FLC 120 no crank, new batteries, starter, solenoid

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Showhauler03, Jun 7, 2025.

  1. Showhauler03

    Showhauler03 Bobtail Member

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    The two big fuses you referenced are marked sleeper, both of those are good. the four lugs are all hot when the ignition key is on. I appreciate your help immensely!
    there is a slim possibility One of the wires could be mislocated on the starter. it had to come on and off multiple times because Fleet Pride failed to mention to me it requires a spacer. I did my best to zip tie the positive wires and keep them separated, but there was a lot of cussing and sweat in my eyes. Keep in mind crossing over the firewall solenoid or sending power to the signal wire fires it right up so it can’t be too far off I don’t think.
     

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    Last edited: Jun 8, 2025
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  3. Big Road Skateboard

    Big Road Skateboard Road Train Member

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    All is good if you're jumping the firewall relay and it cranks, you've just lost signal between the switch and relay it seems. Any possibility you missed something on the relay wiring when ypu replaced it? Maybe the ground isn't grounded.
     
    Bean Jr. Thanks this.
  4. Showhauler03

    Showhauler03 Bobtail Member

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    no that is right for sure.
     
  5. Showhauler03

    Showhauler03 Bobtail Member

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    Dec 9, 2021
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    If freightliner doesn’t have any wisdom tomorrow I’m tempted just to run a new wire from the IGN on the ignition switch to the signal post on the firewall solenoid and add a 15 amp in line fuse.
     
  6. Big Road Skateboard

    Big Road Skateboard Road Train Member

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    Well, really need a diagram then, but I'd check all those fuses first. You can pull them out enough that they still make contact, but you can get a probe to them. Just go through each one, no matter what they're labeled.

    Key on, should have power both sides of most. Any that only have power on one side are bad
     
  7. Showhauler03

    Showhauler03 Bobtail Member

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    Dec 9, 2021
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    I tested every fuse, all good. I then ran a wire from the ignition switch to the signal terminal on the firewall solenoid. When I turned the key, it started cranking the motor so I let go of the key and it kept cranking. I had to hit the battery master to get it to stop. That circuit must be controlled by the ECM or a relay I guess. I’ll be at freightliner when they open tomorrow. I’ll let you know what I find out and once again thank you so much for your help!
     
    Bean Jr. Thanks this.
  8. Big Road Skateboard

    Big Road Skateboard Road Train Member

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    You wired it to the ignition side of the switch instead of the starter signal side. You really need a test light. Super simple to determine what each position of the switch does.
     
    Bean Jr. Thanks this.
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