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Negative Relay Switching?

Discussion in 'The Hokey Ass Message Board' started by GOGREEN, Aug 15, 2020.

    Joined: Aug 26, 2015
    Posts: 4


    I am planning installing my own DIY fuse/relay panel in the glove compartment of my 50 Chevy with SBC, Alternator and HEI. I want to use halogen headlights and electric fuel pump so I'm upgrading to relays.
    I don't plan on any other fancy accessories, no power windows, no A/C...
    I was just about ready to dive in, when I started reading about Negative Relay Switching? My question is, with a 60 amp alternator and minimal accessories, do you think I need to go Negative Relay Switching for increased safety? I'm not a master in electrical but I am doing my best to figure it out. I just don't want to burn my truck down.
  2. That wasn't very helpful.

    Truckdoctor Andy likes this.
  3. fabricator john
    Joined: Mar 18, 2010
    Posts: 86

    fabricator john
    from venice fl.

    A: be nice
    B: i agree with Phil
    C: run a one wire alt, to the hot side of the starter, then pull main hot from there to the dash FUSED , then to your fuse box (and your safe and double fused ) ground switching works and can complicate things a bit , id skip it ..
    D: aftermarket kits from american auto wire, painless,ron francis,coach controls etc,, are not really expensive and have all the potential problems sorted out and id use one ,,, unless you REALLY REALLY want to make our own as a project, you will spend more time and money making your own ,,
    fabricator john
    miss you dad
    Hnstray likes this.
    Joined: Aug 26, 2015
    Posts: 4


    I guess I'm not too brite. I posted in the wrong place. My first time here I didn't expect to be treated like that. I came here because I am trying to learn from those more experienced than me. There's enough of that crap on F-book.
    But Phil and John, thank you for the advice.

  5. grumpy65
    Joined: Dec 19, 2017
    Posts: 378


    Do a search for "WIRING 101"

    Hey @GOGREEN , welcome to the HAMB.
    The thread above is a wealth of info on wiring.
    Maybe @Crazy Steve will chime in here personally to help you out.
    Last edited: Aug 16, 2020
    Hnstray, VANDENPLAS and GOGREEN like this.
    Joined: Aug 26, 2015
    Posts: 4


    thanks for the welcome, @grumpy65, I have been studying, and will keep doing so, but after a little while it gets a little muddy for me and that's why I ask the more experienced, like yourself.
  7. Wrench97
    Joined: Jan 29, 2020
    Posts: 142


    Negative switching is one of the engineering answers to the NHTSA call for less vehicle fires.
    It works because if a wire rubs through to ground it simply energizes the circuit and turns the light on, the circuit still has to have a fuse on the hot side. Another benefit is lower amp loads on interior switches if you mount the relays outside the passenger compartment and are running low amp ground circuits to operate the relays.
    My opinion apply the KISS principle and use relays only if needed to lower the load on a switch if it's a simple circuit keep it simple, for a high speed high amp fan circuit use a high amp relay to shorten the wire run and take the load off the switch
    Last edited: Aug 16, 2020
    Hnstray, GOGREEN and pprather like this.
  8. grumpy65
    Joined: Dec 19, 2017
    Posts: 378


    Not sure I am one of the "more experienced" when it comes to auto electrics. Basic stuff yes, but I am pretty much in the dark with anything above that (pardon the pun). Like you, I educate myself the best I can and have a go with most stuff. By the time I know enough to do all I need to do, I will probably be too old to do it anyway.
    Last edited: Aug 16, 2020
    GOGREEN likes this.
  9. corncobcoupe
    Joined: May 26, 2001
    Posts: 5,064

    Staff Member

    I moved your post to the Main Message Board
    GOGREEN and ffr1222k like this.
  10. mark latham
    Joined: Oct 24, 2018
    Posts: 104

    mark latham

    Whenever possible I switch relays with the ground for the reasons stated above.
  11. Mr48chev
    Joined: Dec 28, 2007
    Posts: 27,510


    I've spent the last fifteen minutes looking for real info on it from a number of sources and what I primarily find is that BS artists try to out BS each each other about it.

    Simply put you are switching the ground side of the relay rather than with 12 V from the switch.

    What we know as a normal positive switched relay in the top image with power from the switch going to the 86 pin.
    The lower image showing the Negative switched relay with the switch between the relay and the ground on the 85 connector and the 86 wire hardwired to the relay rather than switched.

    To me this would be a real pain in the ass for anyone who followed behind you and had to trouble shoot the wiring including any mechanic you have work on it. It goes backwards of normal systems and normal trouble shooting procedures.
    Personally I just don't see the point of doing it just to be doing it.

    Some systems such as a temperature controlled fan on the radiator are negative switched because the temp switch grounds the relay.

    Switched ground.jpg
    GOGREEN and TrailerTrashToo like this.
  12. I always switch the ground side of relays when I use them.

    I’m such a hoodlum....

    Sent from my iPhone using H.A.M.B.
    razoo lew likes this.
  13. WB69
    Joined: Dec 7, 2008
    Posts: 1,277


    Welcome aboard first off. Agree with the KISS method myself. But, there are some good, reasonably priced after market kits out there that make it much simpler and have good support.
    GOGREEN likes this.
  14. jaracer
    Joined: Oct 4, 2008
    Posts: 494


    In heavy truck, many circuits are fused and/or switched on the ground side. Kenworth is the first one that comes to mind.
    62SY4 and Truckdoctor Andy like this.
    Joined: Aug 26, 2015
    Posts: 4


    @Mr48chev , Thank you for the diagrams and your input, I'm starting to feel confident enough to wire up the headlights/relays.
  16. birdman1
    Joined: Dec 6, 2012
    Posts: 994


    I think the late 50's Ford's used switching the ground for the power windows. I guess to reduce the chance of fire? I'm shooting in the dark here.
  17. GTS225
    Joined: Jul 2, 2006
    Posts: 1,173


    Just to clarify something. Modern electronics use the "negative switching" technique because the ECU's and computers really like their grounding, so the processor is hard grounded, and most, if not all the inputs are fused hot leads. Even the injectors are a constant, fused 12v+. That way, the processor does the operations by switching the ground, thus creating additional grounding for the processor.
    Seems the processors don't really like the voltage spikes of hot switching all that much, and it shortens their life. Not a good thing for a production-for-the-masses vehicle.
    I suspect this is where the idea of "negative switching" comes from. It's not necessarily wrong, just doesn't really have a place in the vintage automotive world.

    Just my 2-pence......Roger
  18. I'm personally not a fan of switching grounds. There are times when it's unavoidable (such as horns when using a steering wheel horn ring) but as anyone who has had it happen to them, when the wire grounds ahead of the switch you don't have a controlled circuit anymore (blaring horn that you usually end up pulling the wire off of because that's quicker than crawling under the dash to find the fuse/relay). Not all circuits will announce that they're still on like a horn, the possibility for 'issues' is more than I care for...
    TrailerTrashToo likes this.
  19. Wrench97
    Joined: Jan 29, 2020
    Posts: 142


    I believe you'll find they swap positive and negative at the switch to make the motor run in opposite directions.
    Hnstray and seb fontana like this.
  20. fabricator john
    Joined: Mar 18, 2010
    Posts: 86

    fabricator john
    from venice fl.

    if i may chime in , ive wired dozens and dozens of cars at work (28yrs) for other people ,customers .and never had any problems ,if things get goofy ya sit down and draw it out ,, If your gonna keep the car relatively simple then do youself a favor and buy a kit . they are kinda like paint by numbers and well instructed .. may i suggest a american autowire highway 15 ,(if ya want more circuits theirs other choices), (no relation or attachment by me) they come in a big box , instructions are clear and you just follow the steps the wires are bagged a,b,c,d,e, etc and thats how you install them one circuit at a time charging,lighting,gauges ,accessories etc.. even makes layout easy as you are not tied to a pre wired box so you can run them how you want comes with all the switches accept door jamb and wiper and all the wires are individually labeled every 10" or so makes going back in easy ,, they get like 400 for the kit if ya count your time ya cant get that much wire together for that ,,, you wont have any problems from trying to reinvent the wheel ,which is what gets novices in trouble ,, it the EASIEST and SAFEST ROUTE ,, just my 2 cents ,,,,,,,,,, and dont buy cheaper kits you wont be saving anything (allways pissed me off crap wiring on a 20k+easy car. and cheaper kits take longer because somthing is allways goofy)
    Fabricator John
    miss you dad
    Hnstray and Weedburner 40 like this.
  21. plym_46
    Joined: Sep 8, 2005
    Posts: 3,997

    from central NY

    Pretty common in overdrive circuits where the governor divides the solenoid path to ground. Many Japanese vehicles have lighting circuits with switched grounds.
  22. 1oldtimer
    Joined: Aug 21, 2003
    Posts: 7,586


    I've done both, I see the benefit IF your running it through the firewall or something like that. But, remember that the same relay has power wires going to it and coming from it that still can ground out and possibly burn.

    A lot of the negative controls for newer cars are nice for keeping arcs and spikes down to a minimum from highly sensitive electronics, which isn't a concern on the cars we have.

    The best bet is to make sure ALL your wires are protected properly and run away from heat or sharp edges.
  23. Budget36
    Joined: Nov 29, 2014
    Posts: 4,209


    Hang on a second with the splicing of the power wire, in general when using a relay say for energize the relay with a smaller gauge wire, I/e 22 gauge (check relay current draw and do some googling), then power to the device is run with a larger gauge wire to handle the load that the relay is taking.

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