This is easier to describe with pictures and drawings so if you give me a few days, it's worth adding to my web site, but in the meantime, to clarify how an amp meter works, what you described is how a voltmeter is connected. Voltage is electrical pressure just like pressure in a water pipe or compressed air line. Nothing goes through a pressure gauge to read the pressure, and nothing, (very little), goes through a voltmeter to read voltage, so when you connect one meter lead to the battery positive post and the other lead to the car body, which is connected directly to the battery negative post, no current flows through the meter, but it does measure the pressure, in your case 12.47 volts.
Amp meters measure electrical current flow similar to a municipal water meter inline with the water pipe coming into your house from the street. All water goes through the meter with very little resistance, or opposition, to that flow. In an amp meter, all electrical current goes through the meter. The better the meter, the less effect it has on the circuit and current flow. It should look like it's not even there. If you install an amp meter like you described, from the battery's positive post to the negative post, or car body, you'd have a direct short, smoke, melted wires, and lots of excitement.
Most inexpensive digital volt-ohm-amp meters have internal fuses to protect the meter on the 2 amp and lower scales but very often there is no fuse for the 10 amp scale. The amp meter has to be connected in series, or inline, with where you want to measure current flow. In this case it's the current flow through the battery we're interested in so we would remove a battery cable, then connect one meter lead to the cable clamp and the other lead to the battery post it came from. Since current stays the same in a series circuit, whatever current leaves one battery post is the exact same amount that comes back in on the other post so the amp meter can be connected to either post / cable. We typically insert it in the negative cable only because if the wrench should hit anything metal on the car while it's touching the cable clamp's bolt, no sparks will occur.
This setup would work fine on older cars without computers and on cars with computers up to about the early 1990s, but there's a clinker that showed up around the mid 1990s that makes measuring "ignition-off-draw" more complicated. Some computers take up to 20 minutes after the ignition switch is turned off to go to "sleep" mode. During that 20 minutes the electrical system can draw up to three amps. You must wait and make your measurements after that 20 minutes but anything you do to open the circuit, even for an instant, will wake those computers up again for another 20 minutes. You have to break the circuit to connect the amp meter. That means another 20 minute wait. If you start out on the 2 amp scale, 3 amps would blow the internal fuse so now you again have an open circuit. If you start out on the 10 amp scale, most meters require you to unplug one meter lead and move it to a different jack for the lower scales needed to get the necessary accuracy after the decimal point. That opens the circuit again and makes for another 20 minute wait.
To really confuse the issue, if you start out on a higher scale, then after 20 minutes you're able to switch to a lower scale for better accuracy, amp meter switches are always "break-before-make" styles meaning as you turn the knob, the circuit is broken first to the setting you're leaving, then it makes the connection to the new setting you're switching to. That very short break in the circuit from just switching ranges is enough to wake the computers up again and potentially blow the fuse in the meter.
The secret on these cars is to remove the battery cable, then connect a small jumper wire from the cable to the battery post. Current can flow freely now for the computers to go to sleep mode after 20 minutes. During that 20 minute wait you connect the amp meter to the same two points, battery post and battery cable. You can start out on the lowest scale without fear of blowing the meter's fuse because all current will be flowing through the easiest path which is through the jumper wire.
Once you've waited for the 20 time-out of the computers, remove the jumper wire. Now any current has to flow through the meter and can be measured. If you should have to switch ranges on the meter, simply reconnect the jumper wire first, switch to a different scale, then remove the jumper wire again. In that way the circuit was never broken so the computers will stay in sleep mode.
The magic number you're looking for once the computers have timed out is.035 amps. Most cheap digital meters have a 200 milliamp scale meaning they will read anything from 0 to 200 milliamps. You can also use the 2 amp scale but you'll lose one place after the decimal point. 35 ma. Is the industry standard maximum unless the manufacturer specifies differently. Some allow up to 50 ma. (.050 amps). At 35 ma, Chrysler says a good battery will still start the engine after sitting up to three weeks.
The clamp-on amp meter you mentioned won't work for this problem. Instead of measuring current flow THROUGH the meter, they work by measuring the strength of the magnetic field around a wire. The one important characteristic that makes all motors, generators, and ignition coils work is that whenever current flows through a wire, it sets up a magnetic field around that wire. More current equals a stronger magnetic field. Clamp-on meters measure that magnetic field and convert it into a current reading. They work great for large currents like the 150 amps to run the starter motor or the high current leaving the generator (alternator). They lose their accuracy on very low currents such as what you'll be looking for. Also, I was unpleasantly surprised to find my clamp-on meter only works in AC circuits so all I can do is tell if my electric hot water heater is working! The lowest scale is 20 amps, not even close to being accurate enough for automotive use even if it could read DC current. The advantage to a clamp-on meter, besides not having to break the circuit anyplace, is there's no internal fuses to blow. It will simply read over range if the current is higher than the meter can display.
For what most people need, you can find a dandy digital meter at Harbor Freight Tools for less than ten bucks. They often go on sale for $2.99. They even have the batteries installed already.
I'll try to get some drawings up on my web site to better show the connections for measuring your current drain. I've typed this description enough times that sending you to one page would be faster and more effective.
Wednesday, April 18th, 2012 AT 7:23 PM