I replaced an HDD with and SSD in an old 2009 iMac, disabled the CDROM. Since the SSD does not have a temperature sensor and runs cold, the iMac fan runs at full tilt.

Manually set the “cpu”, “hdd” and “odd” (The CDROM) fans to minimum with:

./fan.sh odd 0
./fan.sh hdd 0  # if using an SSD
./fan.sh cpu 50 # for the iMac8,1

Ref: https://github.com/JuampiCarosi/fan-control

#!/usr/bin/env bash
#

CMD="${0##*/}"
sysdir="/sys/devices/platform/applesmc.768"

declare -a control_file label_file output_file label

# Match labels with fan number and get control files
fan_info() {
    local fan="$1"
    control_file[$fan]="$sysdir/fan${fan}_manual"
    label_file[$fan]="$sysdir/fan${fan}_label"
    output_file[$fan]="$sysdir/fan${fan}_output"
    read -r label[$fan] < "${label_file[$fan]}"
    label[$fan]=${label[$fan],,}                  # lowercase
    echo $(cat $sysdir/fan${fan}_label) "auto="$(cat $sysdir/fan${fan}_manual) "rpm="$(cat $sysdir/fan${fan}_output)
}

sensors|grep -e Core -e ODD -e HDD -CPU
fan_info 1
if ! [[ "${label[1]}" =~ ^(exhaust|master|main)$ ]]; then
	fan_info 2
	fan_info 3
fi

# fan() - set fan
# $1 is fan number (starting from 1)
# $2 is percent to apply
fan_function() {
    local manual max min
    local -i fan_100 fan_net fan_final
    local fan="$1"
    local percent="$2"                            # "auto" or 0-100

    # Getting fan files and data from applesmc.768
    read -r manual < "${control_file[$fan]}"

    read -r max < "$sysdir/fan${fan}_max"
    read -r min < "$sysdir/fan${fan}_min"

    if [ "$percent" = "auto" ]; then
        # Switch back fan1 to auto mode
        echo "0" > "${control_file[$fan]}"
        printf "fan mode set to auto"
    else
        #Putting fan on manual mode
        if [ "$manual" = "0" ]; then
            echo "1" > "${control_file[$fan]}"
        fi

        # Calculating the net value that will be given to the fans
        fan_100=$((max - min))
        # Calculating final percentage value
        fan_net=$((percent * fan_100 / 100))
        fan_final=$((fan_net + min))

        # Writing the final value to the applemc files
        if echo "$fan_final" > "${output_file[$fan]}"; then
            printf "fan set to %d rpm.\n" "$fan_final"
        else
            printf "Try running command as sudo\n"
        fi
    fi
}

usage() {
    printf "usage: %s [fan] [percent]\n" "$CMD"
    printf '  fan: "auto", "master", "exhaust", "hdd", "cpu" or "odd"\n'
    printf '  if fan is not "auto", percent is "auto" or a value between 0 and 100\n'
    exit 1
}

if (($# == 0)); then
    printf "Available fans:\n"
    printf "  %s\n" "${label[1]}"
    if ! [[ "${label[1]}" =~ ^(exhaust|master)$ ]]; then
        printf "  %s\n" "${label[2]}"
        printf "  %s\n" "${label[3]}"
	fi
    exit 0
fi

# fan type and value
command="$1"
if [[ "$command" != "auto" ]]; then
    if (( $# == 2 )); then
        percent="$2"
    else
        usage
    fi
fi

case "$command" in
    ### AUTO CONTROL
    auto)
        echo "0" > "${control_file[1]}"
        if [[ "${label[1]}" != "exhaust" ]]; then
		    echo "0" > "${control_file[2]}"
		    echo "0" > "${control_file[3]}"
	    fi
        ;;

    ####  HDD/CPU/ODD CONTROL
    hdd|cpu|odd)
        for i in 1 2 3; do
            if [ "${label[$i]}" = "$command" ]; then
                fan_function "$i" "$percent"
            fi
        done
        ;;

    ### EXHAUST/MASTER CONTROL
    exhaust|master|main)
        fan_function 1 "$percent"
        ;;

    *)
        printf 'unknown command %s\n' "$command"
        usage
esac