Hey guys I run a very small shop in a Podunk town... And I'm usually very good with electrical, but this one has me stumped. 2015 express 3500 bare chassis model (it's a delivery van with a box back end). Have no backstory of when or how the lights quit working as their employees don't say anything til it breaks down or they get pulled over. This one has no turn signals or 4 ways or high beams. According to every diagram I found theres no actual fuses that control any of those. I see it goes from a magical fuse with no position number (non existent) to the bcm, relay appears to be solid state in the bcm as well. Not a single blown fuse in either panel. I replaced the bcm, because it should still allow those functions without programming, and no dice. I then replaced the multifunction switch, because I noticed certain wiper functions didn't work either. No change. I can command the turn signal lamps on individually with the scanner and rears work. But dash indicators don't come on or anything. Front marker lights work fine, low beams work fine. But no high beam, turn or 4ways. Also the battery gauge sweeps fine but shows 9v all the time and says service charging system. Not paying 175 to have the new bcm programmed when it makes no difference old to new and everything points to it should function without programming, minus other functions like starting. Any advice would help.

 

I've heard that the board in the fuse block can go bad from corrosion (semi-common issue) causing stuff to not work.

 

Problem with the fuse box idea is short of just throwing new/used parts at it, I don't find any means of actually testing the circuit boards, and this customer isn't going to pay for more and more parts to hope and pray, and I certainly don't have the capital to do that either. I need someone to tell me what to test to verify thats the problem. My primary issue is I have yet to find one wiring diagram that shows which fuses actually go to the blinkers/flashers. The fuse box labels literally don't show anything runs those. And the one diagram I found for them says there's 3 10amp fuses that run directly to the bcm then to the switches, but doesn't give a location or fuse position number or anything, so it's as if they don't exist and that's not possible. Very frustrating.

 

It's been a long time but I did watch this years ago and your answer might be in one of these 2 part videos.