Help sanity-check an odd (but simple) electrics / wiring issue

growse · 3 May 2015 at 12:30

I'm having a bit of a problem fitting a new numberplate light to the back of my bike (a '12 Triumph Speed Triple). The replacement unit is an LED unit from R&G and I've had 3 brand new ones shipped from them and none of them seem to work.

The specific light looks like this:

So, two simple connectors and as the pic shows the kit comes with two sockets which I've crimped onto the existing loom. I've tested this with a voltmeter and get a nice 12V across with the engine running. However, none of the three LED units I've got will light up when plugged in - yes, I've tested them both ways round.

I checked the resistance of the LEDs with a multimeter (actually, two, just to be sure the multimeter isn't faulty) and they all appear to have infinite resistance in both directions. I'm no expert, but shouldn't an LED have infinite resistance in one direction, and a lowish resistance in the other?

This should be such a simple thing, and yet I can't quite believe that R&G have shipped me three brand new, identically faulty lights.

Any ideas what to check next?

Drangueos · 3 May 2015 at 13:45

Do they light up if you connect direct to the battery?
red to + and black to -, if it doesn't light up, it is not working.

growse · 3 May 2015 at 21:04

Is it safe to try directly against the battery? Don't want to effectively short the battery out...

kaku · 3 May 2015 at 21:07

What's the spec of the LEDs? It should be okay to connect directly to the battery; the only way it would short would be if something's SERIOUSLY wrong and the LEDs are bypassed entirely.

growse · 4 May 2015 at 20:34

kaku said:
What's the spec of the LEDs? It should be okay to connect directly to the battery; the only way it would short would be if something's SERIOUSLY wrong and the LEDs are bypassed entirely.

Thanks - I tried it across the battery and managed to prove that there was an issue with my crimp on the loom. Strange how it was giving me a voltage across it, but put any load on it wouldn't work at all.

Re-crimped, and fixed

Adam_151 · 4 May 2015 at 21:37

growse said:
. Strange how it was giving me a voltage across it, but put any load on it wouldn't work at all.

Re-crimped, and fixed

Not really strange, a high resistance connection will do this.. with a high impedance voltmeter there is no significant voltage drop across the resistance, add a load thouh and the volatge drop across the resistance is much higher (and could even be most of the supply voltage)