Thermal Sensors

This page aims to summarize known information about the locations and properties of thermal sensors on ThinkPad laptops.

Accessing the sensors

The primary means of accessing the thermal sensors is through the ibm-acpi module, which provides up to 11 thermal sensors (some of which may be inactive) when loaded with the experimental=1 option. Up to eight thermal sensors are exposed at /proc/acpi/ibm/thermal:

# cat /proc/acpi/ibm/thermal;  
temperatures:   44 41 33 42 33 -128 30 -128

On some models there are three additional sensors at Embedded Controller offsets 0xC0 to 0xC2, which can be read by parsing /proc/acpi/ibm/ecdump:

# perl -ne 'm/^EC 0xc0: .(..) .(..) .(..) / or next; print hex($1)." ".hex($2)." ".hex($3)."\n"' < /proc/acpi/ibm/ecdump
40 48 43

The Active Protection System accelerometer also reports a temperature (but it is not certain that the sensor location is inside the HDAPS chip):

# cat /sys/bus/platform/drivers/hdaps/hdaps/temp1
41

Finally, the hard disk temperature can be read through the SMART interface:

# smartctl -A /dev/hda | grep Temperature
194 Temperature_Celsius     0x0022   145   097   000    Old_age   Always       -       31

Sensor locations

This information is model specific.

ThinkPad T43

The following image, prepared by Shmidoax, summarizes the results of his experiments using cooling spray to cool down components and observe the effect on the sensors.

(click to enlarge)

(click to enlarge)

Summary:

EC offset   Index in "thermal"   Location (estimated)
0x78        1                    CPU
0x79        2                    Between PCMCIA slot and CPU (same as HDAPS module)
0x7A        3                    PCMCIA slot
0x7B        4                    GPU
0x7C        5                    Battery (front left)
0x7D        6                    n/a
0x7E        7                    Battery (rear right)
0x7F        8                    n/a
0xC0        none                 Bus between Northbridge and DRAM
0xC1        none                 Southbridge (under Mini-PCI card, under touchpad)
0xC2        none                 Power circuitry, on underside of system board under F2 key

ThinkPad T40

The location of one of the sensors is identified here.

EC offset   Index in "thermal"   Location (estimated)
0x78        1                    CPU
0x79        2                    System board under rear left corner of Mini-PCI module
0x7A        3                    ?
0x7B        4                    GPU
0x7C        5                    Battery
0x7D        6                    n/a
0x7E        7                    Battery
0x7F        8                    n/a
0xC0        none                 n/a
0xC1        none                 n/a
0xC2        none                 n/a

ThinkPad R51

The ibm-acpi documentation includes the report by Thomas Gruber:

EC offset   Index in "thermal"   Location (estimated)
0x78        1                    CPU
0x79        2                    Mini-CPI
0x7A        3                    HDD
0x7B        4                    GPU
0x7C        5                    Battery
0x7D        6                    n/a
0x7E        7                    Battery
0x7F        8                    n/a
0xC0        none                 ?
0xC1        none                 ?
0xC2        none                 ?