Ok ive posted this 3 times and no one seems to have any idea, ive got a laptop that shuts down straight after it boots ive sussed out that it is the cpu overheating because the fan does not turn on i have removed the fan rewired it to another computer and it works fine so i need to know what can cause the fan not to work?
If the fan itself is functioning as you say then it has to be the circuit on the motherboard try and install speedfan http://www.almico.com/sfdownload.php it's an excellant tool and you can use it to control the fan speed,download on another computer and put it on a flash drive, have some one use a can of dust off and spray lightly to spin the fan or an airhose from a compressor would be better and should keep the computer on long enough to install it. You can also try and update the bios it is possible for it to become corrupt and thus causing the fan not to come on. If either do not work motherboard replacement is most likely necessary.
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Re: fan not working
What can cause the fan not to work, is the voltage not getting to it from the motherboard meaning there are bad components on the motherboard that isn't allowing it to get there such as bad IC's and because of this the laptop will get very warm or hot to the touch and you can mainly feel it from the keyboard and moreso from the mousepad and its buttons.
I've seen this problem before and replacing the motherboard is the best option in this case unless you want to take it to a electronics shop and they may be able to troubleshoot the problem component level and see where the problem is but they will probably tell you to replace it as well since they are complicated to troubleshoot.
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The keyboard and the bottom of your laptop are very hot when the laptop is working.
The CPU fans are working all the time at maximum rotation speed and operate much louder than before.
The laptop suddenly shuts down by itself without warning. When it just started, the laptop was shutting down after 1-2 hours and how it shuts down after 5-10 minutes of operation.
The laptop works fine when it runs idle, but shuts down as soon as you start using any memory demanding applications (DVD player, image editing software, video editing software, etc.).
Solution: If the CPU heatsink is not clogged with dust and lint completely, you can use canned air and just blow it inside the laptop through the openings on the bottom and on the sides. It’s nice as a precaution measure, but it might not work if your laptop already has a problem and the heatsink is completely clogged.
Open the laptop case, so you can access the CPU fan and the heatsink. In some cases you can access the heatsink through the latch on the bottom of the laptop. Sometimes (for example Toshiba Satellite A70/A75) you have to open the laptop case all the way down.
Carefully disconnect the fan cables on the system board and remove the fan. If the fan makes unusual sound when it spins (grinding sound), I would recommend to replace the fan.
Clean the fan and the heatsink with compressed air.
I would also recommend removing old thermal grease from the CPU and applying new grease for better heat conductivity.
You could have a faulty fan and/or dust clogging the fan and heat-sink assemble. This is the most likely cause of the laptop shutting down. When the CPU overheats it goes into thermal shutdown and if this problem is not fixed, eventually the CPU will fail.
Check the fan to see it it spins freely and check if the fan works when the laptop is powered up.
If the fan and heat-sink is ok, then to give the laptop addition cooling then purchase a laptop cooling pad and place the laptop in this pad (These pads have cooling fans and helps cool the laptop).
Before anything make sure the air vents are not clogged. You can used canned air to blow out the air vents to make sure there is no buildup of dirt or dust cause that can cause overheating problems.
Look to see if the CPU fan turns on at all during the whole bootup process. The CPU fan won't run all the time but as long at its turning on at all during the bootup process then you know the fan is good.
If the fan never turns on at all and the laptop gets very warm or hot and then shuts off then the motherboard is bad cause that means to much voltage is going thru the motherboard and it can't cool down properly.
Hope this helps. If you can't physically see the fan turn on then where the cpu fan is located put your hand there and see if you can feel air flowing thru, hopefully it won't be hot air but slightly warm.
If you need any other info, let me know and I'll help you further.
when booting immediately after the first chime before the grey screen hold down keys (apple,option,p,r) allow the computer to restart if it still goes to that blank screen restart again and this time hold down only the (option) key or maybe (option,s) on that model try option key first then select the main start up disk from the menu and click the arrow underneath
You probably put more than a pin-head size drop of heat grease compound on CPU, thus "bleeding" over on the CPU, effectively overheating processor (very common). In any event, your problem is overheating -that's *the* place to look for now.