IMac shuts down...overheating? Fan not working?

Question

Our daughter reported that her iMac G4, 1.25gHz, had randomly shut down on her a week or so ago. I got it up and going again with a PMU reset, but it needed another one the next day.
Daughter's now moved onto a different machine so no worries about data loss, but I'd like to fix the iMac if possible.
It checks out okay with the AHT, and Drive Genius successfully repaired some disk errors. However while continuing to run tests, it shut down on me again. I noticed though that the case was pretty warm, so I just unplugged it and left it, and when I came back a few hours later was able to restart it. And then I noticed that it was running very quiet...because the fan does not seem to be running.
I have it going now to see if the fan kicks in, but so far nothing. Looking through the top, the fan blades are covered in dust. I can make them turn by poking a paperclip in.
Unfortunately none of the utilities that would allow me to investigate this on a more recent machine (Temperature Monitor, smcfan) work on the iMac.
I'll try blowing out the dust, and I know that the machine can be disassembled and cleaned, even the fan bearings oiled, but of course I wonder if there are any other options.
The iMac is superfluous for us at this point and we'll probably be giving it away, so I'd like it to work.
Thanks for any help!

Answer 1

In the iMac G4, the fan runs constantly.
From the initial startup in a healthy machine,
the cooling fan is set to run; and as the heat
increases, eventually the fan should run faster.
Except for when the computer is asleep, off,
or if there is an issue in the hardware inside.
So, a loose wire, a failed sensor, a circuit;
or the fan itself may have a bad motor. A
wire inside the winding could be cracked.
If you can find a computer with other defects
for a low price or free, you could try another
fan from a another similar computer. This
would be much simpler if the computer were
not an all-in-one with the display attached...
A basic box with a fan, holes and ports.
Some older iMac G3 owners made headless
Macs several years ago, when their display
went bad; the whole thing fit into a small box.
And cooling fans, accessory peripherals, &
other items were added to expand function.
In my iMac G4 1.25 17" USB 2.0 computer,
in use right now, the sensor overview for the
Temperature Monitor shows as in the hard drive.
The version of this Monitor is 4.3.
In Temperature Monitor preferences, note the
Sensors click-on icon, if you click on that, see
what if anything appears. It should have a name
of the computer in there. Click on the disclosure
triangle to show more info. Look into the Temp
Monitor's preferences: Window and if there is a
name for a sensor there, click on it.
It is possible in more modern computers with more
sensors, to choose from those which may appear.
Also, if the General preference panel is set to get
a temperature sample to frequently, that may be
not a good idea, mine is set at 20 minutes, but
may be OK to sample more often.
In the preference item labeled Menu Bar in T.M.
you can choose to have the main Temperature
Monitor quit and also have TM Lite, launch.
Since I launch TM Lite version, and then use it
to see the main application if needed, I don't
need to look into the main one directly.
I am not sure what kinds of drive sensors each
brand of hard disk drive uses, but among my
Macs, this software works and I see the HDD
sensor primarily. These are basic, so the CPU
temperatures would be a guess, not sampled.
There is a widget iStat, which shows temp info
from the same sensor, apparently. I have the
widget's readout turned off on this point, so I
can have the menubar in Finder showing info
and not have to launch a widget to know it.
The rest of the iStat pro's applicable info shows.
The opposite of having a vacuum pull air out
of the top vents in an iMac G4, when the fan
has gone bad, would be to set the computer
on a semi-recessed base with air pushing up.
The air intake slotted holes around the base
can be slightly pressured into from below &
a furnace filter can be used to keep debris out.
Have you looked in a local craigslist for a failed
iMac G4 (near the vintage of yours) to see if
you can get one for next to nothing, for parts?
Sometimes, you can get lucky. Some get tossed
and are full of great parts...
Good luck & happy computing!