For LED headlamp bulb hints and tips click HERE


I thought I'd write this page to benefit those that may be experiencing problems when fitting LED bulbs and light boards.

Just so you know that it's not a faulty bulb.

So, you fit nice new low power consumption LED bulbs or light boards and they don't all work but the original filament bulbs did.


For example, you've fitted our stop/tail bulbs or light boards.

The stop lights work fine but the tail lights won't work at all. 

Or the stop light won't work with the tail lights on. 

If you test the LED bulbs on a battery they work fine?

This points directly to a simple vehicle wiring problem that wouldn't have shown up with filament bulbs and is actually to your advantage to fix as it means your wiring will be as originally intended.


First and foremost, where combined stop/tail bulbs are concerned, fit both bulbs before testing. Fit one and you may find it won't work. Fit the other and they will both work.

Still got issues??? It'll be a bad earth somewhere in that circuit or one or more lamp is wired the wrong polarity

This problem tends to show up more with lampholders that are insulated from the vehicle or where the number plate lamp is separate to the stop/tail lamps

A cruddy earth connector can restrict the passage of current and because LEDs use so little current a cruddy connection can mean no connection.

LEDs are all polarity conscious. Our bi polarity LEDs work with either positive or negative earth but every bulb on a particular circuit MUST be connected the same polarity or one will conflict against the rest and result in problems. Dedicated positive or negative earth bulbs work on the same in theory.

For example - A positive earth vehicle has 2 Lucas stop/tail lamps with our bi polarity or positive earth stop/tail LED bulbs fitted. The number plate lamp is separate and is connected to one of the tail lamps so that it comes on with the tail lamps. The vehicle being positive earth means the positive feed goes to the cap of the stop/tail bulbs not the terminals. The feed to the number plate lamp must be the same i.e. positive from the cap of the stop/tail bulb MUST be connected to the cap of the number plate bulb holder(s). 


A customer complained that neither my bi polarity or dedicated positive earth LED bulbs would work properly on his positive earth Fordson van. Stop lamps were fine but the tail lamps wouldn't work at all. If he fitted the original bulbs, all was well. The vehicle was freshly restored and fitted a dynamo that I had rebuilt and a regulator that I'd converted to 6v positive earth electronic. It had restored lamps and a new wiring harness. Everything worked as it should bar the tail lamps.

On investigation, the tail lamps were wired positive earth but the separate number plate lamp had been connected for negative earth. Simply swapping the 2 wires around on the number plate light cured the problem.

LED bulbs can fight against filament bulbs on the same circuit so if you change your stop/tail bulbs you may need to change sidelight bulbs too.

Treat LED bulbs like small children - Whatever you give one you must give exactly the same to the rest or there'll be conflict!

The same goes for flashing indicators, pilot lamps, side lamps, dash lamps etc. 


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