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  •  » Horn re wiring

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    5/09/2021 2:27 PM  #1

    terry
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    From: vandalia, illinois
    Registered: 4/01/2013
    Posts: 1,871

    Horn re wiring

    I rewired my horns a few years ago with a relay to give it a better sound.  What I did was intersect the wires under the hood and installed the relay there.  With the problems I was having with my steering column I decided to rewire the horn button/steering column. So I started retracing the wires so I could rewire the horn button wire to the wiring diagram that BB sent me.   I cut the yellow and grounded the end that goes up into the column and capped off the end going to the light switch.  Then i rewired the relay so that the ground is what is being switched by the horn button, It works great, I  push the button the horn honks!  Out of curiosity I put a volt meter on the contacts of the horn button, positive to the yellow negative to ground nothing (It's grounded to the column now)  put the positive to the other terminal and I get 12 volts???   I retraced my wires all the way back to the relay and disconnected the yellow/blue wire from the relay and checked it.  no voltage coming back from the steering wheel but If I put the positive terminal of the meter to the #87 terminal I get 12 volts????  I have a pretty good working knowledge of wiring but I'm better at AC than DC.  Its hard to describe this scenario and right now I'm grasping at understanding it.  Should I get 12 volt back thru the relay like this??  It's not melting wires or blowing fuses so something must be right???

     

    5/09/2021 7:29 PM  #2

    wsinsle
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    From: Belle Vernon, PA
    Registered: 4/01/2013
    Posts: 1,703

    Re: Horn re wiring

    Simple ohms law Terry. If you have 10 volts and 2 50 ohm resisters in a circuit, each resister will drop 5 volts. If you have a 10 ohm and a 90 ohm in a circuit the 10 ohm will drop 1 volt and the 90 will drop 9 volts. Now in your horn circuit, lets say the relay has 200 ohms and with the horn button closed lets say the switch is zero ohms. All 12 volts drop across the relay and it picks up, you will see zero volts at the switch. Now open the horn button, the resistance across the switch now is infinite. You now have 2 resisters in series in your circuit, one 200 ohms and  the other infinite. Because infinite is so much larger that 200, all 12 volts drop across the open switch. Hope this makes sense. 

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    5/09/2021 8:19 PM  #3

    Bullet Bob
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    From: Montrose, Colorado
    Registered: 4/01/2013
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    Re: Horn re wiring

    Terry:  What Walt says makes.  Explained another, more complex way, you are seeing is the 12V applied to the HOT side of the relay coil.  When you put a volt meter on the non grounded side of the horn button you are seeing 12V applied on the relay coil...just "potential".  Since the volt meter has very high resistance to ground it doesn't allow any current to flow but just reads the voltage at that point.  If you left the volt meter connected and pushed the horn button you should see zero voltage since, as Walt explained, all the voltage will be "dropped" by the resistance of the relay coil and at that point the meter is measuring the difference of potential across the closed horn button contacts...which should be zero, or very nearly so depending on the resistance of the horn button. 
    Simply put: Without some current flow (closed horn button) there is no voltage drop across the resistance (relay coil) so the volt meter reads the potential applied to the other side of the coil. 
    I just used this same process to check the continuity of the Torque Converter Lockup solenoid in the 4R70W without having to drain and remove the transmission pan.  I just connected the VOM to the wire the ECM uses to ground the solenoid coil (like the horn button) and turned on the ignition key which applied a constant 12 volt to the circuit.  Since I saw 12v on the ground side when not grounded, I knew the circuit was complete.

     

    Last edited by Bullet Bob (5/09/2021 8:37 PM)

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    5/09/2021 8:38 PM  #4

    6sally6
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    From: Surfside Beach,SC
    Registered: 4/02/2013
    Posts: 6,853

    Re: Horn re wiring

    So he ain't gonna let the smoke out of his horn???!
    6sal6

    Get busy Liv'in or get busy Die'n....Host of the 2020 Bash at the Beach/The only Bash that got cancelled  )8
     

    5/10/2021 5:36 AM  #5

    terry
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    From: vandalia, illinois
    Registered: 4/01/2013
    Posts: 1,871

    Re: Horn re wiring

    It makes sense!  I thought about it and tested and thought and tested.  I just wasn't grasping the answer.  I knew if it wasnt blowing fuses something was probably right and NO SMOKE.  Like I said I'm better at AC and I know I NEVER switch a ground on AC.  Thanks Walt and BB!!

    Last edited by terry (5/10/2021 5:37 AM)

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