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Curious problem on Asrock motherboard with PWM fan headers

post #1 of 5
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I have two PWM controlled fan headers on my Asrock Fatal1ty Pro Gen3 Z68 motherboard. One is for the CPU and the other is a chassis fan, both with the ability to control speeds as a function of CPU temperature. To minimize noise, I'm wanting to connect all of my chassis fans via PWM. To that end, I purchased one of these fan splitter cables so that I can run all of the chassis fans off the chassis fan PWM header.

I connected the splitter cable to the rear chassis fan header and plugged it in to a molex for the power. Then I connected the rear chassis fan to the main lead on the splitter that has both PWM and RPM lines and set the fan to run at max speed in the bios. Following that, I connected one of the other chassis fans to one of the secondary fan extension leads on the splitter. And here is the curious thing: when I connect a second fan, the main chassis fan slows down from the max speed of 1300 rpm to 600 rpm and refuses to change speed under any conditions. Unplugging the second chassis fan immediately causes the rear chassis fan to spin back up to max speed.

Thinking that I may have a cable problem, I then used an unpowered PWM splitter cable like this. Exactly the same thing happened. Ok, so it's not the cable. So I tried the same experiment, but this time using the CPU fan header rather than the chassis fan header. And I got exactly the same results. With both cables. I could understand this happening with the 2-splitter as perhaps too much load is being placed on the fan header, but for this to happen with the 5-splitter that is powered from a molex is very curious.

Anyone have any ideas as to why this is happening?
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post #2 of 5
Do they use a common RPM line? It is possible that if both fans' signal lines are connected that they may be experiencing interference. The way these fans rely on a small magnetic sensor known as a Hall effect sensor to tell the position of the blades at a given time. The fan circuit then switches power to its coils accordingly. This being said for cost savings and simplicity the signal lines often tap off that sensor in the same place that the fan's control circuit does. Thus if you back feed that signal line it causes the fan circuit to think that the blades are at a different position causing it to trigger the coils at the wrong time. Hence the fan would run slowly.
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post #3 of 5
Your best bet, is to get something like this,

http://www.newegg.com/Product/Produc...82E16811995075

This fan controller takes PWM signal can translate it to voltage control. Allow regular 3 pin fan to be controlled in PWM faction.
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post #4 of 5
Quote:
Originally Posted by DSP1 View Post
Do they use a common RPM line? It is possible that if both fans' signal lines are connected that they may be experiencing interference. The way these fans rely on a small magnetic sensor known as a Hall effect sensor to tell the position of the blades at a given time. The fan circuit then switches power to its coils accordingly. This being said for cost savings and simplicity the signal lines often tap off that sensor in the same place that the fan's control circuit does. Thus if you back feed that signal line it causes the fan circuit to think that the blades are at a different position causing it to trigger the coils at the wrong time. Hence the fan would run slowly.
The particular "splitter" linked to, should have an sense RPM wire in only one of the connectors, not in all 5.

@OP, if you look at the fan plugs on the splitter, can you tell if the blue PWM signal cables are on the correct pins in the plug? The PWM control wire should be on PIN #4, which is on the opposite end of the plug from the ground wire comming from the molex connector.

don't want this to sound insulting, but are you sure that you have plugged into the motherboard in the correct orientation (it's hard not to), and that you have caught all 4 pins? (it's easy to offset the 4 pin connector onto 3 pins only). Also another silly question... Are you absolutely sure that your fans are PWM fans?
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post #5 of 5
Thread Starter 
Quote:
Originally Posted by xandypx View Post
The particular "splitter" linked to, should have an sense RPM wire in only one of the connectors, not in all 5.

@OP, if you look at the fan plugs on the splitter, can you tell if the blue PWM signal cables are on the correct pins in the plug? The PWM control wire should be on PIN #4, which is on the opposite end of the plug from the ground wire comming from the molex connector.

don't want this to sound insulting, but are you sure that you have plugged into the motherboard in the correct orientation (it's hard not to), and that you have caught all 4 pins? (it's easy to offset the 4 pin connector onto 3 pins only). Also another silly question... Are you absolutely sure that your fans are PWM fans?
So yes, the splitter does in fact have an RPM sense wire on only one of the connectors as you surmise. I used three different PWM splitter cables to test that the cable was not the issue before I posted my question, one with power from a molex and 5 attached connectors for fans, and two other 2-connector splitters that take power directly from the motherboard header. All of these cables have an RPM sense wire running to only one of the connectors.

With regards to the questions
  1. Is it plugged in correctly? Categorically yes, and all four pins are definitely in the connector. I've plugged all cables into two different motherboard PWM connectors and the same thing happens. So, it isn't the header at fault and it isn't the specific cable at fault either.
  2. Are these PWM fans? Categorically yes. The fans are Thermalright TY-140s and one Scythe Slipstream 140 PWM off the Scythe Grand Kama Cross CPU coooler (fan not available as a standalone product).

I will double check the wiring and pin-outs on the splitters, but it's hard to believe that Akasa and Phobya both would have messed up the wiring on three separate PWM splitter cables...
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