stevieboy308:
i’ve only used ones that you need to turn the side lights on as well!
contraflow - the ones that go off hydraulic pressure require a specific height, and will only be accurate if the trailer is loaded uniformly, was calibrated with a uniform load and was on level ground both times. plus i’ve never seen them on a artic bulker, only a farm tractor trailer.
The old hydraulic ones did just that-some digger drivers were wise to it and would load the back end first so when your load was meant to be spot on, you were overweight (unless your tipper body was low sided). I got around this by having mine setup with the load biased towards the back.
Contraflow:
Dan Punchard:
It could be that lazy fitters energise them via the side lamps to save time wiring them properly to a separate power supply.
But all fitters are lazy… yet my weigher has nothing to do with my side lights? 
My weigher is switched on and operational until I switch it off at the end if the day.
It comes in useful sometimes, because even when the body is down, it still displays either a negative weight (when empty) or a few hundred kilograms (when loaded) and I can judge when the trailer wheels are about to lift off the ground (when traversing Woodhead on the limiter, for example) by the fluctuations in the weight displayed.
You’re better off leaving the weigher switched on all the time as switching it on and off constantly knackers the cells up 