Thanks Jim, I pressed the reset button only once, yes - also, everything else on the iMac works perfectly, somehow.
I'll try dismantling the system when I get a few hours spare. (Perhaps swapping out the hard disk while I'm there - it's a bit small) Best wishes, Josh Kelly Plymouth, UK On 8 Jan 2017 4:41 a.m., "Google" <[email protected]> wrote: Joshua, No, sorry, I didn’t address your issue directly. The fan shouldn’t start until you press the power button. I just dove into the Apple service manual for that iMac, and it doesn’t say anything about that issue. (I’ve messed around with old cars for years, and quite often I run across a problem that’s not covered in the factory shop manual because the vehicle had aged way past the point of the existing knowledge base when the manual was written. Same thing applies to computers.) However, it does say that the fan is a variable speed unit that increases speed as the iMac gets hotter, which I had forgotten. That means there is at least one temperature sensor on the logic board that controls fan speed. So the fact your fan starts running before the power button is pushed suggests that there is something shorted on the logic board — maybe the temperature sensor(s), or the power button/switch, or maybe the fan. Or maybe all of them. Or maybe it’s just a short circuit in the wiring to the fan. With older electronic equipment, the first thing I do is clean inside and out. Then I start looking for “tin whiskers,” which are the little growths from solder caused by oxidation created by ambient moisture in the air. I’ve brought many Macs back to life by simply scrubbing the logic board with a stiff brush in a pan of isopropyl alcohol, then letting it dry in the sun for a day to get rid of the water in the solution. Quite often a PMU reset will sort out problems like this. Did you take off the bottom cover and press the PMU button once? Or more than once? Here’s what the Apple manual says: Resetting the PMU on the Logic Board Resetting the PMU (Power Management Unit) on the logic board can resolve many system problems. Whenever you have a unit that fails to power up, you should follow this procedure before replacing any modules. 1. Disconnect the power cord. 2 Remove the user access panel from the bottom of the computer. 3 Press the PMU reset switch [it’s to the far right of a raised rectangle underneath the Airport card connector; this assumes the RAM slot is on the left side of the opening as you look at it] once on the bottom side of the logic board and then proceed to step 3. Do NOT press the PMU reset switch a second time because it could crash the PMU chip. WAIT ten seconds before connecting the power cord and powering the computer on. If the computer powers on, go to the next step. If the computer does not power on, there is something else wrong with the computer, refer to the symptom/cure, “No Power” in this chapter. Note: This entire procedure resets the computer’s PRAM. Be sure to reset the computer’s time, date and other system parameter settings…. So it could be that your PMU has crashed. In which case, take the iMac apart, remove the PRAM battery — test and replace if less than 2.8 volts — put it all back together and press the PMU button once. Good luck. Jim Scott Eureka, CA > On Jan 7, 2017, at 6:47 AM, Joshua Kelly <[email protected]> wrote: > > Thanks for your reply, Jim. > > The fan turns on the moment I plug it into the wall, though. (It only started doing that about a month ago as far as I can tell.) > > So it's definitely meant to do that before I even press the power button? > > Joshua Kelly > > On 7 Jan 2017 3:54 a.m., "Jim Scott" <[email protected]> wrote: > All iMac G4s have only one fan, which starts running as soon as the computer is turned on and continues running at the same speed until the unit is turned off. As iMac G4 cooling air enters from the bottom of the case and exits at the base of the chrome stalk, it acts as a vacuum cleaner. If you take it apart and get rid of all the dust bunnies and crud buildup on the fan blades and in the entrance and exit ports for air, you'll get a quieter iMac. On the other hand, perhaps the fan bearing, which is probably getting close to 15 years old, is wearing out and adding to the noise. If you do take the case apart, be sure to remove the old thermal pad material and replace it with thermal paste so that heat generated by the cpu and gpu can be piped up to the fan properly. Good luck! > Jim Scott > > On Fri, Jan 6, 2017 at 5:58 PM, Joshua Kelly <[email protected]> wrote: > Hi, > I've been having a very strange issue with my original 700MHz iMac G4 flat panel recently in which the fans decide to start spinning at low speed the moment I provide power to the system - even before pressing the power button. > > I've reset PRAM, NVRAM, and even the PMU to no avail. > > I'd really appreciate any help or ideas related to this. While the fans are remarkably quiet, it does get a little annoying after a while, haha. > > Joshua Kelly > @thejsa_ > > -- > You received this message because you are a member of the iMac Group, a group for those using Apple iMacs and eMacs. > The list FAQ is at http://lowendmac.com/imac/list.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml > To post to this group, send email to [email protected] > To leave this group, send email to [email protected] > For more options, visit this group at http://groups.google.com/ group/imaclist > > --- > You received this message because you are subscribed to the Google Groups "iMac Group" group. > To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. > For more options, visit https://groups.google.com/d/optout. > > > -- > You received this message because you are a member of the iMac Group, a group for those using Apple iMacs and eMacs. > The list FAQ is at http://lowendmac.com/imac/list.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml > To post to this group, send email to [email protected] > To leave this group, send email to [email protected] > For more options, visit this group at http://groups.google.com/ group/imaclist > > --- > You received this message because you are subscribed to the Google Groups "iMac Group" group. > To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. > For more options, visit https://groups.google.com/d/optout. > > > -- > You received this message because you are a member of the iMac Group, a group for those using Apple iMacs and eMacs. > The list FAQ is at http://lowendmac.com/imac/list.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml > To post to this group, send email to [email protected] > To leave this group, send email to [email protected] > For more options, visit this group at http://groups.google.com/ group/imaclist > > --- > You received this message because you are subscribed to the Google Groups "iMac Group" group. > To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are a member of the iMac Group, a group for those using Apple iMacs and eMacs. The list FAQ is at http://lowendmac.com/imac/list.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to [email protected] To leave this group, send email to [email protected] For more options, visit this group at http://groups.google.com/ group/imaclist --- You received this message because you are subscribed to the Google Groups "iMac Group" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout. -- You received this message because you are a member of the iMac Group, a group for those using Apple iMacs and eMacs. The list FAQ is at http://lowendmac.com/imac/list.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to [email protected] To leave this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/imaclist --- You received this message because you are subscribed to the Google Groups "iMac Group" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
