Ambient Temperature Display

Introduction  |  Common Faults & Fixes  |  Part Information  |  Wiring & Tech Info

Common Faults & Fixes

The ATD sub-system is comprised of three main parts :

Faults are not uncommon in all three components. My own S2 has a dead display module and (maybe) a burp in the wiring loom out to the temp sensor (which seems to be in spec).

The most common failure within the display module is that the illumination bulb simply burns out. The backlighting is always on when the engine is running and its just a matter of time before the bulb gives up the ghost. This bulb can be replaced at very little cost as long as you aren't averse to a little bit of fiddly soldering inside your display module.

I used a sub-miniature 12V bulb from Maplin electronics (Part PG77J) with success. The operation is a little fiddly and I certainly wouldn't advise doing this yourself unless you have some prior soldering experience. An alternative device would be a super-bright sub-min 12V LED (also available from Maplin as CJ70M). The LED has the advantage of an easier install but you have to check the polarity is correct before proceeding. Photo coming soon...

It is not uncommon for the temp sensor to fail due to its perennial exposure to the elements. It's location on the S2 is underneath the front left headlamp. It is secured by a moulding in the bumper whilst a 2-pin cable assembly separates it from the rest of the wiring loom. It can be accessed reasonably well from the top of the engine bay with the cover of the main radiator removed. Its easy enough to unclip blind as long as the engine is cold. Link to photo...

The important component in the temp sensor loom is a simple thermistor which varies its resistance in response to the temperature at which it is exposed. This sensor has a negative thermal coefficient (NTC) which means that resistance increases in response to a decrease in temperature and vise-versa.

The thermistor has a nominal value of 1000 ohms at 25 degress celsius. Click here for further specification.

With the temp sensor removed, it is easy to verify that the thermistor is working correctly as the resistance can simply be measured across the two terminals with a DMM. Take a few readings around the house (in the fridge & freezer even) to check the thermistor is alive and well.

Replacement thermistors are available from a couple of sources in the US according to this info at 20V.org. Currently Maplin do not stock a suitable replacement item.

Details of the ATD pinout are provided at this link. Watch this space for info on how the ATD module's wiring makes its way from the back of the instrument cluster out the temp sensor....