Am I right that ALL the loads for the 12v system go through this meter- it has an internal shunt and that is how it measures the amperage flowing through the electrical system? What size cable do I need to use to safely wire it into the electrical system? Do these things catch fire? Any advice is appreciated.
Well, to be technical all loads
except the starter of course, plus the alternator output. That means in particular that the battery will be on one side of it, and the alternator output will be on the other side
connected to the loads. (This is the concept that somebody at SPF didn't get for the first few cars).
That way the current that flows through the ammeter represents what is going into or out of the battery (which is exactly the intent: amps from the battery's point of view). You could if you want leave out of this a big load like your cooling fans but I don't see any reason to do that unless you ammeter max range is way smaller than the fanload. (eg. on my SPF the fans are 40A total and the ammeter reads to 50A, so if were to run the fans and ignition with the engine off the needle would swing all the way to the left.
Yes the needle is deflected by the magnetic field created by the current flowing through a heavy piece of metal inside the gauge, so there is essentially no voltage drop across the ammeter and no heat produced. They don't catch on fire by themselves AFAIK (although I'm sure there's a Lucas story out there somehwere). However, if you were to make a loose connection to the ammeter the loose connection could heat up. More on that later.
The cable size could be whatever cable size is in use at the point where you insert it unless you are lengthening that run a lot. Just for reference the SPF docs say it uses 8 sq. mm wire at that point, which is (oh, how convenient) 8 gauge wire. That sounds right from what I recall messing with that wire.
Advice?
Disconnect the battery, of course.
Solder whatever terminals you use; do not attempt to crimp that connection. That is, you can crimp them or not, but you
must follow up with solder.
Make absolutely sure that between that ammeter and the battery is a big (40-80 A) fuse as near to the battery as you can practically arrange it. On my SPF, for example, the battery is in the rear of the RH sponson, and the 80A fuse is mounted to the vertical brace just forward of there. If you don't have one, the right fuse and fuse holder to get is a "maxi-fuse". These also do, and should, take a lot of effort to install and remove from their holders. I usually use a pair of pliers to pull them because I can't really get a good enough grip with my finger tips.
Carefully carefully insulate around the terminals (pref. heat shrink, maybe two layers). Make sure the connection to the ammeter (screw terminals? 3/8" wide, thick spade lugs?) are really tight and will not come loose. If they are spades, pushing the female terminals (that you install on the ends of the wires) onto the ammeter should be difficult. If not, squeeze the females (OK, I hear it coming) until they go onto the male terminals with substantial effort (like you really have to push with your fingers and maybe wiggle them side to side in order to get them started onto the male). Don't break the ammeter of course, but it might feel like you're getting close.
When the ammeter goes back into the dash be absolutely certain there is no bare metal in the vicinity that could shift or flex and come in contact.
Having said all the scary stuff, if the bad thing happens but you have that big fuse, it's just going to make a big "snap!" and spark and immediately blow the fuse. Probably no harm done.
I'm wondering: wouldn't the "standard" CAV harness have the terminals, etc. for an ammeter already there? If so they might just be joined together by a chunk of copper or otherwise shorted together right behind the dash, so all you would have to do is separate them and plug them into the ammeter? Wouldn't that be sweet.