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Dell Inspiron 1720 CPU fan not spinning, wrong pins give power on mobo

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March 23, 2011 6:36:11 PM

So there are a ton of "CPU fan not spinning" threads out there, but none of them seem to address this exact situation, nor have any of the users tested the fan connectors on the fan and motherboard. I apologize for the long post, but I figure it would be best to give as much detail as possible without rambling.

My Inspiron 1720 went out on me a few weeks ago. I tried to bring it out of standby, and instead I got flashing LEDs for 45 seconds or so. My Google search led me to the conclusion that it was either my motherboard or my RAM. For a little backstory, I had been getting the following message at boot-up periodically since I bought my machine:

"The AC power adapter type cannot be determined. Your system will operate slower and the battery will not charge. Please connect a Dell 90W AC adapter or higher for best system operation."

I have always used the correct power supply, and since my system performance never suffered and my battery always charged, I just always figured that this was some negligible power-related issue on the motherboard. I ignored it for a while.

Then, about 18 months into ownership, my power adapter started shorting out and eventually bit the dust. I got a new one and after working fine for about 3 months, it started doing the same thing. Refusing to blame my motherboard, I bought 2 more adapters, which again worked fine.

Over these next few weeks I would get random blue screens and quirky system performance. Then, one night I couldn't bring the machine out of standby, as detailed above. Again, after searching Google I learned that it was either my motherboard or my RAM. Remembering my motherboard's history concerning power, I figured it would be best to get a new motherboard.

As fate would have it, there were some newly-listed, genuine Dell 1720 motherboards on eBay for $150 with shipping. I went ahead and got one. When it arrived, I installed it and turned it on, only to experience the same issue as before. Well, I took out on of my RAM DIMMs and it worked again, with one exception: The CPU Fan.

The CPU would not spin at all, not even for a split second during POST. I could get all the way into Windows like normal, but the fan still wouldn't spin. I shut my laptop down to prevent overheating and I figured that the old motherboard's power issues had shorted out one of my DIMMs and my CPU fan. So, I RMA'd my RAM and ordered a new CPU fan.

The parts come in and I install them. The RAM works, but the CPU fan still does not spin. I'm not very knowledgeable of raw electronics, so I got my dad to help me test both of my fans as well as the fan connectors on both motherboards. After hooking the fans to batteries, they BOTH spun, no problem. Then, using a voltmeter, we tested the fan connectors on the motherboards, and BOTH of them supplied power. So, everything works, but for some reason they won't work together. Upon further testing we found the following:

The fan connectors on the fans and motherboards are 3-pin connectors. To make the fans spin when connected to a battery, we had to supply power to the two outside pins slots, 1 & 3. This is the only pin configuration that worked; neither 1 & 2 nor 2 & 3 worked. Then, when testing the connectors on the motherboards, we could only get readings from the pin configurations of 1 & 2 and 2 & 3; not 1 & 3 like on the fans. The opposite!

So, WTH? My guess is that there is some sort of function during POST or perhaps a BIOS setting that tells the motherboard which pins to supply power to in order for the fan to run. I looked over the BIOS for fan-related options but didn't find anything. (I have the latest BIOS, A09, on both motherboards.) I also looked over the motherboard for any jumpers I could change, but didn't find any.

I'm stumped. Does anyone have any input?

Thanks...
a c 97 à CPUs
a c 95 V Motherboard
March 24, 2011 3:19:15 PM

I read your post completely! Good communication!

Did you get a non-Dell fan? Dell fans have their own "proprietary" pins configuration, and a standard fan will not work in a Dell computer. I found this out when I wanted to replace a Dell fan. The only option is to re-wire the pins.

Other than Dell, the place to go is Skyline Graphics. http://prosites-skyline.homestead.com/files/index.html

The gentleman is very professional and helpful.
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March 24, 2011 4:56:48 PM

Thank you. :) 

The original fan doesn't say Dell on it, it says Sunon. I don't remember what the new fan says on it (it's in my laptop right now and I'm sick of taking it out) but I was sure to get the exact part number. Even if I didn't, both the fans have the same pin config. I'll rewire the fan as a last resort, but there's got to be something else wrong, since the fan and motherboard worked at one point.

I suppose it is possible that my new motherboard just happens to have the same issue as my old one (perhaps that's why it's parent laptop was parted out) but I still don't understand why a motherboard would just all of the sudden change the pin config on the fan connector.

Thanks for the response!
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a c 97 à CPUs
a c 95 V Motherboard
March 24, 2011 5:07:49 PM

BTW, the fans in Dells don't always say Dell on them. They may be Delta fans modified (pins) for use in Dell computers. Replacement generic Delta fans may not work because of this pins modification by Dell.
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March 24, 2011 8:44:33 PM

I see. Again, though, the ORIGINAL fan that came with my computer and that had been working from the beginning works only with the outside pins, 1 & 3. It is a Sunron fan. I'm not sure of the replacement fan's brand, but it, too, works only with the outside pins, 1 & 3.

I don't think the fans were ever the problem. The motherboards are MUCH more likely to have changed the pin config than the fans are, since the motherboards have information stored on them that can change the operation of the boards. The fans are just fans and I don't see how they would have changed their pin configs. This said, There's got to be a reason why my original motherboard changed its pin config to 1&2 / 2&3 and why the replacement motherboard behaves the same way. Does no one know why the motherboards would have changed?

Also, are there any Dell Inspiron / Vostro 1420 / 1520 / 1525 / 1720 (or any other similar number) laptop owners out there that can perhaps test the CPU fan connector on their motherboard the same way I have tested mine? I suppose this is a big favor to ask, as it involves taking the laptop apart and finding a voltmeter if you don't already have one. I would really appreciate it though. This would tell me if my boards have in fact changed. It's pretty obvious to me (and highly likely) that they have, but I've been wrong before.

If not, someone HAS to know something further about this pin config issue. My best guess is that if there's not a BIOS setting regarding this, there has got to be something that happens during POST that puts power through the outside pins, 1 & 3, on the fan connector.
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a c 97 à CPUs
a c 95 V Motherboard
March 24, 2011 9:15:32 PM

Dell could very well have gone through another revision on the motherboard without changing the part number. This could account for changes like the one you are experiencing.

Unfortunately, getting to the CPU fan in laptops just to check voltages is not an easy task. Given the poor tech support from Dell, I don't think you will get answers from them. But, it is worth a try. Maybe from this, Dell's tech support can learn something new.
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March 24, 2011 9:55:12 PM

My machine is long-since out of warranty so Dell won't talk to me. This is OK though, because those jokers wouldn't know the first thing about my issue.

Any revisions from Dell on other motherboards wouldn't affect my original motherboard, nor would it explain why the original and replacement motherboards have the same pin config when tested. Since the fan used to work on the original motherboard, something had to have changed on the board, which, perhaps coincidentally, had also changed on the replacement motherboard at some point.

In any case, it's looking like I'll just have to re-wire one of the fans... :( 
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a c 97 à CPUs
a c 95 V Motherboard
March 24, 2011 10:05:19 PM

Either re-wire or contact Skyline Graphics for replacement fans.
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April 20, 2011 5:53:16 PM

It's been a while, but I would just like to give a quick update. I tried re-wiring one of the fans to match the pin config of the fan connectors on the motherboards. For some reason it still doesn't work though. It makes absolutely no sense to me.

I've also just contacted Skyline Graphics. Hopefully they can help.

Also - thanks for the Skyline Graphics recommendation, Ubrales, and sorry if I came off as a jerk in my previous posts. I was just SUPER frustrated and stressed about the work I was missing without a computer. I do appreciate your help though.

Hopefully my computer will work again soon! I miss it. :( 
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June 11, 2011 7:19:07 AM

OMG, I am so happy (and sad at the same time) that I found this post. I have THE EXACT SAME PROBLEM with my Inspiron 1720 and have never been able to fix it. I have replaced everything but the motherboard, including the fan, hard drives, memory etc and have basically given up and decided I would need a new motherboard if I ever want to use my beloved 1720 again. But now after reading your post, I am not so sure a new motherboard is the answer. I am not as literate as you with the electronics of it all, and have no idea about pins and such, I can however replace every component myself, just don't know how to rewire.

I would be VERY, VERY APPRECIATIVE if you would update me if you do find a resolution to this. After reading your post, I honestly believe that this is a major problem with this model and may not be able to be fixed. I haven't given up all hope, just most of it.

BTW, mine will actually boot up. I just get blue screens and other various errors and problems and the fan stopped working a while back. It works, I just can't trust it enough to use it.

My direct e-mail is sales@walstonet.com

Thanks!!!
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