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  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
    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.

  5. #5
    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.

  6. #6
    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/

  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
    '82 T3 Turbo FABombjoy's Avatar
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    Since the threading matched, I have mine in the return Y pipe in place of the vacuum switch. Seemed like a decent place to get an accurate read.

    If it fits tho, I might move the sensor to the water pump as I'm considering putting my turbo water return and oil heat exchanger returns into the Y pipe instead. Right now it's an untidy H pipe into the heater return hose, but I could install fitting in place of the kjet sensors and run the hoses that way.

    The Y pipe is a return, right? Am I looking at the flow path correctly? Coolant flows up, out the mini hoses into the heads (and tstat once open) and then returns from the Y pipe and heater hard pipe?
    Luke S :: 10270 :: 1982 Grey 5-Speed :: Single T3 .60/.48 Watercooled :: Borla Exhaust :: MSD Ignition :: MS3X In Process :: DevilsOwn Methanol Injection

    Console5.com - Game console parts, kits, games and more. [shop] [wiki] [RSS] [f] [t]

  9. #9
    EFI'd Member dn010's Avatar
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    Quote Originally Posted by FABombjoy View Post
    Since the threading matched, I have mine in the return Y pipe in place of the vacuum switch. Seemed like a decent place to get an accurate read.

    If it fits tho, I might move the sensor to the water pump as I'm considering putting my turbo water return and oil heat exchanger returns into the Y pipe instead. Right now it's an untidy H pipe into the heater return hose, but I could install fitting in place of the kjet sensors and run the hoses that way.

    The Y pipe is a return, right? Am I looking at the flow path correctly? Coolant flows up, out the mini hoses into the heads (and tstat once open) and then returns from the Y pipe and heater hard pipe?

    I used the water pump location because that is where the sensor was originally on the B280F. Didn't feel like messing around with more work when a swap is already enough work and wiring, etc.

    Attached are coolant routes, one is the B28F, the other the 3.0 and they're pretty much both the same (heater core and expansion tank differences). B280F engine that I'm using has the oil heat exchanger lines running #1 from the block & #2 a line running to the the steel "return" pipe (stock D's engine steel return pipe is different with only one enter/exit passage). If you need sensor information or resistor values I have them depending on which sensor you're using.





    IMG_1513.jpg
    IMG_1514.jpg
    -----Dan B.

  10. #10
    '82 T3 Turbo FABombjoy's Avatar
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    Thanks for posting that! I'd seen it months ago but could not find it again.

    So the Y pipe for supply and not for return, so much for my hypothesis

    Looks like I will put together a better solution for the turbo & exchanger return lines.
    Luke S :: 10270 :: 1982 Grey 5-Speed :: Single T3 .60/.48 Watercooled :: Borla Exhaust :: MSD Ignition :: MS3X In Process :: DevilsOwn Methanol Injection

    Console5.com - Game console parts, kits, games and more. [shop] [wiki] [RSS] [f] [t]

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