iMac Late 2009 HDD replacement, Fan ramps up to almost max rpm.

If I recall 2009 iMacs had hard drives with the temp sensor built into the circuitry of the drive itself, they did not have external stick on sensors. You cannot purchase a drive with that circuitry so you have to add an external in line sensor like the one sold by OWC

https://eshop.macsales.com/item/OWC/DIDIMACHDD09/

+1 on getting the OWC temp sensor recommended by SeaPapp.

I owned a 2009 iMac which I retired back in 2018. I upgraded the hard drive to an SSD and from I recall, I installed a jumper wire in the HDD temperature sensing cable. It's been a long time but consider researching this a bit more. I had zero problems with the fans and the my iMac continued running happily until it finally went kaput in 2018.

Be very careful though. The 2009 iMac had a serious design flaw with the LCD sync cable. It was a very delicate flat-ribbon cable that after multiple LCD panel removals began to literally fall apart from one to many re-insertions into the motherboard connector. A hugely bad design decision by Apple. Thank goodness they removed that design in later models.

It was a solid machine. Good luck.

Yes. I wouldn't call it a "short circuit". The wire is essentially completing the circuit. A temperature sensor senses heat by the increase in electrical resistance. The hotter it gets, the more resistance, resulting in lower voltage. When the sensor is cool, the is little to no electrical resistance.

So by putting in a jumper, full voltage with no resistance travels the circuit and essentially tricks the system into thinking that the phantom HDD is running cold, so not necessary to spin the fan.

The LCD ribbon connector fell apart after the 3rd removal. After that, it was rolling dice if the contacts actually made contact. Terrible design.

Replacement hard disks from the generation require the in line sensor. I too am surprised that it doesn't ramp up to full speed right away.