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    #1

    Diodes?

    I'm wiring up my electric fans, and I was going to use 3 different relays. 1 for low speed, 1 for high, and 1 for when the A/C kicks on to power the high fan as well.

    Is it possible to just use 1 relay for the high speed, and connect the 2 positive signal wires from the A/C and the thermostat switch together to trigger the "High" relay, and put some sort of diode in line with the A/C signal wire, so when the High temperature switch is sends an electrical signal to the relay, it wont backfeed and turn the compressor clutch on? I tried some research..but it's a little over my head.
    -Phil

    sigpic

    +1982 Ford LTD-S Police Car. Built 351w, Trickflow 11R 190 Heads, Holley Sniper EFI, RPM Intake+ Hyperspark dizzy, WR-AOD, Full exhaust headers to tails. 3.27 Trac-Lok Rear. Aluminum Police Driveshaft. Speedway Springs+Bilstein Shocks, Intermediate Brakes, HPP Steering Box.

    +2003 Acura CL Type S 6-speed
    Tags: None
    #2
    A diode will do exactly that, prevent the current going the other way. If you intend to splice a signal from the AC signal wire, you should install the diode to the spliced in wire to the fan relay, so it won't backfeed the AC clutch or the ECM. Are you sure you want the fans to go high every time the AC kicks on?
    Last edited by Arquemann; 06-12-2021, 01:28 PM.
    1985 Mercury Grand Marquis LS, "Maisa"
    1995 Chevrolet Caprice Classic STW, "Sally"

    Comment

      #3
      Originally posted by Arquemann View Post
      A diode will do exactly that, prevent the current going the other way. If you intend to splice a signal from the AC signal wire, you should install the diode to the spliced in wire to the fan relay, so it won't backfeed the AC clutch or the ECM. Are you sure you want the fans to go high every time the AC kicks on?
      Excellent! Saves a relay and some wiring. I assume I don't need anything heavy duty, any ideas of what I should be looking at exactly?

      No, I'm not sure what OEM fans do... It's a dual fan setup, so 1 High speed and 1 Low speed. Is the low speed enough? I know it would be quieter
      -Phil

      sigpic

      +1982 Ford LTD-S Police Car. Built 351w, Trickflow 11R 190 Heads, Holley Sniper EFI, RPM Intake+ Hyperspark dizzy, WR-AOD, Full exhaust headers to tails. 3.27 Trac-Lok Rear. Aluminum Police Driveshaft. Speedway Springs+Bilstein Shocks, Intermediate Brakes, HPP Steering Box.

      +2003 Acura CL Type S 6-speed

      Comment

        #4
        I'm looking through the Holley Sniper I/O's...they activate the fan relays and A/C signal through grounds, not power. So the diode setup might not work. I'm not sure if the system sends power to the A/C clutch, or grounds it.

        I don't have the system yet, and the fans are running off of a temperature switch in the intake, however I want it to be wired in a way that it's easy to switch it over to the EFI setup... So i may not be able to use a diode if it's just grounding things

        Click image for larger version Name: Screen Shot 2021-06-12 at 1.50.03 PM.png Views: 1 Size: 77.9 KB ID: 1289172
        -Phil

        sigpic

        +1982 Ford LTD-S Police Car. Built 351w, Trickflow 11R 190 Heads, Holley Sniper EFI, RPM Intake+ Hyperspark dizzy, WR-AOD, Full exhaust headers to tails. 3.27 Trac-Lok Rear. Aluminum Police Driveshaft. Speedway Springs+Bilstein Shocks, Intermediate Brakes, HPP Steering Box.

        +2003 Acura CL Type S 6-speed

        Comment

          #5
          Factory setups often use an AC thermostatic switch to control the fans, or a pressure switch in some cases.
          If your dual fans come on both at once, i'd wager the low speed is adequate for cooling the condenser, if you have the condenser that's almost the size of the rad.

          I know that diodes are rated by amps atleast, some by volts aswell. Since in your case we're talking about a 12V signal wire, there won't be too many amps. I don't know too much, so definitely ask someone that knows better
          Holley support might be able to assist you, if you're using their Sniper stuff.
          1985 Mercury Grand Marquis LS, "Maisa"
          1995 Chevrolet Caprice Classic STW, "Sally"

          Comment

            #6
            If the relays are controlled by ground then I would say no diodes are needed. If the relays are controlled by switched hot then you need two diodes. It does not matter to the control circuit if you are actually switching hot or ground. You want high fan when compressor is running. That is the standard setup anyway even though it might not be needed in cold weather.
            Jay
            03 Marauder DPB, HS, 6disk, Organizer Mods> LED's in & Out, M&Z rear control arms, Oil deflector, U-Haul Trans Pan, Blue Fuzzy Dice
            02 SL500 Silver Arrow
            08 TC Signature Limited, HID's Mods>235/55-17 Z rated BFG G-Force Comp-2 A/S Plus, Addco 1" rear Sway, Posi Carrier, Compustar Remote Start, floor liners, trunk organizer, Two part Sun Visors, B&M Trans drain Plug, Winter=05 Mustang GT rims, Nokian Hakkapeliitta R-2 235/55-17
            12 Escape Limited V6 AWD, 225/65R17 Vredestein Quatrac Pro, Winter 235/70-16 Conti Viking Contact7 Mods>Beamtech LED headlight bulbs, Husky floor liners

            Comment

              #7
              AC is hot side switching, so you can't share a relay with a ground side switched source.

              Generally diodes are a good idea for isolation. Further good idea are diodes across the relays for snubbers. Nasty spikes happen when relays are de-powered.
              86 Lincoln Town Car (Galactica).
              5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

              91 Lincoln Mark VII LSC grandpa spec white and cranberry

              1984 Lincoln Continental TurboDiesel - rolls coal

              Originally posted by phayzer5
              I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

              Comment

                #8
                I used fuse holders with GM diodes for my relay signals.

                Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
                rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)
                Originally posted by gadget73
                ... and it should all work like magic and unicorns and stuff.
                Originally posted by dmccaig
                Overhead, some poor bastards are flying in airplanes.

                Comment

                  #9
                  would love to find a cheap source for internally-suppressed relays. Make all of this a non-issue.
                  86 Lincoln Town Car (Galactica).
                  5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

                  91 Lincoln Mark VII LSC grandpa spec white and cranberry

                  1984 Lincoln Continental TurboDiesel - rolls coal

                  Originally posted by phayzer5
                  I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

                  Comment

                    #10
                    that wouldn't work for signal isolation. Don't want the low fan temp switch turning on the AC compressor. Hence the in-line diodes to the low speed relay.

                    Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
                    rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)
                    Originally posted by gadget73
                    ... and it should all work like magic and unicorns and stuff.
                    Originally posted by dmccaig
                    Overhead, some poor bastards are flying in airplanes.

                    Comment

                      #11
                      wouldn't signal isolate, I'm considering just for flyback protection. I realize most people don't consider that but the spikes are kind of nasty, and I know my Towncar has problems with the flyback from the headlamp relays sometimes. Pretty sure if I put diodes across them it would stop, but I'm lazy and would prefer a solution of a relay with a built-in flyback diode.
                      86 Lincoln Town Car (Galactica).
                      5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

                      91 Lincoln Mark VII LSC grandpa spec white and cranberry

                      1984 Lincoln Continental TurboDiesel - rolls coal

                      Originally posted by phayzer5
                      I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

                      Comment

                        #12
                        True that. That HUGE ECC diode in the Aeros in the relay pack could be made redundant.

                        Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
                        rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)
                        Originally posted by gadget73
                        ... and it should all work like magic and unicorns and stuff.
                        Originally posted by dmccaig
                        Overhead, some poor bastards are flying in airplanes.

                        Comment

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