11-27-2010, 04:13 PM

For the last few months, my iMac has taken me on a journey.

It has taken to crashing at least once a week, sometimes more frequently. No "kernel panic", no freeze ups, no hung programs - it just shuts off. The aftermath of this is a great deal of wait time to start it up again. If you try to start up shortly after the crash, it will only get partway through the reboot - then you will get into a cycle of it powering off and on. The only solution is to unplug the poor creature.

Once it's been unplugged for 24 hours or so, I can get it running long enough for me to put it through an AppleJack cycle. There is never anything obvious reported from AppleJack: no disk problems reported; always the same files with permissions issues (why don't they get fixed?); and cache files are always removed. But, after running AppleJack - it will keep working again for up to a week, but sometimes as short as a day.

I am trying to evaluate what the best next steps should be ...
- just replace the internal hard drive?
- continue to look for a software cause?
- look for a software solution ... will TechTool Pro diagnose other potential hardware issues?
- use my iMac for a boat anchor, and invest in a new one.

Other comments of interest ...
- OS X 10.5.something - It gets regular updates from Apple
- No HW upgrades ... 2 GB memory; 2.66 GHz processor.
- Hard drive is about 60% full. It was more than that, until I learned about those risks ... so I moved a lot off.
- Temperature monitor reports temperatures that don't appear to be extreme. People seem to think this is a red herring anyway.

Any pointers?

11-27-2010, 04:28 PM

Sounds like overheating to me, like maybe one of the fans is toast or not working properly. Have you run it through the Apple Hardware Test yet?

Intel-based Macs: Using Apple Hardware Test

11-28-2010, 12:01 AM

cwa107 --- Thanks for the prompter ... Now I know something about the Apple Hardware Test.

It generated the error number 4SNS/1/40000000 TpOP-131.000

Googling that - it appears to be flagging an issue with the power supply temperature sensor (anyone confirm or deny that?). Others complaints about this have been related to noisy fans - and that's mine - spinning hard all the time.

I will keep investigating this. Any other pointers are appreciated.

Would a faulty temperature sensor cause a crash?

On my old PCs, I would open them up and take a vacuum and compressed air to clean up the inside ... would this improve the iMac performance?

11-28-2010, 02:27 AM

If it's put of warranty, I would definitely get in there with some compressed air as a first course of action. In the iMac I looked at for a customer, there was plenty of dust build up, over just two years - and it had to have been inhibiting airflow. I don't believe that a faulty sensor would cause a full-on crash, but if the fans are always on full-bore, chances are that it's not able to move enough air to keep it within operating specs.

11-29-2010, 08:57 PM

Well, I opened it up and blew out the dust. I don't think it will make a difference - it was amazingly clean inside.

And the fans still rev up quickly.

It looks like the next step is a new power supply - it seems to be the cure for a bum power supply temperature sensor.

Unless there's other suggestions out there.
---
update: I just installed iMac Fan Control ... it shows the CPU fan is the noisy culprit, running at 3900 rpm. The CPU Heatsink temperature is reading steady at 90 F (32.2 C).
I increased the low limit for the HD fan from 1200 to 2300 - the fan speed went up from about 2000 to 2700, and the HD temperature came down from 105F to 100F - so the HD stuff seems to work.
The DVD fan ... I'll play with that another time.
The graph that iMac Fan Control shows indicates the DVD fan speed changes with DVD temp and the HD fan speed changes with HD temp ... but the CPU fan speed is unrelated to CPU temp (if it were, the speed would be near minimum).

I have to guess this means the CPU temperature sensor is flagged as faulty, so the fan ramps up to max speed.

And I suppose high temperature could be the cause of my crashes --- is there something out there that will collect and save the temperature sensor readings so I could look at where they were when the computer crashed (after the fact)?