iMac A1418 - Fan spins high when turned off, normal when running.

gcain

New member
Hi Guys,

Never worked much on iMacs before. Customer just dropped this one in where the fan runs high when it's turned off.

He said the HDD was upgraded so maybe thermal sensor, except that the fan runs at a normal speed when the machine is running.

No idea where to start; I don't even have files for this Mac yet.
 

dukefawks

Administrator
Fan runs off 12V_S0 rails usually which of course should never be on when machine is off. Also these suck to open. If HDD has been messed with and they did not use original tape to close it back up you are the one that will crack the LCD. I always inform the customer that if the machine was opened before I will NOT take responsibly for cracking the screen if generic tape or Chinese fake tape kit was used.
 

gcain

New member
The screen was already removed for me, which is super convenient.

Originally it (just the logic board) was given to me with a blown L9001 (from memory, backlight filter anyway) maybe this is all related.
 

gcain

New member
Aren't the fans PWM?

So that means dumping 12 volts to the rail the fan is on wouldn't necessarily make it spin right?
Something needs to be telling it to run.
 

Sykulski

Member
Had the same issue a while ago. It was the Q7870. I did not have a FET in same package so i used one from another board and mounted it upside down with wire and conformal coating due to the customers impatience. Customer never came back. If you use an alternative FET then remember it must have a high drain-to-source voltage. I used one that had a maximum rating of 30V.
 

Attachments

dukefawks

Administrator
Seems fine to me. But these FETs need heat sinking to the board, just running some wires to them like this it will probably get pretty hot and die. This should only be done for a quick test, then get a FET that fits correctly on the pads.
 

dukefawks

Administrator
"The fan still spins high when the iMac is turned off. :'("

This is why you measure before just throwing parts at it. I see no reason why an S5 FET would cause the fan to stay on anyway. Fan runs off 12V_S0, so that bus is staying on. So Q7950 is a more likely candidate but measure first the enable and gate. Use brains, no plz Bro solutions.....
 

gcain

New member
"The fan still spins high when the iMac is turned off. :'("

This is why you measure before just throwing parts at it. I see no reason why an S5 FET would cause the fan to stay on anyway. Fan runs off 12V_S0, so that bus is staying on. So Q7950 is a more likely candidate but measure first the enable and gate. Use brains, no plz Bro solutions.....
I can't bench test an iMac board which means I can't measure it. This is maybe the third iMac I've worked on.

That said, I understood the problem differently to you; not that S0 was staying on but that S5 was sending 12v to a rail it shouldn't. But then fans run off PWM anyway so it would require a signal and not just 12v.
 
Top