Question on Cummins 4BT M -what does the magnetic switch do?

Discussion in 'Diesel Engines' started by Northeaster, Dec 22, 2013.

  1. Northeaster
    Joined: Sep 2011
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    Location: Eastern Canada

    Northeaster Senior Member

    Hi Folks,

    Just taking some parts off of my (new to me) 1992 Cummins 4BT 3.9M, in order to clean it up, prime and repaint. Have removed starter, alternator, wiring harness, cooling hoses, expansion tank, etc - but I don't know what function one part performs( which I think is the magnetic switch - located below starter mortor and attached solenoid - it has its own bracket that shares a mounting bolt with the starter motor, and it has about 5 or six wires on it from the wiring harness).
    Does it have something to do with the push button start? The pushbutton does not currently work, and there is a couple of corroded/ cut wires in the harness. All other gauges / panel functions work ok.

    Any help would be appreciated!
     
    Last edited: Dec 22, 2013
  2. whitepointer23

    whitepointer23 Previous Member

    sounds like a start relay. can you post a photo.
     
  3. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    Cant say for sure but many engines do not energise the Alt and other parasitic loads during engine start.???

    This is far easier with the start key switch ACC position and if needed a relay .
     
  4. BerettaRacer
    Joined: Dec 2013
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    BerettaRacer New Member

    On my Cummins 6BT's I've got something a little different. I've got magnetic senser drilled thru and mounted in the bellhousing area, it counts the teeth on the flywheel for govener and ECU control.
     
  5. Northeaster
    Joined: Sep 2011
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    Northeaster Senior Member

    thanks for the help! I think it may be the starter relay.
    I do have the sensor on the bellhousing as well. I thought it counted the teeth for the tachomter, as it does in my little Yanmar 2GM. You got me curiuos, is it used for this as well, or does the tach on the cummins usually work off the alternator, or another method?
    I am pretty mechanical, and am familiar with how the solenoid and starter work, but never really looked deeper, into the function of the starter relay. Just googled it, and think I understand now.
    I will not be putting the engine into the new boat for months, at least, so no hurry. But, I will want to find out why the push button doesn't work, or how the PO jury rigged itm when a bad wire or something caused the push button to not work. I noticed the key switch is not original, as it has the start position, just clockwise of turning it to on/ accessory, like a car. I can start it by turning key on, and then just crossing two temporary wires to the solenoid / effectively shorting it. Also noticed that the yellow wire with red stripe, which is connected to the push button is unhooked / cut in a couple of spots as it makes it's way along the wiring harness to this part, likely starter relay.
    I do not yet have a wring diagram for it.
    If anyone has further insight fire away! Is a starter realy normally used only when you have a oush button start like a marine or generator model, or is it also used when you have a car-like key switch?
     
  6. BerettaRacer
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    BerettaRacer New Member

    I don't have tach's on my Cummins, lol. Mine are gensets, only ECU's, electronic governors, and frequency meters on swbd. :D
     
  7. Northeaster
    Joined: Sep 2011
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    Location: Eastern Canada

    Northeaster Senior Member

    Here's a couple of pics - if someone could confirm this is the starter relay, and if it is needed for button or key start, I would appreciate it!
     

    Attached Files:

  8. boatbuilder41
    Joined: Feb 2013
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    boatbuilder41 Senior Member

    Its a start solenoid, these are installed on a lot of boats just to keep the start solenoid wire from over drawing the wire rating on long wiring harnesses. The typical marine starter is delco 40mt or newer style 42mt. Which draws a lot of amperage. You might say it is just a safety to insure the solenoid doesn't over draw the rated wire capacity of the start circuit
     
  9. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    into the function of the starter relay

    Some are set so the starter can not be engaged if the engine is already operating.
     

  10. Northeaster
    Joined: Sep 2011
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    Location: Eastern Canada

    Northeaster Senior Member

    Merry Christmas all!

    Not trying to beat this to death - but I compared the pics to my on-line part manual and it falls under the catagory of Engine instrument panel, and is properly called the Magetic Switch.

    Boatbuilder41 - I am confused by you calling it a start solenoid - the part I (perhaps incorrectly) normally call the starter solenoid is the small round cylinder part that is attached to the starter.

    I would just like to understand the proper name and function of these parts, for future troubleshooting.
     
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