Temp Sensor Failure

2011 iMac 27" Mid 2011 3.4ghz i7 32gb 1333ram AMD 6970M 2GB. Factory SSD and HDD.

I'll just start by saying I NEED to know at what temperature each fan should kick on. Why? Because I'm manually overriding the temperature sensors with Macs Fan Control - great free app by the way.

Here's what happened. The aluminum shell of the iMac had cosmetic damage, took it in to Apple, new aluminum back was installed. Sweet.

BUT Apple tells me the smart control for the hard drive is dead...Hard drive is $150...But worse, temperature sensor is dead...need new logic board...$700. I say no thank you to both. Take my iMac home.

When I get home, from the moment it boots, fans are running at full throttle - LOUD. Yikes. Never happened before.

I do some research. Install Macs Fan Control - liked it better than SMC Fans.

Did extensive tests. Macs Fan Control regulates everything perfectly. All temperature read outs are present and all fans kick on when things heat up under manual control. So keeping this iMac cool will not be a problem as long as the App is running.

Here's where it's interesting. The ODD fan runs full blast non-stop in auto mode. Weird. But if I use the App to regulate it, boom everything is great.

Next, could not get CPU fan to rev up in Auto. Got CPU Core 1 all the way to 75 and the fan did not speed up at all. So I manually set up the parameters, ran terminal code to max all four cores, and the app regulated temp perfectly.

So you guys can decide to explain to me what's going on exactly, haha, or skip all that, and just tell me the optimum temperatures for the fans to rev up.

Here's what I came up with on my own.

ODD: told it to rev up when the SSD hits 50, and to max out when the SSD is 70. My thinking: who cares what temp the ODD is? So let's just kick on this fan too when the SSD gets warm...
SSD: told it to rev up fan at 45, and max the fan at 58
CPU: told it to rev up fan at 65, and max out fan at 75, this might be too soon, maybe 85?

Thanks so much!