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I have been dealing with a parasitic battery drain for over two years and two batteries.
Battery drains down overnight and won't start in the morning. I performed the following test;
First, all door & hood latches were closed during each voltage test and engine turned off and waited one hour for all sensor to fall asleep.
1. Voltmeter (mv) test across all fuses checking each one for presence of any voltage = Some had minor 0.1 mv.
2. Voltmeter (mv) test on negative battery and positive posts 3.5 mv present = Should be under 0.5.
3. Removed each red wire, one by one and found the one powering the main fuse box sparking when connected/unconnected. This one red wire killed all electrical in the vehicle.
4. Had the alternator tested for charging etc. = No issues
5. I performed a diode test on the alternator and it passed with no issues.
6. Voltmeter (12V) on the ECM in/out puts at the 70 amp fuses - No issues.
7. Have not flashed ECM, but considering it, or possible replacing it.
Not sure what else to do at this time. Any suggestions welcome!
 

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Here is how I look for parasitic drain: Disconnect the negative battery lead. Hook an ammeter (measuring current in amps or milliamps - not voltmeter) between the disconnected cable and the negative battery post. This is where you are going to monitor the current flow. Any vehicle now is going to have some current flow, so the trick is to find the circuit that is drawing the most.

So, remove one fuse at a time and monitor the current flow change. As you mention, you do need to wait for things to go to sleep to get a true reading. One by one remove each fuse until you find the one drawing a suspect amount of current. Then figure out what is on that circuit. That is the big step, but the next step of figuring out exactly what the problem is can be difficult depending on what circuit you find the current draw on.

It's a long process usually - because you need to wait in between each test. When you think you've found the problem circuit - double check your observations by repeating the test with the suspect fuse in and out.
 
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