1979 K20 Speedometer Question

Discussion in 'Tech Discussion' started by Project79, Oct 11, 2018.

  1. Project79

    Project79 Junior Member

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    Joined:
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    Location:
    Hector, Minnesota
    First Name:
    Cody
    Truck Year:
    1979
    Truck Model:
    K20
    Engine Size:
    350
    Can anyone tell the name and function of this? I'm assuming it is for the speedometer cable but i have no idea what it does or how it works.
    1005180946.jpg
     
  2. Blue Ox

    Blue Ox Turning Diesel Fuel Into Fun

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    First Name:
    Derek
    Truck Year:
    1985
    Truck Model:
    K20HD
    Engine Size:
    6.2L
    I'm only guessing, but my first guess would be cruise control.
     
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  3. WFO

    WFO Full Access Member

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    Location:
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    First Name:
    Dan
    Truck Year:
    1986
    Truck Model:
    K20
    Engine Size:
    350
    Yeah, that's cruise control.
     
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  4. Project79

    Project79 Junior Member

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    First Name:
    Cody
    Truck Year:
    1979
    Truck Model:
    K20
    Engine Size:
    350
    Thanks guys. I appreciate it. My speedometer isnt working, trying to figure out what it is that is causing it.
     
  5. mrburitto

    mrburitto Junior Member

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    Location:
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    First Name:
    Cody
    Truck Year:
    1979
    Truck Model:
    K20 Cheyenne Camper Special
    Engine Size:
    350
    Same truck and issue. I am planning on looking into it tomorrow. Ill let you know what I find.
     
  6. 75gmck25

    75gmck25 Full Access Member

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    Location:
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    First Name:
    Bruce
    Truck Year:
    1975
    Truck Model:
    K25
    Engine Size:
    5.7
    Its the cruise control servo. One cable goes from the transfer case to the servo, and then a 2nd cable from the servo to the speedometer. You could get a non-cruise cable and just connect it directly from your transfer case to the speedometer. I found one for my '75 K20 at Autozone for about $18. However, its a PIA to get it connected up to the back of the speedometer.

    1. I would unscrew the cable that runs from the servo to the speedometer (the one running over the firewall), and chuck the end into a drill. If you can spin it with a drill and get the speedometer to work, then the problem is lower in the system. IIRC, I think you have to run the drill in reverse.
    2. If speedometer worked in step 1, then put the upper cable back on the servo and disconnect the cable at the transfer case. Do the same drill test to determine if that cable is good.
    3. If the first two steps work, you have to fix the connection at the transfer case. The speedometer gears are plastic or nylon, and available from most parts places.

    Bruce
     

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