Time to get this alternator disabled!
I did a bunch of digging on this one. The Metro uses a 'self exciting' regulator. So, you can't just cut power to a logic or low power wire. Instead, you have to cut the main power connection between the alternator and the battery.
To do this, I got a beefy relay. The Metro alternator is rated at (I think) 55A. I happen to have a 70A relay kicking around.
Here is a link to the one I used. Its for automotive use, works directly off of 12V, perfect for this project. The only issue with it, is it uses 3/8" wide blade connectors... which nobody carries. The solution? Bolt it together. So, I drilled out the large connectors for a #10 screw.
I had a ~12" section of 6ga wire laying around as well. I bought some ring connectors at the local hardware store and crimped one on the end. This bolts up to the relay, and the other end will bolt up to the alternator. The original wire that goes to the alternator was bolted to the other terminal on the relay.
I then installed the relay into the engine bay. Unfortunately, my 12" piece wasn't long enough to mount the relay up higher (out of water's way more), so I had to mount it on the brake distribution block. I'll figure out some way to waterproof if later on.
I have not hooked up the coil wiring to the relay yet, so I can't actually turn the alternator on.
I'll get to that sooner or later.
A while back, I needed a new battery, so I got a group 24 deep cycle battery. This is necessary anytime you want to drive around for extended periods with the alternator disabled. A regular starting battery will not hold up to the deep cycling at all and will die a fast death. Also, this battery was actually cheaper than a new starting battery for the Metro, so win-win!
With the alternator disabled, you also need a way to charge the battery. For this, I am reusing a Soneil 5A charger that I had used for an alternator disable mod on my previous car. Its fully automatic, just plug it in and it charges. Once its done, it goes into a low level float mode which uses very little energy. So, I plug in once I get home, and the next time I leave I unplug and go. The wires aren't run all nice and pretty yet, but I'll get to that too.
I've made just a few trips with the car since the mod. I can definitely tell a difference. First off, when I use my kill switch, my engine idles down much slower. I've actually not held the kill switch long enough once or twice and its restarted on me because the rpms weren't low enough. Second, I do seem to have a little more oomph. I hit my target speeds on roads a little earlier than before. Lastly, the mpg is hard to say, but I did hit 60.0 mpg for the first time this year on my morning commute. That is pretty darn good considering I nailed a couple red lights pretty bad and it was only 35F out. My previous best was 58.9 and not hitting as many red lights. Of course all of this is anecdotal, so take it with a grain of salt.