Fans stuck at high after singleuser boot

Question

I just finished installing 10.4 (archive and install), the 10.4.11 combo updater, and various updates that popped up in Software Update.
After doing a few things I booted into singleuser mode to do some cleanup stuff.
I did this a lot in 10.3. In 10.3 when I booted singleuser, I'd get to the prompt, run fsck and then mount the root filesystem. At some point during fsck the fans would ramp up, and then would go back to "normal" after issuing the reboot command.
This time, first singleuser boot under 10.4, the fans ramped up immediately, possibly even before the first prompt. They did not ramp back down when rebooting, even after rebooting multiple times and even after going through "Shut Down" through the GUI and restarting using the power button.
Some other threads I've browsed have recommended a few procedures including zapping PRAM and (I think) resetting SMU (which I think is accomplished by holding the power button in for 5 seconds with the power cable unplugged). Haven't tried those yet out of concern that my backup battery might be old (machine was purchased late 2004, both logic board (2005?) and power supply (2008) replaced, but no idea if backup battery was replaced) and a PRAM zap or SMU reset could break stuff.
Advice?
Thanks.

Answer 1

Thanks again to both.
Miriam -- I am familiar with the fan behavior you describe. When I would boot singleuser under 10.3 the fans would indeed ramp up, and it was indeed normal. But as WZZZ notes, the fans would immediately ramp down after rebooting -- in other words, extremely soon after I would reissue the "reboot" command. I never tried rebooting singleuser twice in a row, as far as I recollect, so I can't say for certain that the fan rampdown occurred *before* any boot code was loaded on the reboot.
This problem occurred precisely with my installation of 10.4 -- or even more precisely, the first time I booted singleuser under 10.4. The problem never occurred when I was running 10.3, and my last singleuser boot was probably within several hours -- no more than a day or two -- of installing 10.4. When I installed 10.4 over 10.3, I did an Archive and Install on my 10.3 boot partition, with Preserve Users and Network Settings checked. I'm guessing that probably reduces the probability that it's a hardware problem.
Background on my hardware: This is a Rev A 20" iMac (1.8 GHz), purchased refurbished from Apple. It has had both the logic board and the power supply replaced. Logic board was replaced under AppleCare, I believe not long after purchase (within a year, so probably sometime in 2005). And I believe the power supply was replaced in the summer of 2008. IIRC this particular unit had known problems with both the logic board and the power supply. Both have been replaced on my machine. I don't know if there's a good chance that the power supply would be failing again or not.
I have two clones of my old 10.3 system on external FW drives (cloned using Retrospect's Duplicate feature). I haven't cloned the new install yet. If there's a way of booting singleuser off a clone on an external drive, then that would make for an interesting test. I'm betting there isn't, though, unless choosing the external drive in Startup Disk causes the hardware to look there first even in singleuser mode (and if the FW bus is active in singleuser).