MP 1,1-5,1 Mac Pro 2008 MCP Diode Temperature Sensor Stuck at 64°C

[AphoticD]

About a year ago my trusty MacPro3,1 decided to start running it’s IO fan at a constant full speed. I tried the usual SMC and PRAM resets, and renewed the thermal paste in attempt to resolve, but it made no difference.

It looks like the issue is the MCP Diode temperature sensor which reads a constant 64°C at all times. I believe this is triggering a hardware override to Macs Fan Control and other similar software (including the old SMC Fan Control under Snow Leopard). Technically, the Mac runs well and all other sensor readings are showing good temps, but it is loud and can’t be soothed with the usual software control solutions.

What's interesting is ASD returns a MOT failure on the IO fan itself (Failure Code: 4MOT/1/40000003: IO-2786), but I know for certain the fan spins at (near) full speed and the MCP Diode reading appears to be suspicious as it never changes.

I reached out to the developers of Macs Fan Control to enquire about a possible override in their software, and they were good enough to write back to me to say the only solution for a hardware issue like this would be to either replace the hardware (logic board and/or IO fan) or unplug the IO fan and add a 3rd party fan instead.

Have any resident MP3,1 specialists here ever encountered such an issue? Or anyone had luck running 3rd party fans or possibly an external fan controller for the stock fans? Any other workarounds come to mind?

Another question before I consider buying replacement parts, is it possible for a MOT error to cause the fan to run at full speed like this? If that's the case, am I looking in the wrong place regarding the MCP Diode sensor, and perhaps the 64°C temp is just safe to ignore?

Thanks in advance!

 

[h9826790]

About a year ago my trusty MacPro3,1 decided to start running it’s IO fan at a constant full speed. I tried the usual SMC and PRAM resets, and renewed the thermal paste in attempt to resolve, but it made no difference.

It looks like the issue is the MCP Diode temperature sensor which reads a constant 64°C at all times. I believe this is triggering a hardware override to Macs Fan Control and other similar software (including the old SMC Fan Control under Snow Leopard). Technically, the Mac runs well and all other sensor readings are showing good temps, but it is loud and can’t be soothed with the usual software control solutions.

What's interesting is ASD returns a MOT failure on the IO fan itself (Failure Code: 4MOT/1/40000003: IO-2786), but I know for certain the fan spins at (near) full speed and the MCP Diode reading appears to be suspicious as it never changes.

I reached out to the developers of Macs Fan Control to enquire about a possible override in their software, and they were good enough to write back to me to say the only solution for a hardware issue like this would be to either replace the hardware (logic board and/or IO fan) or unplug the IO fan and add a 3rd party fan instead.

Have any resident MP3,1 specialists here ever encountered such an issue? Or anyone had luck running 3rd party fans or possibly an external fan controller for the stock fans? Any other workarounds come to mind?

Another question before I consider buying replacement parts, is it possible for a MOT error to cause the fan to run at full speed like this? If that's the case, am I looking in the wrong place regarding the MCP Diode sensor, and perhaps the 64°C temp is just safe to ignore?

Thanks in advance!

SMC Fan Control cannot override the fail safe fan speed from SMC.

MFC can override some SMC fail safe fan speed if that's due to sensor failure (4SNS).

In your case, it's the fan motor failure (4MOT), therefore, MFC cannot help. You have to fix the hardware, no way to work around that via software method.

There are few ways to set fan speed. MFC is already using the Target Fan Speed, which has the highest priority. If it doesn't work, then nothing will work.

Apart from change the fan, there is not much you can do. May be unplug it, check the wiring, and reconnect it. And see if that help anything. Zero cost anyway.

 

[AphoticD]

SMC Fan Control cannot override the fail safe fan speed from SMC.

MFC can override some SMC fail safe fan speed if that's due to sensor failure (4SNS).

In your case, it's the fan motor failure (4MOT), therefore, MFC cannot help. You have to fix the hardware, no way to work around that via software method.

There are few ways to set fan speed. MFC is already using the Target Fan Speed, which has the highest priority. If it doesn't work, then nothing will work.

Apart from change the fan, there is not much you can do. May be unplug it, check the wiring, and reconnect it. And see if that help anything. Zero cost anyway.

@h9826790 thanks for the reply. I’ll try reseating the fan connection again. It all looks clean and undamaged.

I’ll try hunting for a 2nd hand replacement IO fan, but curious about that stuck temp sensor. It will show 64C at cold boot, as well as under load, and never fluctuates (confirmed with iStat Menu monitoring graphs). So my concern is IO fan might just be set at full speed regardless of the MOT error.

 

[h9826790]

You may have multiple hardware failure (sensor + motor) indeed.

But if just sensor failure. And SMC command high fan speed due to that high temperature reading. MFC should able to override it. This technique is widely use on iMac HDD upgrade that lost the HDD temperature sensor.

 

[LightBulbFun]

I do wonder if the fan has failed such that its stopped responding to PMW control and since its otherwise just fed a fixed voltage its just ramping up to Max (or something such along those like that the Fans built in speed controller has failed short and is just allowing 12V through all the time an internal mosfet having failed short or such)

and then obviously the ASD will flag a fail since it wont be able to control the fan speed, it will likely try to command the fan to change speed, and want to see that speed change fed back to it on the tach output, but see no change on the tach output of the fan thus flag and fail

at least I hope its the fan itself thats failed and not the logic board PMW driver...


on the stuck temperature sensor, i'd like to see the same mac fan control screen shot on another 3,1, I have noticed often times Mac Pro's can have phantom sensors or other such anomalies, they dont affect the SMC or such, but do look weird in temperature monitoring programs

doing a quick google search, shows other 3,1's reporting the same 64c MCP diode temp as your machine does (well 63.5c in the link below but close enough)

https://forums.crystalidea.com/viewtopic.php?t=1884

so I think the 64c thing here might be a red herring