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Relay's really that important when wiring lights?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Boss, Feb 27, 2002.

  1. Boss

    Boss 1/2 ton status Author

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    Relay\'s really that important when wiring lights?

    Are they and why?
    Wondering b/c my Hella 500 Driving lights ($55) came with a Relay.
    But my el cheapo Halogen lights ($8) that I'm using for the rear does not.
    Then I purchased a cheap set of ProComp Fog lights ($35) that did not come with them either.
    I'm thinking, as long as I have a fuse from the wire to the battery, I should be ok right? That's how I always done it [​IMG]
    Boss

    Pic of my truck Before N' After
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  2. Blue85

    Blue85 Troll Premium Member

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    Re: Relay\'s really that important when wiring lights?

    If you are using a toggle switch inside to turn then on and off, then it will be OK without a relay as long as the switch is rated to carry the current of all the lights. Where the relay comes in handy is when you have a lot of wire between the battery and the lights or a crappy switch or two in the line (like with the factory headlights). Then a relay can make a lower resistance path for the lights, brightening them up. If you are using heavy wire without a lot of splices or connectors, then you should be fine without it.

    <font color=green>Top signs you don't have a real 4x4:
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  3. greg83k5

    greg83k5 1/2 ton status

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    Re: Relay\'s really that important when wiring lights?

    relays are used to keep the high-amp power loop to the lights short, out of the cab, and relatively free of parasitic loss. when you run it through the switch, there is more resistance than with the relay, and less power gets delivered to the lights- not as bright...

    now you could hook your multimeter up and say, look, a full 14.5 volts coming through my switch, but when you apply the load, or the light, it's a different story....

    relays are good. i recommend them, however your stuff will work either way. party on.

    PS with a relay you can wire your lights right off the alternator output stud really easy, they'll be bright as hell, but won't last as long-

    greg

    * keep the greasy side down *
     
  4. marv_springer

    marv_springer 1/2 ton status

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    Re: Relay\'s really that important when wiring lights?

    Yes... it is important...

    The relay will allow you to run more power to the accessory. That circuit will be controlled by a lower amp circuit that will run thru the switch.

    Without the relay all the amps would go thru the switch and indicator and shorten the life of these things....

    Marv
     
  5. Brian 89KBlazer

    Brian 89KBlazer 1/2 ton status

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    Re: Relay\'s really that important when wiring lights?

    I agree with you Boss! I'm no electrical engineer or anything but the only reason I could see to use a relay is to avoid providing a switch that's rated for the amp draw in the circuit. In other words; with a set of 100W lights; you'd draw about 16.8 amps at 12V so you'd need a 20amp rated switch. The relay; from what i was told, is just to allow the switch to be a low amp cheapie! [​IMG] But now you've got to add a realy to the cost so don't quite follow that logic!
    Beyond that; the relay is only an extra electronic component to fail in the circuit. I think the simple switch like you describe is better to but I stand to be corrected! [​IMG]
    EDIT: .......oops [​IMG] no one had replied when I went to post then couldn't get in cause the site was down again. Good info guys!

    Brian
    89KBlazer

    Build it RIGHT or DON'T Build it at all!!

    <P ID="edit"><FONT class="small">Edited by Brian 89KBlazer on 02/27/02 08:41 AM.</FONT></P>
     
  6. Boss

    Boss 1/2 ton status Author

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    Re: Relay\'s really that important when wiring lights?

    Ok, looks like I'll be getting a relay for the fog lights then (it's only 55watts). Probably won't for the rear lights, since I won't be using them that much, only for reverse at night (and yes, will be wiring them into a switch as well).
    I'm definately no elictrical whiz kid...just know how to cut, crimp, solder and stuff, that's it :)
    Just found it odd how Hella came with a relay and Procomp's did not. thanks again.
    Boss

    Pic of my truck Before N' After
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  7. m j

    m j 1/2 ton status

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    Re: Relay\'s really that important when wiring lights?

    be sure to size the wire big enough to handle the load for that length of run.
    Hella is obviously a much nicer quality to provide the relay.
    with the relay in the rear lights you can use a 3 pos toggle to make it legal.
    1 position from a hot source to trigger rear lights
    another position souced from the green paired wires from the tranny rev switch to only trigger extra rear lights when in reverse
    third position extra lights off

    formerly 77chev
    still a jerk though
     
  8. Blue85

    Blue85 Troll Premium Member

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    Re: Relay\'s really that important when wiring lights?

    <blockquote><font class="small">In reply to:</font><hr>

    I'm no electrical engineer or anything

    <hr></blockquote>

    OK, but some of us are!

    <font color=green>Top signs you don't have a real 4x4:
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  9. Brian 89KBlazer

    Brian 89KBlazer 1/2 ton status

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    Re: Relay\'s really that important when wiring lights?

    Great Blue85! Then you can answer this question that I've wanted answered for years now!

    If I wire two alternators up; is it just a matter of wiring both battery leads to the battery's positive terminal like a single one does? Is there any issues to be had with a common ground?

    Thanks

    Brian
    89KBlazer

    Build it RIGHT or DON'T Build it at all!!
     
  10. Blue85

    Blue85 Troll Premium Member

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    Re: Relay\'s really that important when wiring lights?

    Don't do it. The regulators will never be exactly matched, so one will end up doing all of the work until you have enough load to drop the voltage below the threshold of both. What you really need is a single regulator that controls the fields of both alternators. I don't know if you can buy something like this or not. But this is off-topic now...

    <font color=green>Top signs you don't have a real 4x4:
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  11. Brian 89KBlazer

    Brian 89KBlazer 1/2 ton status

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    Re: Relay\'s really that important when wiring lights?

    One last question then Blue. I'm sure Boss will forgive me for jumping in on his post [​IMG]!

    Dual Alternators are an option on both 1-ton Ford &amp; Dodges. How do they wire them up?? How come they work?
    This is not meant to be provocative; just sincerely looking for answers. [​IMG]

    Thanks

    Brian
    89KBlazer

    Build it RIGHT or DON'T Build it at all!!
     
  12. 83ZZ502_Jimmy

    83ZZ502_Jimmy 1/2 ton status

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    Re: Relay\'s really that important when wiring lights?

    1st, please use a relay. I use a relay on almost everything I can.

    2nd, b/c do you really want 20A of power running through the passenger compartment. With a realy it only is .7A in the wire. This lets you run a nice small wire in the cab.(much easier to mange and hide)[​IMG] Instead of a big wire.
    Your lights will be much brighter and last longer with relay.

    Also, Hella lamps are IMO some of the best on the market. Originally used in germany, and for some reason germans know their lighting very well. The procomps are good but not like Hella. And if they put a relay in the box, you might as well use it.

    My.02,
    John

    <font color=blue>"When all else fails, read the instructions, or get a bigger hammer"</font color=blue>
     
  13. Blue85

    Blue85 Troll Premium Member

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    Re: Relay\'s really that important when wiring lights?

    Actually, I think that it would probably be OK. Just make sure that both alternators are the same. Of course, like I said, they may not share the load equally, but neither one would ever be made to work outside it's intended range. The outputs are diodes, so you wouldn't loose current back through one that was regulating to a slightly lower voltage. What the regulator does is adjust field current and that can only vary within a fixed range anyway (from 0 for no charging up to 5-10A for maximum output). If you had a huge load, the voltage would be low enough on the system to make both work at full output. This should be better than one big alternator because you share the belt load and don't need as much side load on the water pump.

    To make it work, both alternators just get connected the same way the stock one does. The grounds are together by default since both are bolted to the engine. The ouputs should both be tied to the starter or the battery. Of course, you need good ground straps to carry all of the current.

    I guess if you're worried about the extra engine load, put an A/C clutch on one alternator and use a voltage sense circuit to engage the solenoid only when below a certain voltage, say 13V.

    <font color=green>Top signs you don't have a real 4x4:
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  14. Overkill

    Overkill 1/2 ton status

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    Re: Relay\'s really that important when wiring lights?

    I guess if you're worried about the extra engine load, put an A/C clutch on one alternator and use a voltage sense circuit to engage the solenoid only when below a certain voltage, say 13V. Just make sure the circuit doesn't kick on the clutch when the vehicle is not running, or you will need two alternators, haha.


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