Do I need a relay?

i just rewired my single 10" fan with a relay.  it's easy

 

do you need it?  i didn't, but it sounds like you are running two fans.  that is a lot of juice to run through that temp switch.

 

Do you know the amp rating of the temperature switch? If not, use a relay.

 

The temp switches I've seen are between 0.5 and 4.0 amps which could be less than a single fan, much less two.

A 30-amp relay costs, what.........Ten Bucks Canadian?

 

What's your time worth finding and changing blown 10-15 amp fuses or replacing a blown 5-amp capacity thermo-switch because your fans are pulling a combined 18 amps on a hot day??

Relay, for sure!

Relay it is.

Does anyone know how much current the usual Mesa 11" fan draws? 

I don't know what it is for a Mesa fan, but my two smaller Setrab fans draw 7.0A (combined) at start-up and only 1A during operation.

Many oil cooling fan makers don't advertise their current draw, although I would assume it's marked on the fan housing.  However, 10-15 amps is not uncommon for Spal and other makers, with some cars that use serious fans, ala BMW, drawing over 20 for very high torque models.  With sealed ball bearing types, when comparing apples to apples, more cfm = more current draw.  A relay makes good sense.

I've downloaded a couple of wiring diagrams off the net.

 

 

The second diagram makes more sense to me.  What do you guys think?

Actually I would wire it up like the first diagram. You don't necessarily need to tie the ground for the fan and temp switch together. There's no problem in doing so as they both need to be grounded and will work with either diagram you choose to use.

I think they both say the same thing except the first diagram assumes you know the body of the thermostat probe needs to be grounded. It might be handy for you to know that the relays are available in "dual 87" format too. Having two 87 outputs is handy if you're wiring two headlights for instance. You have two fans and that's why I mention it.

Yeah, the diagrams are the same, but the second is a lot clearer on the specifics.

Look at the bottom drawing (or the top drawing).  Tell me again, what is the advantage of the relay?

A relay is used so that you don't burn your switch. Your switch turn on the relay and when it is on the relay turn on the fan. So your switch doesn't have to handle high amp.

My battery is up front and my headlights were very dim. A voltmeter check showed my lights were getting about 9.something volts with the existing wiring on my older IM. Two relays were installed ( high and low beam ) with dual 87 output and my headlights now get just over 13 volts. I now drive at night with confidence.

I clearly understand what the intent of the drawings are.  But is the intent valid?

 

In order to be valid, the thermo switch contacts would have to be insufficient to directly handle the load of the fan.  But I am under the impression the thermo switch contacts are plenty capable of handling the load of the fan.  And if that is the case then the relay is a pointless redundant component (and therefore poor electrical design).

 

So, are the thermo switch contacts insufficient to carry the fan's current load?

 

Oh, and just another minor point:  In the drawing the thermo switch /relay coil circuit is fused with 20-35amps  --  the same fuse as the relay contact / fan circuit.  If the thermo switch cannot handle the fan then it doesn't belong on a circuit fused at 20-35 amps.  The switch should be on a separate circuit that is fused at less than the switch's load capacity  --  otherwise the switch blows before the fuse; instead of the other way around.

Switch protection is only one small feature of a relay.  A relay allows a small current to control a larger one.  With the use of a relay, you are in effect moving the on/off control of an electrical appliance from the dash to a spot very close to the appliance itself.  This means that you can use smaller wire and still deliver more current.  High current that used to run through the switch now is a low current that runs to the relay, where it only has a short run to the appliance, either headlights, fans, etc.

 

David's post above illustrates the value of automotive relays.  More power to the lights, since the effective wire run, with attendant current drop, is almost eliminated.  12 gauge wire from the relay to a high-drawing appliance is almost always sufficient, since the shape of modern relays allows them to be mounted on bulkheads, firewalls, engine compartments, almost anywhere, thus close the power draw.  Relays are not fuses, so properly-sized protection still needs to be in place.

So Jim, you're saying the relay isn't necessary as long as the thermo switch can handle the load, right?

 

And mentioning, of course, that we are talking specifically about the fan circuit (not headlights) and the wire between components has to be of sufficient gauge.

 

I think we agree.  Because it really doesn't matter if the on/off switching is done by the thermo switch contacts or relay contacts -- contacts are contacts; as long as either set of contacts are sufficient to carry the load.

 

Example:  When you turn your wall light switch On at home, do you have a relay between the switch and the light bulb?  No, of course not.  That would be pointlessly redundant because the wall switch contacts can carry the load of the light bulb.

 

I'm not trying to fan a debate of semantics here.  But there seems to be an overwhelming support for putting a relay in the fan circuit and I'm wondering why?

If someone knows the thermostat switch (that is probably the same one all us are using) is insufficient to handle the current of the fan, please speak up.

 

If that is the case, then I'm going to the garage right now and installing a relay.  But so far (22,000 miles) I still haven't fried my original thermostat switch.

Thanks for all the advice/comments.  I bought a relay yesterday, so needed or not it's going in.

I'm pretty sure the oil cooler fan that Henry installed on my IM didn't have a relay-just an inline fuse.

Mark, The usual fault in a circuit that is underpowered is not the switch, it's the wire size.  Old connections, corrosion, etc. can also increase resistance.  Long runs of undersized wire usually means a voltage drop.  That drop can generate heat.  Headlights are mentioned frequently with relays, since this is a common upgrade, with brighter bulbs pulling a bigger load.  If you wanted to test your fan circuit, check to see what voltage is actually getting to your fans.  Remember that resistance never decreases with age, it only increases as wires and connections corrode.  Kind of like us, eh?

My wire run will be very short.  From the battery, to the relay, to the fans, will only be around 2 feet.  Would 12 gauge wire be enough?

 

What Jim said.

 

But not all switch contacts are created equal.

 

Besides allowing lower gauge cable runs to switches, relays also allow the use of switches with contacts rated for lower amperage (the specs of a switch include voltage AND current ratings).

 

It could be that your thermo is rated to carry the current your fans will draw (if two fans, add the current rating of both to find the current the switch will need to handle). In theory (ha, ha) you should be able to look up the current rating for your thermo's contacts to see if it will safely handle the load of the fans.

It's standard practice in car wiring to operate heavy current items like lights, motors, window defrosters, etc. with relays. And with the bean counters calling the shots at most car companies today, they wouldn't install anything they could possibly do without.

 

Ron,  12 gauge stranded wire will do nicely. 

Different thermo switches have different amp ratings, some of them are quite low (0.5 amp). If you don't know the rating, to be safe, you assume the worst and use a relay to take the load away from the thermo switch. If you do know the switch's rating can handle the load, then yes, it's perfectly ok to use without a relay.

 

The coil on a 12V automotive relay is typically 50-120 ohms. That means at most, you'll get 100 to 280 milliamps of current on the control circuit. If a wire short occurs past the 86 terminal, the relay coil is still limiting the current through it to 100-280 mA so no danger to relay or switch. And past that it's just looping to ground, no current flows, and no fuse is needed. If a wire short occurs ahead of the 85 terminal, it's a short to ground, which will blow the fuse. But in that case, no current would be flowing through either the relay or switch (both 85 and 86 connected to ground), so you don't need an extra low amp fuse to protect those parts.