Freightliner wont start

I'd turn the master switch off so you don't fry the ecm or something, then take the cover off the battery box, and check all the connections between the batteries. Especially the negative ones.

Then check it to the starter. It all terminates on one big bolt on the starter.

 

HIT IT WITH A HAMA

 

There is a bunch of mega fuses under the drivers door on the frame rail, the assembly can crack and cause problems.

 

Dash and everything goes out right?

Sounds like you are loosing mains power into the whole distribution system. You didnt need to buy a radio. You needed to buy a voltmeter. First verify voltage across the battery posts. Say its 12.3vdc. Now go down the chain and see where it takes its first substantial drop. 12.1, 11.9. Good wires, terminals, connections and relays shouldnt drop much voltage. It could be in the ground circuit or in the positive circuit. Theyre both equally critical, its a loop.

If you put it on beep mode you can check continuity however dont put bunches of faith in this, as it is not a true load test. The meter can show continuity but still be a huge voltage drop there.

Get a meter and youtube how to use it then work upstream throughout your power distribution. What you describe is the worst gremlin. Intermittent fault diagnosis.

 

Also check the voltage between your negative lug and any metal in the chassis is pretty much zero. If youve got a 2volt chassis potential theres a big issue. The ecm is wanting to say a difference of potential across the positive and negative of 12+ volts so raising ground to 2v means the ecm sees 10 and flips you the bird.

Look for zero ohms or close to it between frame and ground points. Beep mode should beep when you scratch in. Your negative battery terminal and all your iron should basically be at the same reference voltage of zero, with full continuity. If not, chomp jumper cables from negstive post onto raw metal to create an expedient ground and check again. Ground is a very big deal.

Report back with findings and well go from there.

 

All trucks have a relay that closes a master relay that energizes the truck. I don’t know where it is on a Freightliner but should be around The key area.

 

Mine is located in a small fuse box on the left side under the hood.