Bit of head scratcher here. The light came on on the dash indicating a bulb was gone, turned out both number plate lights were out. Replaced the bulbs, no good. Checked for breaks in the cable at the boot jam, all looks good.

But what I just noticed is when you turn on the lights without the ignition switched on the bulbs work fine. Within a second or so of the ignition being switched on the light comes on on the dash and the number plate lights go out! Any ideas as to what direction to take on this one??? 

Tests on Friday, any help would be greatly appreciated 😁

I would start with checking earth connections for these lamps

The bulb failure check is made when you switch on the ignition. As the bulbs work when you switch on the lights but then go out with the ignition, the bulb checker is saying the current taken is too small. If it thinks there is a bulb failure it will then remove the power to that/those bulb(s).

As mentioned in the post above, it may be a high resistance earth connection but I would also check out the bulb holders too for corrosion due to water ingress.

Clearly the reduction in current/corrosion is not much as you can initially see light from the bulbs. How bright?

I assume you have not recently changed the bulbs for lower wattage bulbs or LEDs.

Thanks for the responses. No LEDs, normal bulbs. I checked the bulb holders already, they look clean.

The lights aren't as bright as you'd expect though, I replaced the bulbs today, need to make sure they're the same wattage as replaced.

 

Is the earth for these bulbs one of those in the engine bay?

 

Check the wiring plug, seen the connectors corroded away to nothing. Worse part is trying to get the remains of the pin out of the plug.

 

Scott

try disconnecting the battery and leaving it for 30 mins see if that makes any difference. I had an odd indicator fault once and this made it go away. 

Edited 2 July, 20196 yr by seriesdriver
spelling

Sorry for the delay in responding, busy week. But problem sorted.

A cracked earth wire leading to water ingress & corrosion left the number plate lights & rear wiper motor (rear wiper had stopped working as well) with a poor earth connection. The wiper motor casing was positive, reading around 10 volts. 

Found the earth point in the boot on the LHS. Bypassed the crack earth wire, ran the new wire to the wiper motor, earthed directly to it. 

Readings back to normal, wiper working & number plate lights staying lit.

Cheers folks!