Toyota Repair: Burning smell, now a short, 4runner sr5, electronic circuit design

Question
I have a 1997 4runner SR5 with a 3.4L and about 212K miles. For the past few years I've occasionally noticed a slight electrical burning smell, not sulfur or ammonia or oil or coolant but an overheated electrical smell like overheated insulation on a wire, or an overheated resistor or possibly an overheated relay coil. The smell is obvious when I get out of the truck and am walking around the driver-side fender in front of the hood and appears to be coming from the front hood area near the battery after routine driving between home and work, about 3 to 5 miles. The smell has appeared more frequently over the past year and recently was occurring after every drive. Upon lifting the hood and inspecting, no damage or discoloring has been obvious and I've never noticed any smoke, just a heated electrical smell.

Now, I have an apparent short that's completely draining the battery overnight. I've taken the 4runner to the local Toyota dealership and after having it for 2 weeks, they haven't found anything wrong other than admitting the battery is discharging and going flat, to less than 2 Volts. They've said the alternator tests good and the battery will hold a charge as it was just installed in January. They've attempted to isolate the fault by measuring current draw while pulling fuses but, they can't find the source of the short and don't see anything obviously wrong. So, it currently appears I have a short somewhere in or near the battery circuit which I'm guessing is the result of overheated insulation on a wire or an overheated component causing an electrical fault in the engine area.

Having worked in electronic circuit design for over thirty years, I know the smell of an overheated component and that's exactly what I've smelled coming from the engine compartment. It's not oil, coolant, steam, snow on the catalytic converter or anything else associated with the exhaust system. Nothing is obviously burned, charred or discolored but, I'd bet that if I were to begin disassembling everything under the hood possibly near the location of the battery I would eventually find an electrical wire or component that over the years may have been repeatedly splashed with water or has been electrically over-stressed which over time has resulted in a short that is now imposed directly on the charging circuit or battery.

I'm hoping that someone who's familiar with the electrical components in the general vicinity of the battery can provide an educated guess as to where a component or possibly a wire might be located that could be responsible for this problem as after 200,000+ miles, I still like this truck and would like to keep it a while longer.

Thanks for the help,
Jon
Moline, IL

Answer
Since you have experience in electronic and electric circuits you should get a wiring diagram for your vehicle and start checking for shorts to ground with the battery out of the circuit, you can also first try to verify what the dealer supposedly did already and connect an ammeter in series and check for current draw, the disconnect fuses and see which circuit/fuse eliminates the excess draw, then concentrate on that area, I have no easy answer for you, even if I had the vehicle it would take me some time to figure out where the current may be, one thing seems for sure it must be a large draw if the battery goes down overnight which makes it a lot easier than finding a small current draw. You may want to start with the fuse box under the hood, remove it from the fender and open the bottom cover and check for loose connections, burned wires, especially the big white wires which supply voltage to the rest of the circuits in the electrical system, these are bolted in to the 120A fuse, check these bolts to make sure they are tight.