Can anyone explain this - LED bulbs

[john2443]

Halogen dip /main beam, press the dip switch, bulb changes between dip and main.

Replace bulb with LED , nothing when dipped, main is OK.

Take main wire off, dip works OK

The car's 1951 so no canbus or anything fancy to confuse it, 3 wires, 1 main 12v, 1 dip 12v, 1 earth (I tried a different earth and it didn't change anything)

I'm mystified!

[mobile chicane22]

Is it a H4 style bulb with a fixed connector or 3 loose wires which are connected, bench test the LED unit and try the 0v and 12 v on different connectors, this shouldn't blow the LED unit unless its cheap rubbish.

[mobile chicane22]

Also photos of what you are replacing would help assess better

[Decky_Q]

It might not support both sections of the LED lit at once (the way the halogen has 2 filaments lit for high beam). This isnt common but definitely a possibility that they are using the same driver chip for both sections thus unable to operate both at the same time.

[finlo]

Could It be a positive/negative earth thing?

[LarJammer]

Crap earth? BTW, 1951 is early for 12v...

[john2443]

Replying to all the comments...

Have bench tested, high and low both work.

It's positive earth.

It was originally 12 v and still is.

Have tried a separate earth - I wondered if it was doing the dodgy tail light thing of trying to earth through another 'bulb'

Both aren't lit at once - the dip switch turns 1 off and the other on.

Wiring is quite new, good condition, I know exactly where everything goes because I made the harness.

I checked the main and dip wire when they were off to make sure there wasn't some stray voltage, and nothing there.

H4, replacing halogen with one like this.

[Vsix and Vtec]

Will a bulb designed for negative earth applications work in a positive earth setting? Sounds to me like the simplest solution is to create a separate negative earth headlamp circuit for them.

[BertBert]

I'm surprised it works at all with a positive earth. With LEDs being diodes, I would think they'd not work at all on the wrong polarity. Positive earth though as said will def be the problem. It'll work ok on incandescent bulbs though,