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Thread: Thermostat & Fan Wiring

  1. #1
    Senior Member JOP33's Avatar
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    Thermostat & Fan Wiring

    In the manual they instruct you to attach the power lead from the chassis to the power to the fan, then cut the ground coming from the fan and connect it directly to the thermostat. As best I can figure this is so that when the thermostat is made at a certain temp it then provides the ground for the fan and it is switched on. First, am I correct in my thinking here and secondly is this how most have installed theirs? If so, what should be done with thermo wire & ground wire from the chassis? Does the radiator need to be grounded additionally and if so how have most done this?

    Thanks
    33' Hot Rod Coupe/Roadster, Fenderless, Ford 302, 350hp, EFI, AOD, 4-Link, Double Adjustable Koni Coilovers, Split Rear Exhaust, Electric Power Steering, AC/Heat/Defrost, Moser 8.8"-3.55, Willwood Front/Rear Brakes, 18" x 8" Fronts/20" x 10" Rears, Ordered: 1.26.17, Arrived: 3.29.17

  2. #2
    RoadRacer's Avatar
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    Havent done mine yet, but didnt i read somewhere that it's preferred to put this switch into engine rather than radiator? more accurate?

    Yep, pg 206 of manual:

    "The thermostat switch works best installed in the engine as explained in the wiring harness manual. If
    this is not an option then it can be installed in the radiator.
    "
    Last edited by RoadRacer; 10-28-2017 at 11:25 AM.
    Aerodynamics are for those who can't build engines - Enzo Ferrari

    FFR33 coupe #997, ordered 2/20/17, delivered 4/15/17, build thread
    Planned: 350SBC, TKO600, hardtop, no fenders/hood, 32 grill, 3 link, GT500 wheels

  3. #3
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    You read it right, the fan temperature switches work by grounding out the relay which kicks on the fan. You get a more accurate (earlier) reading if your switch is mounted in the intake manifold as opposed to the radiator which is the coolest part of the engine. However, mine is mounted in the intake manifold and will continue running after I kill the engine until it reads cool enough which takes ~8min since fluid is no longer circulating so I am debating about moving it back to the radiator with a high flow thermostat to help.

  4. #4
    Senior Member JOP33's Avatar
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    Looks like I need to do a little more reading on all fronts (EFI, manual, wiring harness) before moving forward. Thanks guys for the input.
    33' Hot Rod Coupe/Roadster, Fenderless, Ford 302, 350hp, EFI, AOD, 4-Link, Double Adjustable Koni Coilovers, Split Rear Exhaust, Electric Power Steering, AC/Heat/Defrost, Moser 8.8"-3.55, Willwood Front/Rear Brakes, 18" x 8" Fronts/20" x 10" Rears, Ordered: 1.26.17, Arrived: 3.29.17

  5. #5

    Ol Skool

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    Be careful which manual and which harness your using. I seem to recall a ground wire issue with the directions. Memory isn't good, but I believe the early revisions didn't account for the relay that came with the Ron Francis fuse block. If you use a sensor on the bottom of the radiator just use a lower setting than you would use on the intake for the fan cut in. More important to have some bypass flow so accurate temperature exists at the sensor. If the EFI relies on a particular range it generally will be better at the intake manifold. Mine is wired so the fan can run with the key in accessory and get heat soak moved out until the radiator drops down around 170*. You may also run a different sensor for the EFI than the Fan relay depending on your combination of pieces parts.

  6. #6
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    What revision of the manual are you looking at? The latest revision "S" refers you to the Ron Francis Harness manual for the fan section for the hot rod. My paper manual is rev R, and has a bunch of wording that is irrelevant now. I just researched the crap out of this today because I couldn't understand what the hell I was being instructed to do.

  7. #7
    RoadRacer's Avatar
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    I just tested my fan for the first time, in prep for engine start next weekend. As I expected, it's super simple in the standard wiring harness (no EFI etc) - i'm bog-standard everything on my build, and I put the thermostat in the intake manifold. The wire on top of the engine plugged straight into it.. and that's it. No wiring modifications needed. The fan plugs straight into the wires down there.

    I tested it by pulling the wire off the thermostat and touching to the engine - fan came on, and was sucking not blowing. All is right with the world
    Aerodynamics are for those who can't build engines - Enzo Ferrari

    FFR33 coupe #997, ordered 2/20/17, delivered 4/15/17, build thread
    Planned: 350SBC, TKO600, hardtop, no fenders/hood, 32 grill, 3 link, GT500 wheels

  8. #8
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    Per the 33 supplement (I think) I connected the blue fan lead to the blue fan wire in the harness, black fan lead to the black fan wire in the harness, and green fan sensor lead from the harness to the sending unit in the radiator. Sounds like this way powers the fan through the relay in the fuse panel compared to the complaints that feeding blue wire power to the fan and grounding it through the sensor caused problems.

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