Why use a relay on a dual elec fan? - Chevelle Tech
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post #1 of 10 (permalink) Old May 13th, 05, 4:54 PM Thread Starter
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Why use a relay on a dual elec fan?

Can someone explain why I should or should not use a set of relays on my dual electric Ford Windstar fan I'm getting ready to install in my Chevelle? I'm not quite clear on the purpose of the relay. If the wire is a sufficient gauge it should work fine just running the hot leed to both fan motors with a proper fuse. As I recall the Ford windstar fans are something like 35 amp at max draw.

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post #2 of 10 (permalink) Old May 13th, 05, 5:03 PM
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Re: Why use a relay on a dual elec fan?

Basically you are using the large switch contacts, inside the relay, to operate the fan.
If you just used a switch you would have to use one with very large contacts. Just like your large wire.
It's impractical to run large gage wires inside the car to the switch.
Much easier and cheaper to use a smaller switch and smaller wires to the switch.
The relay coil can close the large relay contacts but the relay coil doesn't draw much current, thus the smaller switch.

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post #3 of 10 (permalink) Old May 13th, 05, 5:10 PM
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Re: Why use a relay on a dual elec fan?

A point John did not point out is that should you run the large wire required to handle the 35A max draw, you would create a very long circuit. Long circuits operated at high current often cause voltage drops and more heat in the wires. Both of which are bad for reliability of electrical components.

Much better to keep high power circuits as short as possible.

For an example compare the battery cables used originally with what is required for a battery mounted in the trunk. The extra distance requires 1-2 size increase on the wire or, at this power level, cable.
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post #4 of 10 (permalink) Old May 13th, 05, 6:33 PM Thread Starter
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Re: Why use a relay on a dual elec fan?

So I'd want the relays as close to the fan motors as possible, and run the switch wire all the way back to the cab, right? I picked up a couple of 30a relays for the fans. These are basically the relays I have...



Also, what amp switches do I want to run?

1978 Chevy Malibu
540, Rons Toilet, BTE PG, 25.5B/backhalf
[email protected], 1.30 60'

Last edited by SS_Sean; May 13th, 05 at 7:00 PM.
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post #5 of 10 (permalink) Old May 14th, 05, 12:22 AM
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Re: Why use a relay on a dual elec fan?

The switch wire size is not especially critical, the relay control coil will draw an insignificant amount of power. However you should also consider the size of the power supply fuse when deciding on what size wire to use. You want to make sure that the fuse blows before any of the wires downstream smoke.

As to relay location, they can be near the power source or near the power consumer (fans) or anywhere in between. Generally just put them where it is convienient and you don't create a significantly longer wire run.
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post #6 of 10 (permalink) Old May 14th, 05, 1:23 AM Thread Starter
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Re: Why use a relay on a dual elec fan?

Sorry, another question. I heard I'm supposed to jumper pin 86 to 87 and wire the thing inline. Does that sound right?

1978 Chevy Malibu
540, Rons Toilet, BTE PG, 25.5B/backhalf
[email protected], 1.30 60'
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post #7 of 10 (permalink) Old May 14th, 05, 9:04 AM
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Re: Why use a relay on a dual elec fan?

You can wire 86 & 87 together. Then you need to move the switch to the ground feeding 85 to turn it on and off.

Hank

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post #8 of 10 (permalink) Old May 14th, 05, 12:23 PM Thread Starter
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Re: Why use a relay on a dual elec fan?

This look correct?


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post #9 of 10 (permalink) Old May 14th, 05, 9:09 PM
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Re: Why use a relay on a dual elec fan?

Like Hank said, (sorry, I didn't see it befor posting)
If you use the ground as the switch leg, you won't need to run hot wires into the cab.
(Assuming that 85 & 86 is the operating coil and 87 & 30 is the normally open contacts)

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Last edited by Dean; Nov 6th, 06 at 12:11 AM.
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post #10 of 10 (permalink) Old May 15th, 05, 4:35 AM Thread Starter
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Re: Why use a relay on a dual elec fan?

Thanks a bunch for your help!!!

1978 Chevy Malibu
540, Rons Toilet, BTE PG, 25.5B/backhalf
[email protected], 1.30 60'
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