3406B coolant temp sensor location

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by mile marker 27, Jan 1, 2022.

  1. mile marker 27

    mile marker 27 Medium Load Member

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    Got the ‘87 379 Pete home, that I just bought yesterday and I’m needing to figure out why my heat gauge isn’t working? Where’s the coolant temp sensor located? 4MG 3406B engine.
     
    Oxbow Thanks this.
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  3. 062

    062 Road Train Member

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    If it’s the same as the E. Should be top right front corner of the head. It may be easier to see looking down from the top.
    @wore out or @Oxbow can tell you for sure.
     
    Oxbow Thanks this.
  4. wore out

    wore out Numbered Classic

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    It’s gonna be in the top of thermostat housing like @062 said. It’s the 1 wire sending unit.

    key on ground the wire gauge should show full hot. Is a short test to check harness and gauge. Around 94 the changed the resistance on the sending unit and gauge. The wrong sending unit can work the gauge but allow it to read wrong so be careful there
     
    Oxbow and 062 Thank this.
  5. mile marker 27

    mile marker 27 Medium Load Member

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    43F8B6F5-DA47-4968-90B2-35A36E78A3C2.jpeg
    I’ve found it. You’re suggesting that I ground that wire, while key is on and if gauge is good, it should go to, “hot”?
     
    wore out and Oxbow Thank this.
  6. Oxbow

    Oxbow Road Train Member

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    Yes, that will test the gauge and wiring.

    What @wore out said about getting the right sender for that specific gauge is true. After rebuilding one of ours I put a new sender in while the engine was out, just cuz. It read about 10 degrees too high with the new sender. I ended up putting the old one back in.
     
  7. bjytech

    bjytech Light Load Member

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    If what I think I see in the picture is right there is no fixing that gauge as it is a gas filled line that is sealed and runs from the gauge to the sender. Once they quit working you throw them out as there's' no fixing them, you will have to get a new gas filled gauge like it has or replace it with a new electric gauge and sender.
     
    Goodysnap Thanks this.
  8. mile marker 27

    mile marker 27 Medium Load Member

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    I probably should start a new thread but since y’all know you’re stuff, I’ll test you. Took the battery box cover off and there were 4 wires chewed through going to a solenoid on the inside of the box. The solenoid has a hot wire from a battery, a brown and black wire going to a 30 amp two post fuse and then the two wires go into the firewall. The brown and black wire are on the same small post on the solenoid and hot wire from the solenoid to one post on the 30 amp fuse. A brown and black wire come from the 30 amp fuse also. Basically there’s 4 browns and 4 blacks, that all got chewed through and I’m not sure which go to which or what their for?
     
    wore out Thanks this.
  9. mile marker 27

    mile marker 27 Medium Load Member

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    That line is rather thick, for just an electrical line, was my first thought.
     
    Bean Jr. Thanks this.
  10. wore out

    wore out Numbered Classic

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    Can you get me a picture of it?

    I have an 88 out in the pasture I can try to help you with it.
     
  11. mile marker 27

    mile marker 27 Medium Load Member

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    32D2A04C-4A13-44AA-9ABA-197C313DEC75.jpeg Here’s that little 30 amp fuse/relay. The hot wire on it went to the solenoid. The opposite side post on the solenoid went to + battery. The solenoid has two small terminals on top. One has a white wire to - ground, the other is not used.
    The other post on this relay/fuse had both brown and black wires stacked.
     
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