2018 Peterbilt Paccar MX - cranks ONCE, won't start

Here is one video. Voltage Drop Testing a Starter Motor (Quick Tips) - Bing video
Here is another. Ford F-150 No Crank No Start: Engine Starter Motor Diagnostic Walkthrough! - YouTube Don't really need the first 7 minutes.

Same for heavy truck. Do voltage drop testing after visually checking for loose or corroded connections. Possibly a bad starter solenoid, relay, cable or connections.
Don't forget the thumbs up.

 

You made sure to clean the contacts on the connection to the battery terminals? Also helps to know if the dash also dims when cranking just like a weak battery. Probably good to look at the wires from the battery and follow it thru see if there’s any part of it exposed and corroded.

 

Likely a bad solenoid, mine did the same thing when mine failed. Could also be a bad starter. To rule out the starter as much as you can, go whack the #### out of it with a hammer while someone cranks the key. If it starts, its defo a bad starter if not keep on going down the chain as suggested above.

Also if it IS the starter then get the mercedes branded starter if you can find it, and if its compatable with the EPA17 mx13. The paccar one uses a bunch of iron and other cheap parts. I have had 5 in a row fail within 2 months of each other though that coulda been the covid shortages causeing them to use garbage parts. The mercedes ones tend to last much longer and this one is going on a year now with a half dozen starts and stops a day.

Lastly and to rule out the obvious, check for a low voltage cutoff switch. Mine had one in a weird place and when my rig had a low battery tripped and killed the whole truck till i found it near the firewall and flipped it. Lot of places put them in or order them with the trucks.