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Old 05-07-2022, 06:31 PM   #1
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Radiator fan resistors

For tonight's dumb question...

What exactly do the radiator fan resistors do? Are they actually resistors? Why aren't the fans just powered straight from a 12v source?

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Old 05-07-2022, 08:31 PM   #2
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I've forgotten all I used to know about PIE and EIR, but in simple terms that I can still understand; Yes they are actual resistors and they enable the low speed fan function by making the electricity going through them get tired.

When they go bad, if you run the fan without them by joining the wires it's on high speed only. Replace them with a 100W .5 Ohm resistor
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Last edited by 78F350; 05-07-2022 at 08:35 PM.
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Old 05-08-2022, 05:41 AM   #3
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... function by making the electricity going through them get tired...
... lol...
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Old 05-08-2022, 02:42 PM   #4
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Ahhh, gotcha!

Here's a fun fact that I discovered while contemplating cools mods last night.....

Our radiators are the same size as those on a Polaris RZR!

I am going to eventually need to up my cooling game and am loathing the process of bumper modification, piecing together the gt3 parts so that I'm not creating additional updroce from a poorly angled setup, yadda yadda..

So I got to thinking - why not just remove the radiator fans and put a second set of radiators in their place? Stacking radiators isn't perfect, but it would be far more cooling surface area than the center rad addition. But thenen of course you have to buy 986/996 rads, either OEM or csf, and neither are cheap.

And then it hit me - just Google 14" radiators! And wouldn't ya know, the RZR rads are nearly identical in size, use the same 1 3/8 hose barbs in the same locations, and you can get a triple row (compared to OEM single row or csf double row) for $100 a piece! That's a hell of a deal in my opinion, even if some minor fabbing is required to get them mounted.

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Old 02-13-2024, 07:51 AM   #5
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Unable to start a new thread so please excuse the hijack..!

When our 986S (2000) went for its MOT last year, during the emissions test, the car overheated and launched its coolant over the workshop floor. The cooling fans are not coming on.

Having jumped the relays, the left/slow does not come on, but left/high speed, right/slow and right/high speed all activated the fans.

The AC does not make the slow speed fans come on, however the AC needs regassing, but we suspect thereís damage to the condensers, so not sure if that will have an impact?

Would changing the ballast resistors potentially work or does anyone have any advice to offer? Iíve tried a forum search so if thereís a thread Iíve missed, please point me in the right direction!
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Old 02-13-2024, 07:03 PM   #6
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The left/slow not coming on sounds like the left resistor is bad. As to why the fans don't come on unless the relays are jumped would obviously seem to be bad relays.
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Old 02-13-2024, 10:57 PM   #7
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The left/slow not coming on sounds like the left resistor is bad. As to why the fans don't come on unless the relays are jumped would obviously seem to be bad relays.
4 relays on the shopping list then! Thank you
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Old 02-14-2024, 02:49 AM   #8
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Bad relays... or not.

When my left fan quit working I simply hooked the two wires up to a 12V source. It didn't turn on. Bad motor on the fan. A new relay wouldn't help. I ended up replacing both fans for good measure.

May not be the relays. Test the fan directly before installing those "electrical" items... subject to return. Good luck.
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Old 03-17-2024, 05:05 AM   #9
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4 relays and 1 ballast resistor changed. All good now

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