PPC MacMini shuts down immediately after boot, LED blinks

Question

A client gave me his PowerPC MacMini which boots up but then shuts down after a few seconds. Unfortunately, there's not enough time to open the system preferences between booting and automatic shutdown.
I've tried to boot the computer from a system DVD and with "T" pressed for target mode, but it always boots up completely from the internal drive. Booting with "Shift" pressed boots in safe mode, but the machine still shuts down after a few seconds.
Interestingly, the power LED blinks about once per second. I wasn't able to find anything about this on the Net.

Answer 1

AndyO wrote:
If, on the other hand, the mini is starting up, and then failing, and the light isn't so much blinking as pulsing, then the system may be suffering protective shutdown as a result of overheating. That can be tested by seeing whether it runs longer after being left unplugged for several hours, thus being cold, than it does if it's been running (or attempted to be run) for some time, and is thus already warmed. In that event, the mini will need to be disassembled to some extent to try and identify whether the fan is running, the CPU heat-sink is firmly attached and the thermal pad which is squeezed between the underside of the logic board and the base of the system is replaced.
I have a Mac Mini Intel Core 2 Duo that is suffering almost the same symptoms (shut-down almost immediately after startup); however, it DOES appear to be for a variable length of time based upon how much time I let it sit and 'cool' down; therefore, I believe I'm experiencing an over-heating issue.
I've installed a matching 1GB pair of RAM about 1.5 months ago and all has been fine until now. The unexpected (which is now expected) shutdown after startup is occurring on both Mac OS X 10.5.2 and Windows Vista (on Boot Camp partition) ... of particular note ... the Windows side shuts down more quickly (likely due to it not having the same system monitoring/energy handling tools that would be available to the Mac OS X side ... so I would suspect it to over-heat sooner on the Windows side).
Successfully completed Hardware tests (simple & extended) reported memory and other system hardware was fine. 'Check/Verify Disk' on Mac/Windows side reported no issues with either partitions.
Attempts to reset the PRAM and SMC have not solved the problem.
There was one instance, after startup, that the fan ran at full speed continuously and noticed that the power light was not on at all ... at the time, I was on the Mac startup disk and it stayed up and running for about 20 minutes or more until I decided to shutdown and test the Windows side. Upon restart, the fan did not run continuously, and the Windows side shut-down shortly after start up.
Overall, I'm fairly convinced it is an over-heating problem. If anyone has any ideas or reference material to confirm or how-to-do "the CPU heat-sink is firmly attached and the thermal pad which is squeezed between the underside of the logic board and the base of the system is replaced" that would be greatly appreciated.
I am not ruling out sending the machine into the retailer for servicing/diagnostics; however, if there's a relative quick fix so that I don't have to identify and remove confidential/sensitive info first, that would be preferred.
Thanks.