What's the function of the thermistor on a Power Mac G5 2.0 dual?

Question

What's the function of the thermistor on a Power Mac G5 2.0 dual?

About a week ago I posted a question about a bad processor on a Powermac G5 2.0 GHz dual (see https://discussions.apple.com/thread/4553743?tstart=30). I got a new (to me) processor and got the machine running for a bit, but then got memory errors (blinking power button) at startup. Using a heat gun on the logic board fixed that so far. So, the machine will run, but the fans go full speed and I cannot get it calibrated with ASD 2.5.8. After calibration start, I get:

ERROR CPU0 - Check that outer door is secure and in place then rerun calibration

Ambient Temperature Sensor (IC2) is located on main logic board.

ERROR CPU1 - Check that outer door is secure and in place then rerun calibration

Ambient Temperature Sensor (IC2) is located on main logic board.

Needless to say, the door is secure and in place, as is the plastic shield (the sensor of which is also working).

Looking around online, I saw mention of a replaceable Ambient Board in PowerMac G5's. However, mine is one of the original 2003 models (bought in May, 2004) - they apparently don't have this board. There is a small thermistor in the same place the board would be (in front of the speaker/fan assembly). Does anyone know if this serves the same function (i.e., is it the main ambient temperature sensor)?

I disconnected it and pulled it out (no mean feat since I didn't want to remove the PCI divider and logic board). One of the leads was crushed a bit, and I thought I could see bare wire under a magnifier. I put it back in, hoping that reconnection would help things, but no luck.

Any chance that replacing the thermistor would allow me to calibrate the machine, or is the Ambient Temp Sensor (IC2) reported by ASD a different component?

Thanks for any help.

-Aron

PowerMac, Mac OS X (10.5.8), original PowerMac G5 2.0 GHz dual

Posted on Dec 6, 2012 1:37 PM

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Answer 1

Oct 20, 2018 1:09 PM in response to Jacumba

Jacumba - Thanks for replying. Your idea that there is a bad fan runs counter to my understanding, but that's likely to be because my understanding is wrong.

I think the fans are working - they all spin, they test okay by ASD's fan tests, and they vary speeds (i.e., they start slowly/normally but ramp up to full speed within a minute or so). There's not much more to a fan than being able to turn - I assume that the fan controllers on the logic board vary the current to the fan motors as necessary.

My understanding is that the temperature sensors are used by the system to control the fans. If sensors are not giving valid signals, the failsafe mode of the machine is to ramp the fans to full to protect against possible overheating. The error messages indicate that ASD cannot get good readings from the Ambient sensor during calibration. That's why I asked about the thermistor, and whether that part is the main ambient temp sensor.

The unexpected shutdowns I'm assuming have to do with failing solder joints, particularly around the DIMM slots and the memory controller. I say this because the heat gun method solved (at least for now) the problem of unrecognized memory at startup.

I'm waiting on the arrival of a used logic board (a crapshoot, I know), but was hoping to get some information on the thermistor as the "new" board will probably come without it and I'll have to use the one I have. That's why I was asking whether that could be the problem.

If I've misinterpreted the situation, I'd be glad to know.

Thanks again for your help.

-Aron

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Answer 2

Oct 20, 2018 1:09 PM in response to japamac

japamac - thanks for your reply. Are you implying that ASD 2.5.7 could do the calibration where 2.5.8 failed?

The errors I mentioned above (Ambient Temp Sensor errors) were from ASD 2.5.8. As I remember, I got the same errors using 2.5.7. At the moment I can't check that as I'm getting incompatible memory errors when trying to start up - the heat gun treatment has run its course. It seems evident that the logic board solder joints are bad, so I'm just waiting for the arrival of a replacement logic board (Monday, I hope). The errors did change when I swapped out what appeared to be the bad processor. The errors changed from EEPROM checksum errors to the Ambient Sensor error. I've since been told by a local repair guy (though not a specifically Apple tech) that a bad logic board can cause errors that appear to be coming from somewhere else, so I'm hoping the logic board swap will be the answer (and perhaps I'll have three good processors now!).

I'll post the results when I get the board.

-Aron

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Answer 3

Oct 20, 2018 1:09 PM in response to japamac

Thought I'd give an update. Got the new (used) logic board today and put it in. Computer fired up and temperature calibration (ASD 2.5.8) worked! Computer's been running fine for a few hours now, and the fans are running normally. I'm keeping my fingers crossed. Only problem so far is that Time Machine didn't recognize the old backup, but I found this: 10.5: Repair Time Machine after logic board changes - Mac OS X Hints and it seems to have worked.

Thanks for the help.

-Aron

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Answer 4

Jacumba

Level 4

2,349 points

Dec 6, 2012 5:22 PM in response to Community User

aronfromtoledo, I just read your first post that you linked

I was hoping that someone could confirm my diagnosis of problems on my Powermac G5 2.0 GHz dual. It's one of the original ones (bought in May 2004). For a while now I've been having problems with the computer not waking from sleep, fans at full speed, shutting down unexpectedly, etc. I got hold of an ASD 2.5.7 disk and ran the tests. Everything passed, so I tried a thermal calibration (I hadn't changed CPUs, but I have had them out for cleaning a few times). The calibration failed with the messages:

The problem probably originated with a bad fan. Most people tend to think if a cpu cooling fan fails it just stops working. Not so. If the fan stays at high speed that is a indication that the fan is bad also. And will cause unexpected shut down. So the thing to do when that happens is to change the fan. If a problem persist it would indicate a logic board.

If the cpu was bad the computer may not even start. If the cpu was overheating they have a built in sensor to shut down before damaging the cpu, which would happen with a fan not running at all or severe dust build up. A fan is not compensating for overheating by reving high, it's more a indication of the fan itself being bad.

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Answer 5

japamac

Level 7

24,400 points

Dec 8, 2012 6:35 AM in response to Community User

Does anyone know if this serves the same function (i.e., is it the main ambient temperature sensor)?

I believe the thermistor is a main temp sensor for the system. The 2004/2005 G5's also have the thermistor, though they additionally have the Ambient Temp Sensor board.

is the Ambient Temp Sensor (IC2) reported by ASD a different component?

Yes and no. The seperate board on later models is one, but earlier models had the same circuitry as part of the logic board. I think that you can find the IC2 stamp on the logic board.

What are the errors that you currently see (if any) with the ASD 2.5.7? A new processor should allow calibration with 2.5.7.

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Answer 6

Jacumba

Level 4

2,349 points

Dec 8, 2012 8:56 AM in response to Community User

A cooling fan thermistor would be a heat sensor built into the fan itself. But I'm not an electronics engineer and don't dig that deep into the computer electronic science. If I suspect a hardware problem I just swap it out.

There is a misinterpretation, you clearly posted twice that the fans were at full speed, now you claim they're working. You seem to be all over the place with this computer from ram to fans to motherboard and I think you said cpu.

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Answer 7

japamac

Level 7

24,400 points

Dec 8, 2012 7:09 PM in response to Community User

Are you implying that ASD 2.5.7 could do the calibration where 2.5.8 failed?

No. I was looking for confirmation of the error (or not). I have seen variances between the tests, but chalked them up to multiple failures in the logic board on the machines that were showing such.

The errors did change when I swapped out what appeared to be the bad processor. The errors changed from EEPROM checksum errors to the Ambient Sensor error.

That would all be consistent with failing logic board circuitry/components.

The earliest G5 had the first moves to use of RoHS compliant solder use and saw various failures due to such.

My belief, with regard to the Ambient Temp Sensor, is that failures were too common to warrant leaving the sensor circuitry integrated in the logic board. A move to the separate Ambient Temp Sensor board in later models allowed replacement of just that part instead of the entire logic board.

The thermistor is a simple resistor providing temperature values based on resistance for the system to base function/fan speed on, but the circuitry supporting the temp sensing requirements for the 4 cooling zones of the G5 is more complex.

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Answer 8

japamac

Level 7

24,400 points

Dec 11, 2012 1:45 AM in response to Community User

Excellent! 🙂

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Answer 9

BDAqua

Level 10

250,259 points

Dec 11, 2012 3:24 PM in response to Community User

Great to hear! 🙂

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