This will be a tachograph unit. Different types but take a Lucas tachograph as an example. 2 different warning lights on dial - one lights up if the tachograph isn't properly closed or if there is no disc in it or if the stylus isn't contacting the disc.
The other light is the pre-set speed warning light - typically this is set at the HGV speed limit - 80 km/hr and illumintaes when you go over that speed.
JD
January 2009
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