what does this part do?

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Coolbreezin, May 19, 2022.

  1. Coolbreezin

    Coolbreezin Light Load Member

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    Hello all. I have a '91 379 with a 3406b engine, all mechanical. I was told this part is the switch for my jake. I pulled the wires and cleaned them as best I could and ran a wire to connect them. Didnt effect the engine at all. I watched a video of a guy with a different year 379. He pulled the wires from his switch and put a test wire in the ends and his jake kicked on. His switch was tucked up under the floorboard by his pedals. Anyone know what this part does and/or where my jake components are? Thank you. thumbnail_Resized_20220519_140204.jpg thumbnail_Resized_20220519_140234.jpg
     
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  3. RocketScott

    RocketScott Medium Load Member

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    I'm not familiar with cats, so I'm just guessing that that is the throttle switch. You should also have a clutch switch

    Both need to be closed for the jakes to work. Step on the throttle or the clutch and the jakes stop working. Both need to be adjusted properly

    The picture I have won't upload but the power to the jakes goes like this:

    Switched Accessories to the on/off switch - 242
    On/off switch to the clutch switch - 289
    Clutch switch to throttle switch - 290
    Throttle switch to selector switch - 223
    Selector switch to jakes - 224 (middle bank) and 225 (front and rear bank)

    ETA: There should be a 1N5404 diode on the throttle switch, it prevents arcing that will wear out the switch
     
    Coolbreezin Thanks this.
  4. AModelCat

    AModelCat Road Train Member

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    Switch on the clutch is so the jake only activates when the clutch pedal is released.

    That switch there I believe is what will shut down the jake once the engine hits idle speed, so it won't stall the engine out.
     
  5. Oxbow

    Oxbow Road Train Member

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    It is a continuity switch that makes connection when there is no pressure on the throttle pedal. It also has a jamb nut on it and can adjust low idle speed.

    # 3 in the diagram that @SmallPackage provided (handy diagram - thanks). If you already bypassed the switch then your problem is elsewhere.

    Some people replaced this switch with a jake switch like what is on the Big Cams, but these aren't very expensive and easy to replace.
     
  6. Oxbow

    Oxbow Road Train Member

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    A test light should help you find the problem. Id neither wire is hot with the key on then it is before this switch. If both wires are hot at the switched when connected the switch is working and the problem is downstream of that.
     
  7. Coolbreezin

    Coolbreezin Light Load Member

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  8. SmallPackage

    SmallPackage Road Train Member

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    Technically the throttle switch and/or the clutch switch isn’t needed for the Jakes to work. You could bypass it and just use the toggle on dash to activate it. But it will be full manual on and off thru that switch only.
    The first generations of the system were like that. The Cummins in my ‘67 is like that. No throttle or clutch switches. Just key power to toggle on dash and straight to Jake boxes on heads.
     
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