Fender electrics have got me this time

Guys and girls I'm really stuck with this one. Defender 200tdi, reversing light isn't working. I know I know it's a dead simple one but get this... Just finishing up fitting r380 so naturally I've got the newer type switch, with is working a treat. I've got power in, power out, and I've got 12v to the lamp at the back (green and brown). The earth is all checking out from the lamp, good continuity, and I've even ran a new earth straight to the battery, but I cannot get the lamp to work!!
The bulb is fine, continuity through the bulb and she shines when it's hooked up direct to the battery but for some reason there doesn't seem to be any amperage at the lamp itself. Even scratching the 2 wires together doesn't produce a spark, but yet test after test is still showing power at the back when in reverse gear with the ignition on. It's giving me all the symptoms of just a blown bulb, but it's not blown.
Please someone "shed some light" on this one for me... (See what I did there [emoji16]) many thanks.

Fit a new unit

they plugs or bullets on the 200? what happens if you connect the wires directly to the bulb? bypassing the unit. also run engine on and try.

They bullets. All worked fine on the old switch before I pulled the box, but like I said new switch is working a treat anyway. I've cut the bulb holder of and stripped wires back, still nothing when touched to the bulb.
I'm wondering now if it's a resistance between the switch and the bulkhead plug before it goes back down the chassis to the rear

300tditoy said:

They bullets. All worked fine on the old switch before I pulled the box, but like I said new switch is working a treat anyway. I've cut the bulb holder of and stripped wires back, still nothing when touched to the bulb.
I'm wondering now if it's a resistance between the switch and the bulkhead plug before it goes back down the chassis to the rear

Click to expand...

wire is old now, it could deffo be resistance. that's why i mentioned trying it with the engine on, to see if the extra juice helps.

go pull the wire from the switch and earth it to the gearbox. then stick your multi on the other end and see what it says res wise. bit crappy as it going via gearbox and chassis.. but will give an idea

I say fit a new unit also, even if the bullets are fine, they usually corrode where the metal tips touch the bulb, it builds kind of a thin film over the tip but enough to stop current passing. You could try scratching the tips with a small screwdriver, just where it touches the bulb.

Even if that works, you will get more intermittent issues over time. So for the price of the units, its worth replacing for the piece of mind of know the job is done for good.

I'm going to replace the unit in the end anyway, but the unit can't be at fault right now because I've already cut it off. I've got 2 wires with the ends stripped back, and even touching them to the bulb nothing happens.
Thanks for the advice so far though, I'll keep looking.