Headlamp aim remains the most common cause of initial annual test failure on heavy trucks. Although this is a
seemingly trivial issue when compared to, for instance, tyres or brakes, it can still cause delays and expense,
and ultimately impact on an operator’s OCRS rating.
VOSA successor organisation the Driver & Vehicle Standards Agency (DVSA) has issued advice on headlamp alignment for those preparing vehicles for test, pointing out that there are two main reasons why vehicles
are presented with misaligned lamps: human factors and equipment factors.
If you find trucks you prepare for test are failing on beam aim then the first thing to do is check that the aim tester you are using is correctly calibrated and adjusted: many go out of true in use.
There is also a human factor. Every individual sees the light spectrum slightly differently, and many men are, to a certain extent, colour-blind. Variations in colour vision at the blue and red ends of the spectrum will mean that individuals may see the cut-off line in slightly different places.
To accommodate these variances, there is a 2.75 per cent margin within which headlamps can be set. The trick is to centralise the beam within the allowance as best as you can.
Not all workshops have access to precision beam alignment technology: some still shine the lamps on the workshop wall. If you have to do this, then DVSA suggests you must do your best to ensure the beams are actually level with each other: they find some vehicles have one lamp pointing up and the other down. It’s also good practice to check headlamp aim every time a bulb is changed or the lamp itself is disturbed.