Relay for two electric fans - Chevelle Tech
Electrical & Wiring Troubleshooting electrical problems.

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post #1 of 6 (permalink) Old Jul 25th, 00, 5:08 PM Thread Starter
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I have 2 electric fans on my car and I've been running them for about a year with out a relay and only a toggo switch. Lately after some hard use the fans stop working and I traced it down to the fuse I have in the power wire. I want to use a relay to wire them up, do I need to use two relays, and when I go to the part store what should I ask for so they don't say "what kind of car is it for", are they normally sued for a/c or something. Also I'm not sure on how to wire these things up, can someone tell me were I might find a diagram on how to wire these up or can you just tell me. Thanks for any help.

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post #2 of 6 (permalink) Old Jul 25th, 00, 5:35 PM
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I've purchased several of the Bosch type 30 Amp relays from Radio Shack online. The part number is 900-2394 for a single pole-double throw 30 amp relay. The price is $2.99 each.

Terminals 85 & 86 are for the coil, so 85 gets power from your switch and 86 goes to ground. Terminal 30 gets your high current power and terminal 87 gets wired to your fan (87 is the normally open contact and 87a is normally closed).

Whether you need more than one relay depends on how much current your fans draw. If you blew a 20 amp fuse, I'd probably go with two. They'll probably last longer this way anyway. Just jump the power from your switch between both 85 terminals.

I don't know what to tell the guy at the parts counter, nor what the price would be.

Hope this helps.

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[This message has been edited by cjlandry (edited 07-25-2000).]
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post #3 of 6 (permalink) Old Jul 27th, 00, 6:30 AM Thread Starter
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I got a diagram from this topic , and I think I know how to do it now, just to make sure could you go over how to wire these up by just using a switch, I don't want them to come on as soon as I try to start the car. Also do the relays need to be close to the fans for some reason or can I mount them under the dash?
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post #4 of 6 (permalink) Old Jul 27th, 00, 2:07 PM
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I drew this out for you. I embellished on the wire size scale to prove a point. The larger wires need to be large to handle the amps the fan OR fans draw. When you use (2) relays you in essence create a "current divider"--meaning when you use the same guage wires leading to and from the relays where they come together from the battery (+) and to the actual fan(s) ---they equally share the same amount of current.

In other words, instead of one relay having to deal with 30 amps all by itself, 2 equal wired relays EACH share 15 amps apiece to help each other out.

Keep the large power wires SHORT, don't be running massive wires under the dash, this:
A) is an ugly pain
B) is a dangerous fire hazard if something goes wrong.

It's okay to run the little switch wires all around and everywhere, why? because the current flowing thru them is so small compared to the fans load. Just include a switch like I showed, then the fans "won't always be on when the engine is".

Find a tap such as the "IGN" terminal in the fuse box for the wire that feeds the switch. That way YOU can only turn them on when the motor's running to prevent battery wear-down.

I'd recommend a fuse close to the battery on the main (big) power (+) lead feeding the input to the relays.

Make sure the grounds have a GOOD connection (the funky little 3 tier symbols mean ground). In fact, what you may want to do is run the big ground from the fan(s) themselves directly to the battery (-) to ensure a good connection, instead of placing burden on your vehicle's ground straps (since they link the battery negative to the car sheetmetal) if you let's say bolted it to a fender. The engine block itself is another good ground choice if it has a good ground to begin with.

The "little" grounds from the relays themselves can be mounted to any good ground, even a nearby fender bolt.

Remember the relays are made of plastic--their casings will melt if too close to an engine, radiator, or exhaust manifold.

Good luck and practice good safety!


[This message has been edited by Coppertop (edited 07-27-2000).]
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post #5 of 6 (permalink) Old Jul 27th, 00, 2:34 PM
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The relays are pretty easy to find; it is the sockets for them that seem to be hard to get. Try looking at the accessory driving light stuff in the parts stores. That is where I have seen some relay kits. Also, JC Whitney has relay kits that are pretty useful.
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post #6 of 6 (permalink) Old Jul 28th, 00, 8:21 AM Thread Starter
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Thanks very much for the information and drawing, it will really help. I'm going to get this progect underway in the near future so I'll let you know if any problems occur. One more question, Since I have my battery mounted in the truck and I have the one wire alternator power going strait to the starter to meet the battery cable, is it o.k. to get my power supply strait off of the alternator? This is the way I've been running it, and since it all goes to the same place I figured it would do.

[This message has been edited by MarkM (edited 07-28-2000).]
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