New here, just trying to get some help on my truck. I have a 2015 F550 4wd 6.7 cab chassis with an IMT service body on it. Ive only had it about 6 months as I just started my own mobile heavy equipment business. Yesterday I got in my truck like any normal morning, started up just fine. Drove to the jobsite, shut it off, went inside to chat with the owner, came back out and started it no problem and moved it into their shop. Worked for a few hours and when the job was done I went to leave and the truck wouldnt start. Sounds like the starter engages, voltage drops and it disengages. Thought I had just killed my batteries. Threw a jump pack on it and it started right up. Not a second thought, drove to another jobsite that was more remote so I let it run for about an hour. Shut it off and went to start it right away and it did the same thing. Threw a DVOM on it while trying to crank and voltage dropped to around 8v. Figured batteries were bad. Called my retired dad to come bail me out and went about my job. Got to a stopping point and decided to mess with the truck some. My service body has an electric over hydraulic crane. I figured if the batteries were shot it wouldnt work well. Unsaddled it and it works fine. Decided to grab my power probe and climb under the truck and see if sending voltage to the start solenoid would crank it over. It did. I turned the key on, took the power probe to the start solenoid and it fired right up without issue. Got the truck home and decided to try and figure out the issue. Swapped the relay labeled "start run" and nothing changed. Removed the terminals that were kind of corroded and cleaned everything up. No change. Checked all the grounds i could see and all checked good. Checked that the starter was grounded. Checked power wire to the starter. No issue. When you try to start it with the key voltage drops so low my PowerProbe shuts off and reboots. It acts like batteries but the fact that it starts no problem with the power probe on the solenoid makes that not make sense. It seems like there is an issue with the solenoid wire but im having no luck finding a schematic for the truck anywhere. I have the expertise to fix it, just not the knowledge of the truck. Any help would be greatly appreciated

 

Maybe this could help, for a 2015 from the Ford manual: