fiat punto varying brightness of lights when driving at night

Yep. Its working properly. When you rev the engine, the alternator provides more current and a raise in voltage which results in brighter lights although when driving down the road it shouldn't be massively noticable.

Get the alternator tested.

If the battery's about had it this can happen. It's because it's not good enough to light the lights at full strength by itself and you only get full strength when the engine's going fast enough to get the alternator to supply the required power. Suggest you get it checked.

Rob Graham

Get a DVM and measure the voltage at the battery (lights on) at idle and with the engine running faster so the lights are at their brightest and report back. Some variation in brightness could be normal on that model. But that could be proved by knowing the voltages are within tolerance.

You could also try charging the battery with an external charger and see if the difference between bright and dim is still the same.

When actually moving, I'd not expect to see any variation in the lights. Alternators tend to reach their peak output at quite low revs.

It will do that to some degree normally, but if it varies greatly I'd suspect the alternator or the regulator contained in it.

If the alternator is under a heavy load, the voltage will drop a bit at tick over and the lights dim a little. If at normal road speeds they are varying, there is a fault on the alternator which needs to be checked out - it could mean a rectifier has failed or the voltage regulator.

Hi folk, Many thanks for the replies, he had the alternator checked out with gauge following advice and yes, the voltage regulator was a duffer, readings up and down, and now stable after the fix. He replaced the alternator and as the regulator was with it, got the fix that way. Also replaced cambelt whilst at it as a wise move and it goes far better now.

Steve