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Thread: I/O - Ground or High Side?

  1. #1

    Default I/O - Ground or High Side?

    Wiring up everything at the moment.
    I have a Meziere Water pump, Spal Electric Fan and stock reverse lock-out on T6060 for some additional outputs, all via relays.

    Got the 558-400 J1A/B add on harness, which plugs straight into the engine harness.
    2 will have to be either G or H, and the other what ever is left.

    Whats the pro's / cons for selecting Ground or High for the output switching.
    I'm assuming Ground means the ECU switches the pin-out to earth, and H means the ECU supplies +12v to the pin-out, is that correct?
    Regards, Dave

  2. #2
    Join Date
    Nov 2013
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    It's really all personal preference.
    I Prefer to run a High side signal to trigger relays, so if the wire shorts out for some reason, it can't accidentally turn on a relay.
    Especially if it's a device that needs to meet specific requirements or it does damage, like a transbrake, or a a nitrous solenoid.
    Which if activated at the wrong time, could make for some very costly repairs.

    Things like Fans, might be better to run a ground side relay for just that reason, if something happens and the trigger wire gets cut(or melts) and grounds out, at least the fan will be turned on.

    You should always use relays with a diode/resistor in them, so that a device (like a fan) can't backfeed and keep power going back through the ignition, which would keep the engine running even with the key-off.
    Last edited by S2H; 01-13-2016 at 08:49 AM.
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  3. #3

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    Good point on the ground shorting and actuating something accidentally. Never thought about that. My mindset was always to let the ECU do ground side switching. Seemed like it would be easier on the ECU instead of switching power.

  4. #4
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    Dec 2009
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    Quote Originally Posted by 3 window View Post
    My mindset was always to let the ECU do ground side switching. Seemed like it would be easier on the ECU instead of switching power.
    It's not any easier on the ECU. The amperage draw is the same on the positive and negative side of a circuit.
    May God's grace bless you in the Lord Jesus Christ.
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  5. #5

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    Quote Originally Posted by Danny Cabral View Post
    It's not any easier on the ECU. The amperage draw is the same on the positive and negative side of a circuit.
    LOL, X2! That's why they are called "circuits". Current is the same anywhere in a circuit "loop."

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