. or will it cause meltdown. just for gen onfo i am running a gm 94 amp one wire alt. any help would be great.

Described above is the proper way to hook an ammeter up, NOT a voltmeter.

Hook +12 to ignition, (-) to ground and that's it IF IT IS A VOLTMETER.

See directions from AutoMeter web site: http://www.autometer.com/download_instruction/247e.pdf

 

[MrMoMo]

"A voltmeter IS an ammeter in the sense that they both measure current." Uh, NO theo you got that wrong.

A volt meter Is NOT the same as an ammeter. A volt meter has to be connected across a potential difference (that's what voltage is) A volt meter hooked from a + to a + would read 0volts because there is no potential difference between +12V and +12V therefore a reading of 0V

Kind of a neat trick, you can use your handheld volt meter to measure a bad switch using this theory. If you put the + lead on the hot side of the switch,and the - lead on the cold sideof the switch with the switch on and power running though it you should read 0 volts. If not, there is a voltage drop across your switch- which could also be measured in resistance. (you can do the same thing to find out ifa wire is too skinny. measure voltage (when on) from one end to the other -should read 0 as there should be 12V at each end. If you have a voltage drop across your wire, you have say, 12V at one end but 11.6V at the other end, your volt meter would read 0.4V. You can the trouble shoot a bad wire or bad connection.

A bit off topic, but may help explain whya VOLT meter HAS to go to ground. (can I suggest perhaps that the instructions that you were reading were for a few different models of meter from the same manufacturer?) I know Blue Sea does that in their instructions.

 

[Ethan]

MrMoMo, Theo is right in that many Analog meters can be used for voltage or amp measurements however the display would need to be adjusted. Don't bash him to hard untill you know for sure.

 

[dacicci]

"A voltmeter IS an ammeter in the sense that they both measure current." Uh, NO theo you got that wrong.

A volt meter Is NOT the same as an ammeter. A volt meter has to be connected across a potential difference (that's what voltage is) A volt meter hooked from a + to a + would read 0volts because there is no potential difference between +12V and +12V therefore a reading of 0V

Kind of a neat trick, you can use your handheld volt meter to measure a bad switch using this theory. If you put the + lead on the hot side of the switch,and the - lead on the cold sideof the switch with the switch on and power running though it you should read 0 volts. If not, there is a voltage drop across your switch- which could also be measured in resistance. (you can do the same thing to find out ifa wire is too skinny. measure voltage (when on) from one end to the other -should read 0 as there should be 12V at each end. If you have a voltage drop across your wire, you have say, 12V at one end but 11.6V at the other end, your volt meter would read 0.4V. You can the trouble shoot a bad wire or bad connection.

A bit off topic, but may help explain whya VOLT meter HAS to go to ground. (can I suggest perhaps that the instructions that you were reading were for a few different models of meter from the same manufacturer?) I know Blue Sea does that in their instructions.

the directions are for differnt models the gauge i have is a voltmeter. i knew it snould not be hooked + to + that is how you hook up amp meter. i hooked my volt meter to ing + then to ground and it is working fine thanks for all the info i just wanted to make sure i was not mistaken.

 

[coiled40gary]

Assuming your wire harness is perfect, the + output of the alternator, the + side of the battery (assuming the fusible link is not bad), the ignition (assuming you stay on the correct side of the ballast resistor), the power buss in the various fuss blocks etc. are all the same point for the purposes of monitoring your voltage. Realizing that wire harnesses are not perfect, the differences between the various points will be the voltage drop in the wires, which you will have in the wire going to the meter as well. There may some advantage to simply connecting to an under-dash fuse block, because that will tell you if certain parts of your wire harness are having an issue. So, unless you are easily alarmed by a few tenths difference in your voltage reading, just connect it to the most convenient point, or to the point you are most concerned with. You can use the meter to probe various potential connection points to see if it will work before perminently mounting. I suspect what you really need in your volt meter is not the most accurate reading, but a repeatable reading and the ability to notice gross changes that indicate something just stopped working, or that something is loading the system down too hard. In the LC, the fuse block in the cab below the dash on the side wall to the left has switched and unswitched 12V connection points on the back side of it, connect there.

If the volt meter is working properly, it should have infinite impedance, and should not smoke anything or drain your battery.

If you are hooking up an ammeter, you will have to break a wire and put the meter in the current flow (unlike a volt meter). Unless you have a way of calibrating the ratio between the current flowing through the meter, and the current flowing the curcuit, and that ratio never changes, you can not just connect an ammeter to the + side of the alternator and ground, and expect it to work properly, like you can with a volt meter.

Hope that helps,
gary

 

[theo]

"A voltmeter IS an ammeter in the sense that they both measure current." Uh, NO theo you got that wrong.

A volt meter Is NOT the same as an ammeter.

1) I admittedly stick my head up my ass at times. That's where it was when I suggested following the manufacturer's advice. My bad.

2) I'm talking about electromechanical gauges here. I qualified my statement by saying they are the same in that they both measure current. This is true whether you believe it or not. They both send current through a coil surrounded by a magnet. The strength of the field generated from the coil is directly proportional to current, so the deflection of the needle is predictable. The main difference in the 2 gauges (not the only difference) is in the amount of current that flows through them. The ammeter goes "inline" and measures all of the current. The voltmeter admits a fraction of the current through a resistor and sends it to ground. So hooking it up to an unswitched lead will put a few millivolts of constant draw on the battery.

3) Once again if #1 wasn't clear enough. Every so often I stick my head up my ass (more often than that if you ask my wife).

 

[Rice]

1) I admittedly stick my head up my ass at times. That's where it was when I suggested following the manufacturer's advice. My bad.

2) So hooking it up to an unswitched lead will put a few millivolts of constant draw on the battery.

3) Once again if #1 wasn't clear enough. Every so often I stick my head up my ass (more often than that if you ask my wife).

Millivolts or milliamps?

(Sorry Theo, I'm too bored over here)

 

[theo]

Millivolts or milliamps?

(Sorry Theo, I'm too bored over here)

Oh sure. I can always use a good kick in the ribs! Feels especially nice lying on the ground like I am.

 

[MrMoMo]

The difference is the voltmeter uses a resistor to shunt a tiny fraction of the current through its coil and lets the majority of the current flow unimpeded.

Ok, perhaps I jumped a little too hard on that one, but could you explain to me how "the majority" of the current is allowed to flow unimpeded. This to me says that there are two paths for the current to flow though the volt meter, one through a resistor (the shunt/coil) and another that goes unimpeded to ground. If electricity follows the path of least resistance to ground, why then does any of it flow though the resistor, instead of bypassing it and taking the "no resistance road" Then, how come ALL of the battery current does not do this? Which in my eyes would be a dead short. (unimpeded, no resistance = no load = short circuit)

It would make more sense to me if it read "The difference is the voltmeter uses a resistor to shunt a tiny fraction of the current through its coil" End of story for the voltmeter, but the majority of the current flows through the loads of the other circuits, it is not impeded by the volt meter, unlike the ammeter which ALL the current must flow through. (however it only draws a tiny amount of current to deflect the needle)

And, to make it more confusing, Ammeters can also have shunts.

Am I getting more confusing, or more confused?

 

[coiled40gary]

If you want to know how much your meter will draw on the battery when the truck is off, look on the spec sheet that came with your meter for a thing called input impedance. I just looked it up for the voltage ranges of a Fluke multi meter, and it is 10Mohms for that. OK, so it is not infinite, but some meters use an op-amp on the input and do have an "infinite" input impedance. The lower the input impedance, the less accurate your volt meter will be (because it loads the circuit with the shunted current path everyone is talking about). To calculate the parasitic draw, divide the voltage by the input impedance. For an example, we will say your meter has 1M of input impedance, and the voltage on your battery when the truck is turned off we will say is 12V. Then you meter will draw 12 microAmps (not milliAmps, but 1000 times smaller). For the life of your meter, you may want to connect it to switched 12V anyway.

Something else to concider...
If I were selling a volt meter, and writing the instructions on how to install it, I would tell you to connect it directly to the alternator output. The reason being that I might sell it to someone who might want to put it on some old truck. And that old truck just might have something wrong with it that the volt meter might make apparent. Well then the person putting it on their old truck is likely to blame my meter for reading "funny". So I am going to tell them to install it in a manner that will have the least potential to be effected by any problems that may exist on the vehicle. With the meter connected to the alternator, you could even open the fusable link, be running on a dieing battery, and your meter will still read 14.5V. Now, being the owner of that old truck, I might be more interested in seeing the voltage level that is getting to my ignition system, than just seeing what the regulator in my alternator is set to. Monitor at the end user, and you will see the sum of all effects. If you do monitor at the ignition, pay attention to which side of the ballast resistor you are on. There is nothing wrong with monitoring on the 9 to 10V side, as long as you know that is normal .

gary

 

[dacicci]

he h ee hhee lot o big words...

just kidding thanks for all the good info i think what i really want out of the gauge is what was said when you can see a big change in the voltage or a eccesive drain on my batt. so i hooked the gauge to the ing then to ground. i do not have any stock wireing left in my 40 so i know all my wireing is good.