relay, two temperature controlled fans

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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=136714
Printed Date: May 14, 2025 at 4:39 AM

Topic: relay, two temperature controlled fans

Posted By: 75blazer
Subject: relay, two temperature controlled fans
Date Posted: May 28, 2014 at 3:26 PM

Hello,

I have tried to figure this out on my own by searching the forums and I find myself getting overwhelmed trying to paste different ideas together. Since it is evident that many on this board are much smarter than I am, I figured I give this a go.

I want to run 2 electric cooling fans. I want 1 to turn on with a temp probe in the cool side of the radiator and the other to turn on with a temp probe in the water neck (different temps, I was thinking 200 on at the water neck and 185 on at the cool side of the rad). Additionally, I want them both to turn on when I switch on the AC. FURTHER, when switching on from the AC, I'd like to have 1 of them on a 1 or 2 second delay to soften the blow to the electrical system.

Am I nuts or does this make sense?

Replies:

Posted By: oldspark
Date Posted: May 28, 2014 at 9:20 PM

Is this for a liquid cooled vehicle? If so, why 2 fans? And why not 2 sensors for one fan?

If you want to limit the cooling (ie, higher radiator-out temperature provided in (engine out) temp is not exceeded), why not PWM the fan? (But normally engine-in temps are not regulated; cooling usually operates based on engine outlet temperatures. Controlling both usually requires control systems with an engine-independent pump.)

Posted By: 75blazer
Date Posted: June 02, 2014 at 1:33 PM

It's an old Chevy V8. The shroud/fan kit I intend to use is a dual fan setup, thus the 2 fans. Regarding PWM, in a nutshell, I think it's just too complicated for me. I can wire up relays, programming chips on the other hand, I'm not so sure.

Posted By: oldspark
Date Posted: June 02, 2014 at 5:27 PM

The easiest is probably to use diodes between the temp switches and fan relays and cross/inter connect them in whatever combination you want.

One output (probably for the outlet fan) could go thru a delay - maybe a 555 circuit or a readymade DEI (DEI 528T?).

FYI - the diodes are used to isolate different circuits whilst allowing interconnection - see DIODES above. (EG - maybe the 'negative door trigger' in your case where driver & passenger door are your in & out sensors, and the brown to alarm -ve input goes to your out fan relay (85) via the delay and another diode goes from your in sensor to the other fan relay.