How hot should a Power Mac G5 be expected to run?

[gsteemso]

So, some background. I have two Power Mac G5 towers (one each with dual single-core CPUs / single dual-core CPU). When I have them booted into Leopard, which these days is most of the time, I use a handy little software add-on called iStat Menus to give me a live display of (among other things) certain important chip temperatures. The CPUs on both machines tend to idle at about 40°C, with CPU A on the dual CPU machine usually hotter by about 5°C under load, and when they get near 80°C (which tends to be an issue in very hot weather) I shut down the machine out of caution. I have noticed that another sensor labelled “U3 heatsink” tends to run 5-10°C hotter than CPU A. Is that normal? Is it something that could be improved by replacing the thermal paste? I don’t want to just try it and see what happens, as I might normally do with less finicky vintage hardware; I am not practiced with applying thermal paste, and I do not want to have a smoke-emitting “learning experience” in a way that would be very hard to recover from. I welcome advice from anyone who’s knowledgeable about this, or at least has experimented on enough of them to imitate it convincingly.

 

[Unknown_K]

Do the fans ramp up when the CPU's get warm? I assume these are air cooled models.

 

[gsteemso]

They are indeed air cooled models, and you know, I've only ever heard the fans really vigorously spool up when the machines were having various boot problems last year. They never really seem to blow very hard, no matter how hot they get.

 

[Unknown_K]

AI says :
38°C to 55°C (100°F to 131°F) when idling, and can rise between 60°C and 80°C (140°F to 176°F) under heavy load. Liquid-cooled and high-frequency 2.5GHz–2.7GHz models run hotter, approaching 80°C before thermal throttling or emergency shutdowns occur. [1]

Thermal paste does get dry over the years and Apples heatsink is just an aluminum block with fins and one heat pipe, so they need good contact. If you do re-paste the CPUs do one at a time and put them back in the same socket so you don't have to recalibrate.

 

[gsteemso]

Good to know, thanks! Bit disturbing that we have to treat anything from a potentially hallucinatory AI as “knowledge”, but better than nothing I suppose. Still curious about that “U3 heatsink”. From what little research I did a few years ago, I got the impression U3 was some kind of motherboard controller chip, like the northbridge/southbridge chips on a peecee.

 

[Unknown_K]

Google says U3 is the memory controller chip on the motherboard that runs fairly hot. I remember a log flat black heatsink in a G5 and wondered what it was for. I assume there is a thermal pad between the heatsink and the chip that might go bad over time but just guessing (heatsink is held on by plastic clips that probably break over time).

I don't know how Apple dealt with overheating on their G5 towers. Generally, on the PC side the BIOS just slows the CPU clock down when running hot to more manageable temps, but PC memory controllers are in the CPU now not a separate chip like in the old days and on a G5.

Is your G5 full of dust and dog/cat hair or nicotine from cigs? If so clean it out. Looks at pictures of your machine on the internet and make sure anything that is supposed to have a heatsink has it. Anything with heatsink compound should be checked and reapplied every 10 years or earlier depending on how hot it gets (CPU. GPU). Very old heating pads tend to fall apart when you take the heatsink off so know the thickness ahead of time and have spares ready before you poke around unless down time isn't a problem. Some machines were designed to run hot and still be reliable, some are not. If the machine in question starts crashing, you should look into why.

Cooling systems are designed to keep the machine running perfectly while under warranty, so the company doesn't lose money repairing expensive things on their dime. They tend to not over engineer the system so it lasts forever because they will lose sales. Hobbyists tend to not run a machine as hard or for as long as they were run when new. Even when running a chip at spec you can expect atoms to move around from the heat and current so eventually, they will fail.

 

[treellama]

U3 is the northbridge on the back of the motherboard: https://thehouseofmoth.com/a-little...-mac-g5-and-what-to-do-with-this-information/

After I converted my quad to air cooling, that chip was running very hot and keeping that bizarre internal fan going at high speed. I eventually took it apart as per the instructions above and cleaned it out, and now it runs much cooler.

These machine are old enough that all the thermal compound is dried out and should be replaced.

 

[gsteemso]

It certainly sounds like I'm overdue for new thermal compound! I shall have to get on that ASAP. Thank you for the instructions about U3 @treellama – I didn’t know about that.