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Thread: Relay Wiring Help

  1. #1
    Variable Bitrate
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    Relay Wiring Help

    Not really CarPC related but I figured you guys could help me understand the wiring.

    If you look at the following digram. The FPSS basically monitors fuel pressure and cuts power to the solenoids if it goes too low.

    What I don't understand though is how does the FPSS cut the relay when it has already been tripped?

    Can someone explain that too me?



    Cheers

  2. #2
    Variable Bitrate
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    Removes the path to ground?

  3. #3
    Variable Bitrate
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    And that would disable the relay? The way i understand relays is the 12V switched is flowing through the relay triggering it then out the other side to the FPSS. So its already triggered the relay, who cares where it goes now.

    Am i understanding that wrong?

  4. #4
    Maximum Bitrate fixerofallthing's Avatar
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    Without seeing the terminal numbers on the relay I am guessing, but the red and white wires go to #85 & #86 terminals on the relay, they are the coil contacts of the relay. They must be 12v+ and 12v- to keep the relay contacts closed. If either one is lost the relay opens

    The other 2 wires go to terminals 87 and 30 and are the load contacts. The are normally open without the coil energized.

    Maybe this will help.
    The Car 2005 Scion xB

    The Truck 2004 GMC Canyon

  5. #5
    Variable Bitrate
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    Quote Originally Posted by SKiTLz View Post
    And that would disable the relay? The way i understand relays is the 12V switched is flowing through the relay triggering it then out the other side to the FPSS. So its already triggered the relay, who cares where it goes now.

    Am i understanding that wrong?
    IMO, yes, your understanding is wrong. Relays are not flip-flop type affairs, i.e., once switched they stay switched until told otherwise. They only stay switched (or triggered as you use it) as long as current flows on the low power side of the relay.

    You can stop reading if the above was enough. The info below may not be well stated technically (for example, the 'coil' I reference may be a solid state device), but hopefully it makes the point. But first, for any circuit to work there has to be a source and a sink (ground), OK?

    A relay generally has 4 leads. On one side is the circuit path is from the low power source, through a coil, and off to it's ground. The other side has a high power source, through a switch, and off to it's ground.

    A relay works by using the low level current flowing through the coil. When the coil has been energized, it closes the contact/switch on the high power side. In this state current is flowing through both elements of the relay - low power on the coil side and high power on the switch side. If by design, or by accident, the current on the low power side is interrupted, the contact/switch opens, and the high power side is shut down.

    In your specific application, the current flowing through the relay from the switched source flows out through the FPSS to ground. If the FPSS cuts out, then the path to ground is interrupted and no current flows. This de-energizes the coil (that holds the contact closed), opens the contact/switch, and current flow stops.

  6. #6
    Maximum Bitrate FusionFanatic's Avatar
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    Quote Originally Posted by SKiTLz View Post
    What I don't understand though is how does the FPSS cut the relay when it has already been tripped?
    FAQ: Relays:

    relays don't get "tripped" like a circuit breaker does. a circuit breaker is controlled by the same circuit that it cuts power to. a relay is turned on and off by a different circuit than the one it's controlling.

    relays are like a momentary switch, meaning the switch has to be 'held down' to remain on. think of it like a standard pushbutton switch where you have to hold it down with your finger to make it stay on, power is cut when you let go. a relay is doing the same thing except it's a magnetic coil holding the switch down for you. when you energize the coil, the switch is 'pressed down'. when you de-energize (cut power to) the coil, the switch is 'let go'.

    in your specific setup, the FPSS simply cuts the ground to the coil. the relay then 'lets go' of the switch (cutting power to the solenoids).


  7. #7
    Variable Bitrate
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    Perfect guys.. All great descriptions and very helpfull. Makes perfect sense too me now.

    I had read the Bcae1 site on relays but the diagrams/terminology is a little "geeky" for my understanding.

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