Same thing here on a 2011 iMac.
Computer is overheating and nothing I do in MFC seems to make a difference. I have added an SSD and upgraded the HDD and previously I used MFC to prevent macOS from spinning the HDD fan at 100% all the time.
Now with High Sierra, MFC tells me the HDD fan is spinning at 1979 rpm all the time (+/- 2).
I tried changing that fan to auto control, or custom (constant value 5500), or custom (constant value 1100), or sensor based (WDC WD40EFRX-68WT0N0, from 42c to 52c -- this is what I previously used before High Sierra).
HDD temp is currently 57c and nothing will make this fan spin faster.
Here's a screenshot:
Here's the technical info:
Macs Fan Control 1.4.8.1
iMac12,2
CPU: Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz [100]
GPU: Intel HD Graphics 3000 [active], AMD Radeon HD 6970M
macOS 10.13.1
Fans:
ODD-min{1150}curr{1345}max{2926}mode{0}
HDD-min{1100}curr{1978}max{5500}mode{1,5500}
CPU-min{940}curr{3145}max{2901}mode{0}
TempSensors:
TA0P () - 30.0234, 30.0234
TA0V () - 32.1133, 32.1133
TA0p (Ambient) - 34.75, 34.75
TC0H () - 54.875, 54.875
TC0P () - 55.5, 55.5
TC0c (CPU Core 1) - 63, 63
TC0h (CPU Heatsink) - 54.875, 54.875
TC0p (CPU Proximity) - 55.5, 55.5
TC1c (CPU Core 2) - 65, 65
TC2c (CPU Core 3) - 65, 65
TC3c (CPU Core 4) - 61, 61
TCGc (GPU PECI) - 65, 65
TCSc () - 62, 62
TCXc (CPU PECI) - 66.5625, 66.5625
TG0D () - 74.5, 74.5
TG0H () - 68.25, 68.25
TG0d (GPU Diode) - 74.5, 74.5
TG0h (GPU Heatsink) - 68.25, 68.25
TG0p (GPU Proximity) - 68, 68
TH0O () - 9, 9
TH1O () - 5.61328, 5.61328
TL0P () - 46.5195, 46.5195
TL0V () - 52.8359, 52.8359
TL0p (LCD proximity) - 58.6875, 58.6875
TL1P () - 58.3125, 58.3125
TL1V () - 49.0664, 49.0664
TL1p () - 58.3125, 58.3125
TL1v () - 49.0586, 49.0586
TL2V () - 55.9375, 55.9375
TLAV () - 55.9375, 55.9375
TLBV () - 51.3477, 51.3477
TLCV () - 52.8359, 52.8359
TO0P () - 53.25, 53.25
TO0p (Optical Drive) - 54.25, 54.25
TPCD (Platform Controller Hub Die) - 65, 65
TS0P () - 49.25, 49.25
TS0V () - 46.2773, 46.2773
TS0p () - 49.25, 49.25
TS2P () - 51.25, 51.25
TS2V () - 48.4844, 48.4844
TS2p () - 51.25, 51.25
Tm0P () - 55.375, 55.375
Tm0p () - 55.375, 55.375
Tp1P () - 57.875, 57.875
Tp1p () - 57.875, 57.875
Tp2H () - 63.375, 63.375
Tp2h () - 63.375, 63.375
Tp3H () - 57.625, 57.625
Tp3h (PSU Primary) - 57.625, 57.625
Disk drives:
WDC WD40EFRX-68WT0N0 - 57
Samsung SSD 840 EVO 1TB - 55
ST2000LM003 HN-M201RAD - 51
ST2000LM003 HN-M201RAD - 52
ST2000LM003 HN-M201RAD - 47
ST2000LM007-1R8174 - 50