The oil pressure gauge thread got me thinking.....
I have both amp and volt gauges in my gauge box. Bith will match the oil pressure gauge that us going in the neon.
Which is more useful, and why?
My only experience is the factory amp gauges in the muscle car era. And those were old and fire prone. Ive always installed volt gauges as aftermarket stuff.
Voltage gauge because you can pull it from anywhere instead of moving the entire output of the alternator through it and burning your car? Plus batteries are volts, so i don’t need to know the amps because if the alternator dies a voltage gauge will still let me know that.
I'm going to say amp gauge because I think it could tell you more, like a thrown belt or bad battery. Over or under charging, unusually high load....
Voltmeter. No question.
Voltmeter. Ammeter is just another source of fire.
So I have to be the weirdo who likes ammeters?
I like them because they make for a nice state of battery charge indicator. Important if you're regularly running at or beyond the limit of your alternator. If you're at 14v and there's no current going into the battery, it's pretty well charged, so I can do things like run the headlights AND the radiator fan at the same time without worrying that I'm going to kill the battery when stuck in traffic because the alternator will not put out anywhere near its rated output at idle.
Incidentally, a lot of newer GMs have amp clamps on the negative battery cable. Current flow in/out of the battery is important enough to them that they bothered to install a sensor.
Ammeters are great in theory, but in practice they're problematic because, well, fire. It's probably better to make sure the alternator can handle peak load :)
Here's a thought. If the alternator can't keep up, the light should come on. And if it's just starting to tickle the limits of the alternator, wouldn't you start to see a little bit of voltage coming into the light? Maybe you could put a voltmeter on that. Or I've misunderstood something, which is quite likely.
NermalSnert said:I'm going to say amp gauge because I think it could tell you more, like a thrown belt or bad battery. Over or under charging, unusually high load....
A voltmeter will tell you the same thing - if a belt comes off, the voltage is going to drop. Same thing with over or under charging, the voltage wil go up or down.
In reply to Keith Tanner :
Dunno. Never see the alternator light come on. I'm not even sure there is one. I could reliably pull the system voltage down below 12v at idle just by turning the fans on. Also pulls the idle down a few hundred RPM.
Maybe the alternator light only comes on if there's an actual problem? AFAIK the system works just fine, it's just overstressed. Confluence of a heavily underdriven crank pulley and some serious fans attached to the radiator. But then, I also don't have heated seats, rear defroster grid, 5000w fifteen speaker audio system, EPAS, etc.
stuart in mn said:NermalSnert said:I'm going to say amp gauge because I think it could tell you more, like a thrown belt or bad battery. Over or under charging, unusually high load....
A voltmeter will tell you the same thing - if a belt comes off, the voltage is going to drop. Same thing with over or under charging, the voltage wil go up or down.
An ammeter will directly tell you if it's charging or draining the battery and by how much, whereas a voltmeter gives you a number that you need to mentally translate into charging or draining. It's not a linear scale, and it depends on what the current charge state of the battery is, so it's a piece of information with much less precision to it. That said, the voltmeter is probably good enough to say "Hm, this is a problem, I should diagnose it when I get home and can put my proper test equipment on it".
A modern ammeter isn't going to be a fire hazard.
Ammeter.
With a shunt, and overload protected.
I was just going to ask why all these ammeters don't just use a shunt...
I've never learned anything about automotive ammeters, but shunt is the easy solution to measure larger amperages.
I'm a fan of simplify and neither I'm a car. Tow vehicle, maybe a voltmeter for running accessories with engine off. But really the light is all you need.
codrus said:An ammeter will directly tell you if it's charging or draining the battery and by how much, whereas a voltmeter gives you a number that you need to mentally translate into charging or draining.
I agree that reading an ammeter is more direct - one glance, and if it's doing something other than sitting steady just above the middle something's going on. On the other hand, if a voltmeter is reading 12.5 volts or lower, there's a problem; if it's reading 13.5 to 14.5 or so things are good, if it's reading greater than 14.5 or so there's a problem. It does take a little more thought but it is pretty straightforward.
K.I.S.S.--Voltmeter all the way.
Knurled. said:So I have to be the weirdo who likes ammeters?
I like them because they make for a nice state of battery charge indicator. Important if you're regularly running at or beyond the limit of your alternator. If you're at 14v and there's no current going into the battery, it's pretty well charged, so I can do things like run the headlights AND the radiator fan at the same time without worrying that I'm going to kill the battery when stuck in traffic because the alternator will not put out anywhere near its rated output at idle.
Incidentally, a lot of newer GMs have amp clamps on the negative battery cable. Current flow in/out of the battery is important enough to them that they bothered to install a sensor.
I'm with you. I kinda miss the days of Ammeters. On the plus side, you knew you were making more juice than you were consuming. On the negative side you knew you had a regulator or alternator issue. On the other negative side, if you upped your amperage you either had to rewire the whole dash or risk fire.
I like the simplicity of voltmeters (and the lack of death from fire) but they only give you part of the picture. You can assume that if the voltmeter says 14.7v that it is actually supplying enough bulk amps to keep up with demand, but not necessarily. Having said that, most alternators these days are ridiculous overkill. Who needs 200 amps anyway?
There have to be modern ammeters that don't funnel all the juice through the gauge. With the common use of amp clamps there has to be some gauge that sources an amperage signal via magnetic sensor.
I remember when I was flying, the plane had three gauges for the juice; a volt meter, a charge-discharge ammeter (like an old car) AND a bulk ammeter that showed amps from 0 to 30. You could pay attention to the bulk ammeter to verify that things like the Magneto weren't on the fritz. Kinda handy when you're 5000 ft in the air.
In my car, if I see the needle to the left while at anything above idle, I'm either about to overheat or the alternator quit.
In a car, for the most part, a voltmeter is enough to know the important things like "is the alternator working and is it keeping up with the current electrical load?" If we're talking about something with deep cycle batteries that are likely to be partly drained on a regular basis, the additional information an ammeter can provide would be worthwhile.
Knurled. said:In reply to Keith Tanner :
Dunno. Never see the alternator light come on. I'm not even sure there is one. I could reliably pull the system voltage down below 12v at idle just by turning the fans on. Also pulls the idle down a few hundred RPM.
Maybe the alternator light only comes on if there's an actual problem? AFAIK the system works just fine, it's just overstressed. Confluence of a heavily underdriven crank pulley and some serious fans attached to the radiator. But then, I also don't have heated seats, rear defroster grid, 5000w fifteen speaker audio system, EPAS, etc.
I'll admit I'm not rock solid on this stuff - but don't you rely on the alternator light to actually energize the alternator? Once the alternator lights off, there's no more current flowing through the circuit so the light goes out. Here's where I get fuzzy - if the alternator can't provide the power needed, I'm pretty sure you start seeing current flowing through the light again and it starts to glow. Someone who's better on charging systems than I has to fact-check this for me!
If it can't keep the car operating under expected operating conditions, I'm not sure I'd describe it as "working fine" 
We live in the modern age. Why not use a shunt and bluetooth and you can have both safely displayed on a digital gauge that gives you volts, current and power consumed in Watts or HP? For a race car, you want to minimize current as much as possible cause it is the big power eater.
I myself an not a huge fan of gauges. Until there is an actual problem to report, I would rather watch where I am going. While the "Idiot" light gets a lot of abuse from the motorhead community, they do the job. Once again, it would not take a lot to make an idiot light a bit smarter ( teach it basic calculus) so that it could alert you to say a decreasing oil pressure rather than waiting for it to be catastrophic. Chances are that even with an analog gauge, the analog operator is going to miss the event until the death rattles start.
Pete
The NA/NB water temp gauge is effectively a smart idiot light. If it's not moving, you're good. If it's moving, it's giving you proper information that 1) something is wrong and 2) here's what's currently happening.
So of course, there are a whole bunch of online tutorials on how to "fix" it.
In reply to Curtis :
My rough understanding is modern alternators are overkill in order to keep the battery and system voltage stable, which keeps the ECU and various other computers happy.
The alternator light is probably the most useful of all idiot lights. It's just a 12V light connected to the alternator positive and the battery positive. As long as those two voltages are the same (and under normal operation they should be), no power flows and the light is dark. If either is not working, power flows through the light and illuminates it. At least that's how it works in old cars. Newer cars with BCM's to operate the gauges, I have no idea.
I'd probably try to get a split-gauge with an ammeter and a volt meter top and bottom of the face. Because more is better.
I got a AMP and Volt gauge on my monster battery charger, and I rely more on the volt gauge, the dummy light on the neons is also pretty good, so I see no need for one.
Keith Tanner said:Knurled. said:In reply to Keith Tanner :
Dunno. Never see the alternator light come on. I'm not even sure there is one. I could reliably pull the system voltage down below 12v at idle just by turning the fans on. Also pulls the idle down a few hundred RPM.
Maybe the alternator light only comes on if there's an actual problem? AFAIK the system works just fine, it's just overstressed. Confluence of a heavily underdriven crank pulley and some serious fans attached to the radiator. But then, I also don't have heated seats, rear defroster grid, 5000w fifteen speaker audio system, EPAS, etc.
I'll admit I'm not rock solid on this stuff - but don't you rely on the alternator light to actually energize the alternator? Once the alternator lights off, there's no more current flowing through the circuit so the light goes out. Here's where I get fuzzy - if the alternator can't provide the power needed, I'm pretty sure you start seeing current flowing through the light again and it starts to glow. Someone who's better on charging systems than I has to fact-check this for me!
If it can't keep the car operating under expected operating conditions, I'm not sure I'd describe it as "working fine"
I'm pretty sure the alt light will go out only when the alternator is not 'making power'. Theoretically the light is hooked up to 12+ volts on one side and the alt output on the other. When the alternator is not making power, the 12v flows from the battery into the alternator and the light is on - this 12v into the alternator is also what 'energizes' it. When the alternator is making power, there is 12v+ on both sides of the light (0v across the light) and it goes out.
The light cannot tell you if the alternator is 'keeping up' with system demand however. because if system voltage (while engine running and alternator working hard) is 14.2 then the same 14.2 is on both sides of the light. If 5 minutes later the system voltage is 14.0, again same on both sides of the light and no light
In reply to Curtis :
200A is not overkill in the face of things like electric power steering, heated seats, heated steering wheels, heated mirrors.... Supposedly the film-type rear defroster on some Caddys represents an 80A load all by itself!
Keith Tanner said:I'll admit I'm not rock solid on this stuff - but don't you rely on the alternator light to actually energize the alternator? Once the alternator lights off, there's no more current flowing through the circuit so the light goes out. Here's where I get fuzzy - if the alternator can't provide the power needed, I'm pretty sure you start seeing current flowing through the light again and it starts to glow. Someone who's better on charging systems than I has to fact-check this for me!
That's my understanding for how a traditional alternator light works.
On an NB Miata (and I suspect on many other ECU-controlled alternators), the alternator light is just a light the computer turns on if the alternator performance isn't keeping up with what it expects. The ECU monitors the system voltage and drives the duty cycle on the field winding in the alternator in order to increase or decrease that voltage. If it's pushing it as hard as it'll go and the voltage is still too low, then it turns the light on.
You'll need to log in to post.
