Question on my planned car electrics - please see diagram

Electrical & Ignition

  1. David Womby

    David Womby Active Member

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    This is for my 360-powered Bristol 408.

    The goal is for the electric cooling fan to run under two circumstances

    1 - whenever the thermostatic switch B triggers it because of high coolant temperature. This should trigger the relay at D regardless of the ignition being 'off' or 'on'.

    2 - whenever the manual switch A is turned on but only if the ignition is turned on.

    Electric-fan-rewire.jpg

    Sorry - I was born and raised in the UK and still write 'earth'. US translation = 'ground':usflag:

    My main question is: is there any danger if the ignition is 'on' and both the thermostatic switch B and the manual switch A are 'on' simultaneously. I think it should be OK. It means the relay at D will be getting +12v from both sources but I don't think that's a problem and I don't think I need diodes to protect the two switches, do I? Or do I? Any help will be appreciated.

    David
     
  2. Don Frelier

    Don Frelier FBBO Gold Member FBBO Gold Member

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    That will work.
    DC- or DC-Com would be more accurate but you have the right idea.
    A good sized fuse will be in order for the fan probably 25-30A and stranded copper wire 14gauge or larger.
     
  3. black64

    black64 FBBO Gold Member FBBO Gold Member

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    If your manual switch is closed and your temp switch is closed and you turned ignition switch off you would back feed ignition switch circuits until the thermostat switch opened.
     
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    • David Womby

      David Womby Active Member

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      Thanks. yes, will be using appropriate sized fuses and wiring.

      Darn. I hadn't though of that. Thanks. Would a diode between switch A and the relay stop that?

      David
       
    • black64

      black64 FBBO Gold Member FBBO Gold Member

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      A diode would work since relay coil doesn't draw much current. Also your fan will run after you shut ignition off until the thermostat switch opens.
       
      Last edited: Feb 22, 2021
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      • Don Frelier

        Don Frelier FBBO Gold Member FBBO Gold Member

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        Maybe just add another relay:
        upload_2021-2-22_12-33-6.png
         
      • David Womby

        David Womby Active Member

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        Good. That's what I want. Thanks.

        David
         
      • vance.dykes

        vance.dykes Well-Known Member

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        it takes less voltage to ground a circuit than to power it. most new cars fan control's are done that way for longer life of the fan and relays.
         
      • David Womby

        David Womby Active Member

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        Thanks. I see but I'm not sure I can work out how to do it that way and have my initial objectives met - thermostatic switch to operate fan with ignition either off or on but manual switch to only operate fan when ignition is on. Any ideas how to wire that?

        David.
         
      • 747mopar

        747mopar FBBO Gold Member FBBO Gold Member

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        Just another thought, if the key is off why run the fan? I know it will cool what water is in the radiator but without the engine running to circulate the water it won't cool the engine unless of coarse you have an electric water pump?
         
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        • David Womby

          David Womby Active Member

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          Ah! I hadn't realized that. I assumed some degree of convection would continue a low circulation after engine shutdown. My last car was setup from the factory this way to run a fan after shutdown if it was above a certain radiator temperature. Maybe I am trying to be unnecessarily clever?

          David
           
        • Derwud

          Derwud Well-Known Member

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          here is how I do it.

          cooling fan.jpg
           
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          • vance.dykes

            vance.dykes Well-Known Member

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            gm / chrysler made a ground temp switch use as trigger for on of the fan and the fan can be powered all the time look on you tube for wiring a relay with ground trigger on. using 2 relays can be used for the way you want or with the right relay you could use 1 relay.
             
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            • David Womby

              David Womby Active Member

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              Thanks for the replies and suggestions ( and circuit diagram ). I will think this through tonight and may be back again with more questions. I really appreciate all the help.

              David
               
            • 1968rt

              1968rt Well-Known Member

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              What you need is a relay with multiple contacts. The normally closed contact will will have 12vdc that has a switch in the circuit. It will send power only when you apply the switch. The normally open contact will have 12vdc from a separate circuit. When the coil is energized the contact closes and sends the 12v through. The coil power will come from the temperature switch. Make sure all power leads are fused. I can draw you a circuit but Im pressed for time right now
               
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              • Nacho-RT74

                Nacho-RT74 FBBO Gold Member FBBO Gold Member

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                I already have diagrams and a thread for these ( even being dual fan setup ) to make it work automatically and also manually ( override switch ) ... and with AC compressor sensing too. Will search and post. Even using a lighted override switch which will turn on when fan is working ( either automatically or activating light switch ) lol

                I made it being triggered by ground. No need for diodes.
                 
                Last edited: Feb 23, 2021
              • 451Mopar

                451Mopar Well-Known Member

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                What Derwud posted above is what you want, switches trigger on low / ground side, and relay controls power to fan, not the fan ground side.
                 
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                • Nacho-RT74

                  Nacho-RT74 FBBO Gold Member FBBO Gold Member

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                  couldn't find the thread but here is a diagram. Is posted for dual fans but works the same with single fan just using what you requires.

                  If ammeter is being used, relays shouldn't be conected to batt but alt

                  dual fan option FULL.jpg

                  here is a set of temp sensors could be used in need of a dual temp setup. These are not Chassis grounded so need to be grounded at the input. It could be used for positive too, but since I made the diagram to be triggered by ground, well...

                  IMG_1065.jpg
                   
                • Nacho-RT74

                  Nacho-RT74 FBBO Gold Member FBBO Gold Member

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                • Don Frelier

                  Don Frelier FBBO Gold Member FBBO Gold Member

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                  The relay for AC here will never be energized? There is no power shown to the coil
                  The lighted switch option is not shown very clearly?
                  dual-fan-option-full-jpg.jpg
                   
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