dual electric fans ii
Printed From: the12volt.com
Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=133809
Printed Date: July 13, 2025 at 9:28 AM
Topic: dual electric fans ii
Posted By: 8t6k5
Subject: dual electric fans ii
Date Posted: March 08, 2013 at 8:23 PM
Gentlemen,
A search of the internet for help in wiring 2 electric auto fans led me here:
https://www.the12volt.com/installbay/forum_posts.asp~TID~114321~PN~1~TPN~1
However, I would like a little more clarification on some of the diagrams provided. Unlike the OP, I prefer my fans on a manual switch; #1 Fan (Large) and #2 Fan (small-backup). Aditionally, I would like both fans to run automatically when the Air Cond. is on. Off with ingition off.
There is a diagram on pg. 4: https://www.the12volt.com/installbay/forum_posts.asp~TID~114321~PN~4~TPN~4 that I was planning on duplicating, only I would substitue the thermal switches to manual. However nothing is ever as easy as it seems so Im asking if that would be correct?
If I may throw a wrench in the plan, is there such a switch, rocker?, where say...left would be #1 ON - center both OFF - right both ON..... with the option of the A/C auto on?
As far as AC power its a basic GM 350 so the AC should be positive..?
Thank you,
JMG ------------- My Super Custom Signature....
Replies:
Posted By: 8t6k5
Date Posted: March 08, 2013 at 8:26 PM
LOL, Ive been instructed to provide more vehicle info:
1986 Chevy K5 Blazer 4X4
Tune Port Injected 350 from 92' Z28
auto trans - AC....ect ect...
Installing Dual Ford Windstar Fans, 15" and a 13" single speed each, max 4400cfm ------------- My Super Custom Signature....
Posted By: Ween
Date Posted: March 08, 2013 at 9:29 PM
you need:
1 SPDT center-off switch, 2 SPST or SPDT relays (40A contacts), 3 1N400* diodes. wire the input (center) of the switch to ignition, the output (outer) terminals to terminal 86 of the relays, one to each. ground terminals 85. connect one diode across the outer terminals of the switch, banded side of the diode to whichever relay is for the larger fan. this diode is for the single or dual fan selection. next take the other diodes, connect the banded leads to the terminal 86 of each relay. connect the non-banded leads together and connect to the AC system positive. this is for your auto-on with AC. terminals 30 connect to battery power, terminals 87 to each fan positive. don't forget to fuse your power wires.
all the diodes can be connected at the switch switch, just route one control wire to each relay. fuse the ignition feed to your switch also.
mark
Posted By: 8t6k5
Date Posted: March 08, 2013 at 9:45 PM
Perfect...just about. Im still pretty green here.... I have all my accessories are on their own relay's, but have not quite grasped the concept of multiple relays or Diodes.
Meaning, I know what they are, look like, what they are supposed to do (I think)..but not how to install them, arrows are supposed to go with the current?
I drew the diagram out and it looks pretty simple..compared to what I was expecting.
Im lost with the banded side diode.... is that the small white marking on the diode? ------------- My Super Custom Signature....
Posted By: Ween
Date Posted: March 08, 2013 at 9:54 PM
yes, the banded end is the marked end. think of the banded end as the point of the arrow, positive current flows to the point.
Posted By: 8t6k5
Date Posted: March 08, 2013 at 10:11 PM
Ween]a wrote:
l the diodes can be connected at the switch switch, just route one control wire to each relay. fuse the ignition feed to your switch also. mark
Not quite sure what you mean there.
Very crude, but I believe this is it.....
[IMG]https://i1359.photobucket.com/albums/q792/8T6K5/12VOLT_zpsc13f2a8f.png[/IMG] ------------- My Super Custom Signature....
Posted By: 8t6k5
Date Posted: March 08, 2013 at 10:12 PM
 ------------- My Super Custom Signature....
Posted By: 8t6k5
Date Posted: March 08, 2013 at 10:13 PM
Dunno why it wont show... you can copy/paste ------------- My Super Custom Signature....
Posted By: Ween
Date Posted: March 08, 2013 at 10:18 PM
you'll have four wires at your switch; ignition, AC system, relay 1, and relay 2. ignition and AC accessible under dash, relay 1 and 2 under hood.
reverse the diode on your switch...should make primary come on, not secondary.
Posted By: 8t6k5
Date Posted: March 08, 2013 at 10:21 PM
So switch will have 4 tabs too....or ignition/AC on same tab?
------------- My Super Custom Signature....
Posted By: Ween
Date Posted: March 08, 2013 at 10:32 PM
three tabs on switch, call them ignition, relay 1 and relay 2, ...AC connects to relay 1 and relay 2 through the diodes.
Posted By: 8t6k5
Date Posted: March 08, 2013 at 10:43 PM
Right, banded ends to R1 #86 and R2 #86, 1 each to outer tabs on switch, 12v to center tab. That should be how I have it on the diagram...?
Basically, I wire each as if they are on own switch and merely add 3 diodes to do the rest?
#1 diode jump between outer tabs on switch
#2-3 diodes to R1 #86 and R2 #86, connect and run to AC positive......
Not nearly as complicated as I thought, thx. ------------- My Super Custom Signature....
Posted By: Ween
Date Posted: March 08, 2013 at 10:51 PM
correct, you're welcome.
Posted By: oldspark
Date Posted: March 09, 2013 at 2:52 AM
If you use ground switching, no didoes are needed.
Just parallel all the grounded sensors and switches that you want to turn a particular relay on.
Posted By: 8t6k5
Date Posted: March 09, 2013 at 6:58 AM
Im sorry, I dont understand
------------- My Super Custom Signature....
Posted By: oldspark
Date Posted: March 09, 2013 at 7:11 AM
When you have multiple inputs using +12V, you need isolation diodes.
If the inputs are GND, you don't.
EG - instead of having sensors & switches supplying +12V via diodes so that a relay or load will be on with any or all, if they instead supplied GND to a "hot" relay or load (ie, the other end is to +12V), you just join them together (eg, to relay #85).
However, if you need to isolate one grounded signal from another, you'd still need diodes. But that isn't required if the sensors or switches ONLY control the relay or load.
Posted By: howie ll
Date Posted: March 09, 2013 at 10:31 AM
Grounds from switch to relay 85, 86 to ignition, job done.
------------- Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.
Posted By: 8t6k5
Date Posted: March 09, 2013 at 11:03 AM
I appreciate the help guys, but honestly I'm more confused. I'm more of a visual learner, would it be possible for you to modify the diagram I posted?
------------- My Super Custom Signature....
Posted By: Ween
Date Posted: March 09, 2013 at 11:34 AM
Still has AC system, which has positive control. Just as easy to go positive, plus wants isolation of aux fan. Would need DPDT switch plus additional relay to invert AC control output.
Posted By: oldspark
Date Posted: March 09, 2013 at 3:44 PM
Then stick to the +12V switched system as is.
It was merely a consideration. I have occasionally used GND switching with the occasional polarity converter for the odd input - eg, AC sensor if it can't be connected to GND or of it controls another circuit that likewise cannot be converted (often they switch relays).
GND switching often overcomes the need for diodes, and fuses for switches. And it's great when switching circuits with different voltages or from different +ve supplies - eg, from different fuses.
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