Pin_Head said:
Maybe I should have qualified it. What he says about volt meters is true: they are easy to hook up and they tell you the voltage. It is what he says about amp meters that is bull****.
Specifically:
1. You don't need to run high current into the cab with modern aftermarket amp and meters found on cruisers after '78.
2. An amp meter does tell you if your alternator is not working properly, because it will read a negative current drain. His claim that it doesn't is pure BS.
3. What an amp meter tells you that a volt meter doesn't is whether your battery is fully charged or not and if your battery is in good shape because it can take and hold a charge.
4. An amp meter will also tell you if your alternator is overcharging although it won't distinguish between this and a bad battery by itself.
5. An amp meter will tell you if you have a serious short that will drain your battery with the key off.
6. If your alternator fails, an amp meter will give you an idea of how long the battery will last.
A volt meter is like an idiot light with a needle. Because the voltage is regulated on cars, the voltage doesn't change until you turn the car off or the alternator fails. While it can also detect an over charging situation, this is rare.
While it is true that amps and volts are related by resistance, in cars and trucks it is the amps that vary the most because the voltage is regulated and the resistance is constant.
I think he's partially right about ammeters, maybe not totally correct.
1) True, you can get an ammeter without the high currents inside the cabin. But since these are relying on a very low resistance shunt, in my experience they tend not to be very accurate. A very small change in resistance (either due to connections or old age) will give inaccurate readings. But I think the trade off is worth the extra margin of safety of not having those wires in the cabin.
2) I think what he said was that you couldn't distinguish a working alternator from a different problem, but yes, a ammeter will tell you if the alternator is working in some instances. But a voltemeter will also tell you that by the battery voltage.
3) This one is where a voltmeter would be better. An ammeter will not tell you if your battery is fully charged, only whether there is current flowing into it. Theoretcially a good battery will show now flow when it's fully charged, but a bad alternator or other problems will show the same thing. Whereas a voltmeter will actually tell you what condition the battery is in.
4) Maybe. It might tell you if it's overcharging, but the current flow could also be due to accessories that are running off the battery. It's somewhat vague. A voltmeter will also tell you if it's overcharging based on the voltaage reading, and my gut feeling tells me it's a more accurate indication.
5) True, but an ammeter will only tell you if there's a serious short that's draining a LOT of current. I think you'd also be able to tell by the smoke coming from your engine compartment
A voltmeter will also show a short, and is maybe a more accurate indication of minor shorts
6) Actually, it is the voltmeter will tell you how much longer your battery will last, the ammeter is only telling you how fast it's draining. If your ammeter is showing 10 amps of current flow, you don't know whether the battery itself is good or bad, only that it's draining at 10 amps. A voltmeter will tell you how much life is left in the battery, and you can estimate how long it will last by how fast the voltage is going down. For this situation, I would definitely prefer the voltmeter.
In the end, it all depends on what you prefer. They will both tell you information in different ways. I think a voltmeter will give you the same information as an ammeter and more IF you know how to read it correctly, but it's not as easily understood as an ammeter reading.
