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Thread: Fan Relay Control

  1. #1
    Senior Member Josh's Avatar
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    Fan Relay Control

    So I am just finishing up my wiring harness and I am second guessing my fan relay wiring. I have opted to do a MS controlled circuit dependent on coolant temp and eliminating the otterstat. However, I also want the fans to engage when the A/C clutch is engaged. So this confuses me. I know you can switch a relay using IAC1A and IAC1B (same goes with the IAC2 ports) and they are inverse to one another (one 12V one ground). But the A/C clutch outputs 12V so to prevent going to a double thrown relay I was wondering if I could just use a chassis ground and power the relay with just IAC1A (configured accordingly as a 12V output), or since IAC1B is not used no current can be output? I have attached a schematic for clarity, just wondering if the proposed idea will work. FYI I am using a 2 wire IAC so i am controlling it with the FIDLE circuit.
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  2. #2
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by Josh View Post
    So I am just finishing up my wiring harness and I am second guessing my fan relay wiring. I have opted to do a MS controlled circuit dependent on coolant temp and eliminating the otterstat. However, I also want the fans to engage when the A/C clutch is engaged. So this confuses me. I know you can switch a relay using IAC1A and IAC1B (same goes with the IAC2 ports) and they are inverse to one another (one 12V one ground). But the A/C clutch outputs 12V so to prevent going to a double thrown relay I was wondering if I could just use a chassis ground and power the relay with just IAC1A (configured accordingly as a 12V output), or since IAC1B is not used no current can be output? I have attached a schematic for clarity, just wondering if the proposed idea will work. FYI I am using a 2 wire IAC so i am controlling it with the FIDLE circuit.
    Yes that should work. That is pretty much what the OEM harness did.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  3. #3
    Senior Member Josh's Avatar
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    Thanks for the input Dave. Just glad to get rid of the cobbled together OEM setup!

  4. #4
    Senior Member Josh's Avatar
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    nothing special. just a standard 30/40 relay and a basic 1N4001 diode. Been running this circuit for a few seasons now, no issues.

  5. #5
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by dn010 View Post
    What diodes & relay did you use for this circuit?
    You would probably use the normal fan relay (30 or 40 amp rated). The diode can be a one amp but you want about 200 volts rating when it's switching inductive loads like the relay coil.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  6. #6
    EFI'd Member dn010's Avatar
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    What diodes & relay did you use for this circuit?
    Last edited by dn010; 01-18-2016 at 03:33 PM.
    -----Dan B.

  7. #7
    EFI'd Member dn010's Avatar
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    Thanks Dave & Josh!

    Quote Originally Posted by Bitsyncmaster View Post
    You would probably use the normal fan relay (30 or 40 amp rated). The diode can be a one amp but you want about 200 volts rating when it's switching inductive loads like the relay coil.
    -----Dan B.

  8. #8
    EFI'd Member dn010's Avatar
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    One last question, I promise!

    I was looking at everything last night trying to find the easiest way to do this - the schematic shows that the existing AC & otterstat wiring already have diodes installed so provided they are still there I would leave that alone. So, it seems to me if I hook the otterstat black/orange wire to my MS3X Nitrous switched +12VDC (of course programmed to fan control in TS), could I not just install a 30/40 replay in the blue socket and call it a day? I'd clip the fan fail warning light Black/orange wire and splice it into the NO2 / black/orange wire so the light would come on when fans are tripped. I am using DPI fans so the amp draw should be much less for the relay & related wiring to handle.

    Basically, I am curious if I need to worry about socketing a replay in place of the splice wires in the blue fan fail socket, or should I just take out the blue socket and wire up 2 more relays to control each fan.

    Thanks in advance,
    Dan B.
    Last edited by dn010; 01-22-2016 at 02:26 PM.
    -----Dan B.

  9. #9
    Senior Member Bitsyncmaster's Avatar
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    The fan fail socket is not configured (pin connections) for any standard relay. Are you leaving the OEM fan relay installed? It sounds like all you need to do is cut the wire that was coming from the otterstat and use that wire to connect to your ECU. You would leave the stock diodes connected so when the AC is on it would run the fans or when the ECU wants to run the fans.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  10. #10
    EFI'd Member dn010's Avatar
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    Thanks for your reply Dave, I'm glad I asked first! I do have the otterstat wire already cut and running to my MS, it is just a matter of splicing it with the NO2 switched wire and configuring it. I am looking at the cooling fan circuit diagram and see now that the socket isn't in proper configuration, I should have studied it before asking. I do plan to leave the OEM fan relay installed since it will be getting the same +12VDC as it was originally but from a different source. So unless I get one of your SS relays or I build 2 relays to control each fan, I'm stuck using the ugly burnt splice wires for now.

    Thanks again!
    Last edited by dn010; 01-22-2016 at 02:50 PM.
    -----Dan B.

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