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Posted by: nola914 Jun 28 2007, 10:26 PM

I'm close to starting up my 2.0L out-of-body. I'm going to plug in a few basic guages to keep an eye on what's going on. In my parts bin I have an old Amp meter (old - but never been used). But I got to thinking... the 914's use a volt meter, as do newer cars. I suppose that there is a reason for that.

Seems to me, and this is a very simplified way of looking at it, the amp meter is providing info about the alternator's operating efficency, while a voltmeter is showing the battery's condition. Am I thinking correctly?

In my case, with a new battery, but an old alternator that hasn't been turned in 4 years, to get a reading on the system, should I use the amp meter or look around for a volt meter? Or both?

Posted by: SirAndy Jun 28 2007, 10:35 PM

if you use the amp meter, make sure you use the biggest wire you can find ...

otherwise, it'll melt like butter ...
Andy

Posted by: chris914 Jun 29 2007, 01:53 AM

A battery always has a voltage difference that can be measured even when there is no current draw (unless it is dead). It is the condition of the battery that is important to starting your car and keeping it running. It is only important to know if the alternator is working or not, hence the little idiot light in the dash.

An alternator is not always charging the battery. It charges the battery when the battery falls below a certain voltage. When you car is running it draws almost no current from the battery. Your car could run for days before the alternator would need to charge it again. The starter is the biggest draw of current on your battery.

If you had an ammeter in you dash you would have to run a big a$$ wire from the alternator to the gauge and back again. Look at the wire that is there now. The copper is like a quarter inch in diameter. The amount of current running in that wire can kill.

A voltmeter measures voltage in parallel and an ammeter in series to the circuit.

So yes a voltmeter tells you the condition of the battery and an ammeter tells you the working condition of the alternator, that took me long enough to say, lol.

Do you need one? Depends on that you want.

Posted by: Dave_Darling Jun 29 2007, 02:35 AM

I think Chris may be overstating things, but he is at least partly right.

The voltmeter gives you an indication of the current condition of your charging system, including the battery. The ammeter tells you how much draw your car is putting on the system versus how much your alternator is putting into it.

The main reason cars don't have ammeters these days is the wiring. Ammeters need big wires, and those are a little expensive (particularly for long runs like in a rear-engine'd or mid-engine'd car!) and a little difficult to work with, and pose a not-insignificant risk of "letting the magic smoke out" of your car.

I'd use the voltmeter, because I'm used to it. For out of the car, the ammeter might be worth a look...

--DD

Posted by: Joe Owensby Jun 29 2007, 09:46 AM

The voltmeter is by far the better instrument. It tells you several things: the current charge state of your battery, as well as how much voltage your alternator is putting out (which is an indication of both the condition of your alternater as well as the voltage regulator). You should be able to watch the voltage reading and know immediately if your charging system is working properly. For instance, if the voltage regulator is malfunctioning, it could output too much voltage and overcharge the battery- with resulting leakage of battery fluids, etc.

Joe

Posted by: ruddyboys Jun 29 2007, 10:11 AM

Not to steal this thread, should my volt meter be moving back and forth while the directionals are on?

Posted by: 914-8 Jun 29 2007, 10:14 AM

I have both in my car.

Dave's explanation in the paragraph above is the best, most clear one.

In real life, I'd take the voltmeter if I had to choose between the two. The most impt info, overall, is the condition of your charging system, including the battery. In addition, it seems like the voltmeter, to some extent, also does tell you the net draw that is being put on the battery. For example, when I turn on my AC, my voltmeter drops a bit.

All useful info to help quickly see the most common failures, particularly alternator or regulator failure.

The ammeter never moves. The only time I found it even slightly helpful was when my starter seized up. When I turned the key, the starter would not turn, but the ammeter showed a huge draw on the system.

Posted by: marks914 Jun 29 2007, 10:33 AM

I prefer the voltmeter, but I would get one of these

mark

http://rides.webshots.com/photo/2831499580057582025pRgPDv

Posted by: skips Jun 29 2007, 09:25 PM

You can use an ammeter "shunt" to use an ammeter without using big wires to the meter, shunt and meter need to be a matched set. Google experimental aircraft electric systems, lots of discussions on preference of voltmeter vs. ammeter.

Posted by: Bartlett 914 Jun 30 2007, 10:05 AM

Amp meters work by measuring a voltage across a very small resistor referred to here as a shunt. This resistor is very low in value in ohms and is high in wattage. This can be in the meter or remote mounted. I would never run large wires for this. It would be best using a remote shunt and small wires across the shunt to the volt meter (calibrated to read amps) Not real useful in out cars IMHO.

Posted by: Air_Cooled_Nut Jun 30 2007, 08:27 PM

I run a volt meter in my VW. Amp meter doesn't return as much info IMO.

Posted by: nola914 Jul 2 2007, 08:51 PM

Thanks for all of the replys. I was away from the board for a few days so this is the first time I got to read them.

After reading the msgs, I'm a little leary of the amp meter I have. Like I said, it's never been used but it's old. I don't want to melt some wires.

I think I'll get a new volt meter to start off with. That sounds like the safest route to take, and not be compounding any other startup problems with a meter issue.