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I have a slight drain on my 2000 f250. Appears to be related to fuse 15 which is pcm/gem module. How do I go about finding it from here now that I know it's that fuse.
How much is the drain? The PCM will draw a small amount of current when the truck is off to keep it alive.
It's killing my battery overnight. Not completely but enough it won't start the next morning. I noticed last night when I went to get in the truck the back light on the radio was lit up real dim.
If it's dead overnight we already know the draw is excessive. Trick is to find it. Is the radio or stereo aftermarket? First thing is disconnect anything that isn't factory. Also get battery on charger asap.
(To check for actual current draw set DMM for amperes and install in series with positive or negative cable and battery.)
It can take a half hour or so for all the modules to power down and accurately measure the true resting current draw. Should be approx. 50 milli-amperes or less. Can start pulling fuses individually to see what's what. Alternatively pull all the fuses and reinstall to isolate the problem circuit.
Your comment about the radio backlight - I also can see a small light that stays on inside the radio. As long as your draw is less than 1 mA you should be OK.
With the information you have given so far, there are a couple things to check. The mA draw after about an hour down (like Tedster9 says) if you can. Check the voltage at the batteries with the truck running and off. You should see about 14v with the truck running and somewhere between 12-13v with the truck off. Voltage checks should be very close if not the same at each battery. This will tell you if you should be looking at batteries or alternator.
I've had my 7.3 since 2007. In that time I have had 2 batteries (an AutoZone and a Costco) short out on me giving out only about 10.3 volts. Found out later both companies use Interstate for their batteries. In any case, I assumed that there was some kind of battery drain going on because the voltage would appear normal after I drove it a bit, but the next morning the truck would barely start. This was of course because the shorted battery was pulling the other battery down. I would say charge the batteries, disconnect them and check the voltage the following morning. If one is really low, it's probably shorted out internally.