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Why did power supply fan kick into high after chassis fan swap?

I have an Antec Sonata case with the 380W TruePower power supply. The 120mm chassis fan began having trouble starting on power up, so I replaced it with a Kingwin CFBL-012LB. The original chassis fan didn't have any kind of connection directly to the ASUS P4P800-E Deluxe motherboard - only to the 12V molex connectors from the power supply labeled "Fan Only" (4 conductors). The replacement fan's molex connector only has 2 conductors (pwr & gnd), but it has a separate connector to connect to the motherboard's "CH FAN" connector.

When I plugged the new fan into the PS's "FAN ONLY" 12V connector and the motherboard's "CH FAN" connector, the chassis fan (rated to run at 950 RPM - I'm not sure if it varies at all) seemed to run fine, but the power supply fan only runs at high speed (about 2700 rpm). Prior to this I had only heard the power supply fan go high for short periods of time when the PS got warm - and then it would throttle right back down.

When I unplugged the chassis fan's connection to the motherboard's "CH FAN" connector, the power supply seems to run at normal speed again, but this time the fan of the video card (????) kicks into high gear.

I'm not sure what the problem is - anybody care to guess? Is there a certain chassis fan I'm supposed to use as a replacement or do I just have things connected incorrectly?

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  1. Oops - I guess the way I put the (???) made that unclear. Yes, I have a ATi Radeon video card. There reason I put the (???) next to the mention of the video card is that I'm stumped as to how removing a connector from the "CH FAN" connection of the motherboard could cause the video card to kick its fan into high gear.
  2. Does the fan have any instructions with it, or on the manufacture's website?

    Try plugging it into only the motherboard (I'm assuming the CH FAN connector has three wires).
  3. I do have 4 hard drives in the case, but I've never had any temperature issues that I'm aware of. Normally nothing is over 40°C according to ASUS Probe, PCWizard, and SpeedFan - and the only thing I've changed is the chassis fan. Are there any utilities that can monitor how much power is being drawn from the power supply other than putting something like a Kill-A-Watt at the AC power source?

    This is a early-2005 PC with a Northwood 3.0 GHz CPU and the video card is a Radeon X1650 (uses a 46" LCD as a secondary 1920x1080 display via DVI).

    Happy New Year!
  4. Best answer
    Plug the fan into the molex OR the mobo, not both as your post suggests you did,
    if you use the molex line it will run at 100% all the time,
    if you use the mobo connector the mobo will decide how fast it needs to run

    you could try building your system into to get an idea of the power required
    bear in mind your psu is five+ years old and is possibly due for revision/upgrade anyway
  5. The "Fan only" connector on Antec power supplies is designed to only run a limited number of fans with a maximum total current for those fans. If you connect too many fans to the "Fan Only" connectors, the excessive current on the speed-controlled "Fan Only" connector will cause the variable speed function in the power supply to fault, and your fans will run at either full speed, or will completely stop.
  6. Having homebuilt PC's since 1985, I had always used cases with built-in power supplies and fans, so I wasn't even aware that the mainboard would supply 12v power to the case fan. I also thought the case fan would only draw as much power as it needed to run, and since all sources inside the case have to go through the power supply, I didn't know it would make much difference to the power supply where the power was being drawn from.

    Long story short - I unplugged the connection to the large molex coming out of the power supply, and now everything is as it should be. The PC is quiet once again with all fans running.

    Thanks for the help. I learned something valuable from each of you.

    Can I select multiple best answers?
  7. Best answer selected by tluxon.
  8. This topic has been closed by Mousemonkey
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