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What is your question?
I removed the battery when it read 6V with the multimeter when connected to the system. It was fine after removing (needed a charge), but I also tested the unconnected battery terminals which read 1.2 ohms meaning there is a short somewhere. How do I find where the short is? I have never worked on automotive electrical systems, but I am an electrical engineer with reasonable equipment and I have successfully managed several mechanical repairs with the help of friends.
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Vehicle Owner/Engineer; 1 > A GOOD BATTERY in a state of discharge should not measure less than 9.6 volts, from what you have posted I suspect that you do not have a viable battery. 2 > I therefore suspect that there is no other issue with this vehicle, which, when running with a good battery should have 13.7 to 14.9 DC volts indicated across the battery terminals, indicating a good DC charging system voltage. 3 > To test for a parasitic draw, an inductive ammeter should be used, and individual circuits can be checked by removing fuses to isolate them, 4 > I do not know of any published specification for a resistance test across the ( disconnected ) battery terminals, but with all of the circuits involved, I would not consider that reading a problem. 5 > That is a somewhat simplified explanation, there may be other issues , such as a failed rectifier internal to the alternator, which in some cases may indicate a suitable charging voltage, but can cause other problems, such as a discharged battery, or even engine performance problems, this can be diagnosed by checking for AC ripple across the battery with the engine running, or with a lab scope.
Thank you so much for you response! The battery, when disconnected, read 12.9 Volts. I didn't know this at the time of posting. It wasn't actually a bad battery, but it had been constantly drained. I used the ohmmeter to trace the 1.2 ohm (very small) resistance and found that it was actually at the ECM where all of my fuses and relays were attached. I removed all of the components and tested it again to find 1.2 ohms across the battery input terminals. Since this clearly meant there was some sort of conductive path that was isolated in the unit, I took it apart. I found inside that there was some kind of brown oily liquid which I can confirm was not oil, coolant, transmission or brake fluid. I cleaned it all off, reassembled the ECM and reinstalled the unit. To my surprise, the truck started up again. No repair costs. Crazy. Nerd note: There is a small internal resistance to any battery. When connected to a significantly larger resistance, its effects are not noticed. However, when a resistance that is very small is connected to it, it changes the apparent output of the battery. Voltage is divided by resistances in series. For example, if a battery with internal resistance 1 ohm has a load resistance of 99 ohms attached. 99% of the battery voltage goes to the load resistance and 1% to the internal resistance. If a load resistance of 1 ohm is connected to the battery, each one shares 50% of the voltage. For a 12V battery it divides it into 6V. You can't test the internal resistance directly (i.e. ohmmeter), it's sort of the scientific black magic that happens as a result of the chemistry and physics of a battery. Really high end batteries for advanced applications try to reduce this parasitic resistance as much as possible.