Starter Solenoid AND Mag Switch?

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by RocketScott, May 1, 2018.

  1. RocketScott

    RocketScott Medium Load Member

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    Apr 10, 2013
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    Cool, thanks for walking me through it.

    This truck has so much janky wiring. In stead of using the factory harness they ran wires all over the place. I just figured out the wiring for the transmission and air drier. They had the power for the backup switch going to the air drier and then ran another wire from the starter solenoid to the backup switch. I guess to their credit they put a fuse on it.
     
    Tug Toy Thanks this.
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  3. pushbroom

    pushbroom Road Train Member

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    235083 shows up as a 2013 337. Is it one digit wrong? There is not an option for 2 different trucks for that serial number provided.

    Using a generic diagram it shows the coil side of the firewall mounted solenoid powered from the key by a 10 amp fuse.
    The switched side of the solenoid has power coming from the main power terminal on the starter motor, to the solenoid and back to the starter solenoid s terminal.

    By the math you have done it seems as though the firewall mounted solenoid is not needed. However the solenoid could also be there to isolate the cab wiring, key switch, and shared fuse, from the much more likely to fail starter solenoid wiring and solenoid as they are mounted out side the cab and subject to the elements.
    Also if the starter solenoid was wired directly to the keyswitch, and the solenoid had high current, it would blow the 10 amp fuse. The next step for many people would be to install a larger fuse. Now the cab wiring has risk of melting down to due high current load, as does the key switch, and the other components that share the same keyed power.
    I know that isn't explained the best, but I hope you understand what Im trying to say.

    I would keep the firewall mounted solenoid as it keeps the wiring factory, keeps less load on the keyswitch, and isolates the circuits.
     
    spsauerland Thanks this.
  4. RocketScott

    RocketScott Medium Load Member

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    Apr 10, 2013
    Kentucky
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    Good catch- The last part of the VIN is KD285083
    I was reading the VIN off a fuzzy picture on my phone. The first 8 looked like a 3

    The wiring diagram I have shows the power for the start switch coming off the Main Acc Relay, which comes through a couple circuit breakers right off the positive lug on the starter. I'm looking at SK19517 that I got from super miller. Some things are different with my truck but it's been pretty reliable.

    I understand what you are saying, I'm not the type to put bigger fuses in to "fix" a problem. I'm trying to resolve sins of the past and get everything closer to factory while upgrading/improving where I can. I ordered all the weatherguard and packard parts to repair things correctly. Since photobucket crapped out i haven't been able to post pics, otherwise you would see how bad it is and what I have done so far.

    I'm not opposed to putting a separate mag switch/solenoid in. I just don't like being part of the department of redundancy department. The more moving parts there are the more things that can go wrong. I also have to question everything on this truck. Lots of years of "Just make it work" that I have to unwind. It seems like wherever something didn't work they added wires, circuitry, or air lines till it did.

    Is there a resource that I'm missing or do you guys that ask for the last 6 of the VIN work for a dealer?
     
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