Outer Limits 4x4 Board

Spent far too long on this one today and thought I'd throw it out there to see if any one had any suggestions.

A friend noticed my brake light was working on one side while following me the other day, so I had me a look while I was working the car. Did the usual - checked bulb "looked" fine (ie. both filaments were intact), checked conductivity back to plug under dash, checked fuses, checked for obvious signs of wear along cables. Nothing came up. I then spent far too long trying to bypass sections of the circuit with a known good test wire, to no avail. I finally went back to basics and thought I'd swap bulbs left and right. That's when the strangeness really started. Here's what I found:

| Brakes on | lights on | both on
Left bulb | good | good | good
Right bulb| nothing | good | light

Swap bulbs:

Left bulb | good | nothing | good
Right bulb| light | good | nothing

(where "light" means only one filament on, like when the tail lights are on normally)

I can't make head or tails of it. Firstly, why would swapping the bulbs make any difference, when they both work in at least one situation? And secondly, what the h3ll is going on here?

I don't imagine the bulbs are too expensive (but I've been wrong before!) so I'll try a good known bulb or two soon if I can.

Have a close look at the metal cap, you will see two locating pins on each side of the cap. You wil also notice that they are not directly opposite each other, ie one pin is higher than the other. This is so the bulb can only be inserted one way in the holder - some holders deflect enough to allow the bulb to be inserted incorrectly. There are two contacts on the bottom of the metal cap, with the earth running through the metal cap, if the bulb is slightly off, or inserted incorrectly, the two contacts may connect with the holder contacts improperly and give you problems with bulbs doing 'strange things' as you say.

Each contact on the bulb is for the different filaments in the bulb, one for 5w for taillights, 25w for brake, so if they both contacted the one contact in the holder, you have the problem.

I did notice the offset bayonet base, and am pretty sure neither bulb would go in either socket the wrong way. I'll double check that.

I'll pull them out again this evening and see if I can see any signs of cross circuit conduction. My casual glance suggested the fittings and bulbs were okay, but that would be contradictory to the odd problems after swapping bulbs. I hope the problem is a bad fitting - I don't want to pull all the wires out again! Thanks for the reply.

one word is a dead giveaway.....
"light"

Right bulb| nothing | good | light

A dull light is a good indication of a poor earth,
as area54 said check the metal frame around the globe but also check the earthwire where it attaches to both the globe holder and the body.