Relay and fan quick question - Ford Mustang Forums : Corral.net Mustang Forum
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post #1 of 10 Old 07-04-2016, 10:55 AM Thread Starter
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Relay and fan quick question

I'm trying to figure out my fan wiring. I have an electric fan that I want to run a bypass switch on. Here is what I have so far.....
12V power (+) from battery goes first to a circuit breaker and then to a relay (post#30). Then post #87 (opposite #30) is wired directly to fan (+) wire. Fan and relay (post #85) are "grounded". Post #86 is connected to 12V "ignition on"power. Here is the missing part..........
I have a switch on my dash that is a "bypass" type switch that controlled the previous electric fan. It could be "switched on" to just run the fan continuously, otherwise the fan would come on automatically depending on temperature. How would I attach this bypass switch in the relay wiring so that it could be left on all the time, or "switched" to turn fan off manually until temperature was over say 180? Would this wire lead be attached to relay post #85 (ground) or to post #86 (12V ign)? Here is a picture of what I thinking it should be wired as. Thanks!!!!!

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post #2 of 10 Old 07-04-2016, 07:26 PM
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The Sending Unit is the trigger (#85) for the relay. Put a switch on that.

If you want to work the fan manually, bypass the Sending Unit by wiring 85 to ground, and put a switch on it you you can toggle it on and off.

I do this by using the flood light switch since my car didn't come with flood lights... It was a dummy switch before, but I got one off a GT. It looks stock.

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post #3 of 10 Old 07-04-2016, 07:41 PM
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You can also have it turn on automatically when the A/C is turned on too if you want, maybe with same relay setup as the manual switch (still a good idea)

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post #4 of 10 Old 07-04-2016, 07:44 PM
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Those sending units aren't always that reliable, and are sensitive to failing if you crank them in to tight. I replaced a bunch on my old 86 Grand Am. Just use a thread sealer and snug them in.

The problem with bypassing it is that you always need to keep an eye on your temps. You could leave it in and still use the switch in case the Sending Unit fails by just adding an extra switched wire from ground to post 85 on the Relay.
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post #5 of 10 Old 07-05-2016, 05:34 PM
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Speaking of sending units, what are you using to monitor your coolant temps - factory gauge only?

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post #6 of 10 Old 07-05-2016, 09:51 PM
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those sending units are not sending units

they are switches
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post #7 of 10 Old 07-06-2016, 03:52 PM
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Quote:
Originally Posted by indy2000 View Post
those sending units are not sending units

they are switches
I know that's the case for the oil pressure switch/gauge, but IIRC the fox coolant gauge had some gauge-ability?

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post #8 of 10 Old 07-06-2016, 07:08 PM
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a sender for the coolant temp gauge will NOT work to energize a relay.
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post #9 of 10 Old 07-07-2016, 11:06 PM
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There are two common types of "senders". One is a switch, NOT a sender. The normal thermal switch will check wide open when cold, and closes to a short when the engine reaches a certain temperature.

The other type is a thermistor. It acts like a variable resistance, normally going low resistance when hot. It should NOT be used to drive a relay. It is designed to drive low current electronics like a gauge or computer.

NONE of these switches or sensors are hurt or weakened by normal torque levels as long as you do not push down on the terminal with the socket or wrench while torquing, or have it bottom out on the sensor or switch tip. They are not any more sensitive to torque than anything else you might thread in.

They will fail prematurely if you use them wrong, like using a thermistor to control a relay coil.


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post #10 of 10 Old 07-07-2016, 11:56 PM
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the pic title clearly states switch

the diagram for some stupid reason calls it a sender..........thermistors, rtds........rarely use its housing as a ground.

that is a switch.


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