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Thread: Diodes?

  1. #1
    P31 Pursuit Car Brown_Muscle's Avatar
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    Default 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



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  2. #2
    Lost and driftin' Arquemann's Avatar
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    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 at 01:28 PM.
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    P31 Pursuit Car Brown_Muscle's Avatar
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    Quote 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



    +1982 Ford LTD-S Police Car. Built 351w, Trickflow 11R 190 Heads, Edelbrock 1906 Carb+ RPM Intake, 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

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    P31 Pursuit Car Brown_Muscle's Avatar
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    Default

    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

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    -Phil



    +1982 Ford LTD-S Police Car. Built 351w, Trickflow 11R 190 Heads, Edelbrock 1906 Carb+ RPM Intake, 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

  5. #5
    Lost and driftin' Arquemann's Avatar
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    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"
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    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
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  7. #7
    I'm an air-conditioned gypsy gadget73's Avatar
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    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.

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

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  9. #9
    I'm an air-conditioned gypsy gadget73's Avatar
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    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 SE, triple black (Timewarp) - poly front bushings, KYB struts and shocks, Holley SystemMax1 lower intake, SilverFox AOD valve body,

    1984 Lincoln Continental TurboDiesel - rolls coal

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

  10. #10
    Beater gonna beat sly's Avatar
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    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)
    Quote Originally Posted by gadget73
    ... and it should all work like magic and unicorns and stuff.
    Quote Originally Posted by dmccaig
    Overhead, some poor bastards are flying in airplanes.

  11. #11
    I'm an air-conditioned gypsy gadget73's Avatar
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    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 SE, triple black (Timewarp) - poly front bushings, KYB struts and shocks, Holley SystemMax1 lower intake, SilverFox AOD valve body,

    1984 Lincoln Continental TurboDiesel - rolls coal

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

  12. #12
    Beater gonna beat sly's Avatar
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    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)
    Quote Originally Posted by gadget73
    ... and it should all work like magic and unicorns and stuff.
    Quote Originally Posted by dmccaig
    Overhead, some poor bastards are flying in airplanes.

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