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2004 4.8 Electrical issues, no crank, no headlights, tach bouncing between 0 and rpm, no signals, etc.

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I'm at a bit of a loss on this one. 


Symptoms appeared suddenly on one of the first -15 days


I have verified passlock is properly bypassed by re-enabling it and cranking via relay and getting no fuel. Install bypass back and it fires up by jumping the relay just fine


I'm getting 12v on a crank wire under the dash off the ignition switch harness. I can't find a pinout/schematic for the multifunction switch. 


I have pulled and cleaned all the grounds I can find including the hood ground and strap ground to engine block. I could only find two under the dash and one by the drivers frame rail, I may be missing some


I have pulled and manually tested all fuses for continuity, all good there.


No low beams but bulbs are good

Turn signals quit working, bulbs are good. 


I'm thinking the next step is to pinout the connector under the dash here. It looks like most of the wiring goes through there.


Does anyone have a wiring diagram for the multifunction switch, this bulkhead, and to the underwood fusebox? I'm trying to follow the crank signal through hoping I'll figure out the rest along the way if it's a problem where they're all related. 








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So put an oscilloscope on the can lines. I could see the messages floating overtop another waveform. Traced it down to unstable voltage at BCM.


Went and pulled all fuses but this time ran a lockpick up and down each terminal to break up Corrosion. Lo and behold when I did IGN E, things fired right up. Have turn signals and no gauge issues. Interesting that I was getting 13.1v on the terminal while cranking with multimeter before, but after cleaning it works again.


Headlights still don't work but I'm thinking it's multifunction switch. I'll figure that one out when I get sunlight again

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A BIG THANK YOU for posting the cause and cure. I don't know if folks don't have the time or knowledge or tools to properly diagnose problems like this because most of the time they end up at the dealer and we never hear what the actual problem was or how to fix it 🤞

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