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post #1 of (permalink) Old 09-22-2003, 02:28 PM Thread Starter
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Quick Relay Questions

I just bought an auxillary electric fan from Summit (16" Summit brand). I want to wire it using a simple on/off switch and a relay. I don't need a temperature switch. So I have a few quick questions.

1.) What's the difference between SPST and SPDT relays and which one should I use?

2.) The fan only draws 20 amps at start-up and 10 amps once it gets going. Is a 30 amp relay OK?

3.) Does it matter where I put a fuse at: either the power from the switch wire or the power to the fan wire?

4.) Online source for relays and connectors? Waytek has great selection but steep minimum purchase requirements.

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post #2 of (permalink) Old 09-22-2003, 03:21 PM
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Re: Quick Relay Questions

SPST= Single Pole/ Single Throw
meaning single line in/out
SPDT= Single Pole/ Double Throw
meaning single line in/ double line out
SPST is what you will want

30A should be just fine

You'll want a fuse on both wires. A small fuse on the wire to the switch, 5-10A should be plenty, and a 30A on the actual power wire that the relay is switching.

You can pick all these up at a local auto parts store or radio shack for cheap.

(here is a sloppy diagram of how to wire it)
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post #3 of (permalink) Old 09-23-2003, 11:01 AM Thread Starter
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Re: Quick Relay Questions

Thanks for the reply. I hope to finish the wiring this weekend.
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post #4 of (permalink) Old 09-23-2003, 11:37 AM
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Re: Quick Relay Questions

May I suggest a slight change to the wiring...
Instead of running a fused wire to the switch, run a connection from your ignition to the switch. It is fused as well and will insure that the fan does not run with the ignition is off. The relay is going to pull such a small amount of current that your not even close to putting any unnecessary load on the ignition ckt.
post #5 of (permalink) Old 09-23-2003, 08:28 PM
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Re: Quick Relay Questions

It is also easier and safer to swith the ground side of the relay. That way if your wires do go to ground by mistake, the only thing it will do is turn on the fan. This way you only need one wire to the switch and loop the other to a ground at the switch mounting.

It is very easy to hook up a thermal switch to the same circuit if you use a sending unit from a from wheel drive Chevy. They calibrate something like on at 210 and off at 190. Just hook up the ground side of the relay to the single wire on the sending unit. You can drill and tap a 1/2 in pipe thread to the t-stat housing. If you are running a 258, get a t-stat housing from a 4.0l, and put the GM sensor in place of the cts sensor that is already on the 4.0's housing.
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post #6 of (permalink) Old 09-23-2003, 08:51 PM
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Re: Quick Relay Questions

Ditto on switching the ground and the fan sending unit,you can get them in all thread sizes just look in the NAPA spec catalog.

You always think I'll just keep a eye on the gauge and turn the fan on but trust me you'll forget to do it. I know I did so I added the sending unit for auto fan and my engine liked me again. [img]images/graemlins/grin.gif[/img]

If you switch the ground side you can use a 3-way switch and run one term. to a steady ground, the other through the sending unit. Then you'll have the best of both worlds ON-OFF-AUTO
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