Brake light issue NA8

[stevo]

I have a very simple problem but it's got me stumped. I noticed the boot lid mounted brake light wasn't working ..... "No worries, whatever it is that will be a very easy fix"

The bulb works when I test it on it's own with 12v and an earth supplied, and also when I test it with it mounted in the bulb holder off the car. Not surprisingly it also passes a continuity test with a meter.

I have 12v, earth and continuity at the terminals in the wiring connector at the lamp when I'm supposed to.
With the bulb holder connected to the wiring connector I still have 12v, earth and continuity at the terminals in the bulb holder.

So that rules out any connection problems between the bulb and bulb holder, and between the wiring connector and bulb holder.
Now here's the odd part. Plug the bulb into the bulb holder, plug the wiring connector into the bulb holder, press the brake pedal and hey presto .................NOTHING !

Just to make sure I wasn't going mad I did my testing another 3 times, with the same results each time. All my testing was with a meter not a test lamp. When testing at the wiring connector plug I tested between the two wires, and also between each individual wire and 12v/earth respectively.

I'm sure the answer is going to turn out to be something very simple but at the moment it's got me puzzled. Can anyone point out the glaringly obvious thing I seem to be missing?

[MiGoreng]

Brake light switch behind your pedal. Check it

[manga_blue]

Check that the centre contact of the socket still gives +12V when it's depressed, as if a bulb was in it. Otherwise just try changing the bulb anyway. I've had perverse house light bulbs that work in one socket and refuse to work in another for no apparent reason.

BTW the wiring to the boot mounted stop light is a common failure point in NA8s. The copper inside the wire near the boot hinge loses continuity while looking perfectly OK inside its plastic casing. Sometimes it will work with the boot open but not closed.

[Mr Morlock]

I would change the wedge base bulb to eliminate it. If you have power to the connector with brake down then the fault must be in the bulbholder or the bulb. The other thing to change is the wedge base bulb holder- presumably you could get one without too much trouble or borrow one if you have a mate with same car.

[stevo]

Thanks for the replies.

It can't be the brake light switch MiGoreng, I have power at the wiring connector and the other brake lights work correctly every time I depress the brake pedal.
.
I know what you mean about broken wires Manga, I've had that before on other cars, but the wiring is fine. I even pulled it out of the boot lid and checked it visually, and with a meter while bending it all over the place, all good. There is no center contact on this bulb holder, it has the capless type bulb with one contact each side.

Your reasoning is exactly the same as mine Mr Mortlock, with the pedal depressed if I have power and earth at the connector then it can only be the bulb or the bulb holder. I tried 2 new bulbs, still no good. I haven't tried or replaced the bulb holder as yet, I guess that's the only thing left. I'm pretty sure that's OK but decided to check it again anyway.

So I removed the bulb holder from the lamp and left it connected to the wiring, just hanging down. I removed the bulb from the holder and tested with a meter at the two contacts in the holder, exactly where the bulb makes contact. The meter shows 12 volts with the pedal depressed and zero with the pedal released. So all good up to, and including, the bulb holder contacts. I insert a bulb in the holder and get nothing.

That only leaves a possible bad connection between the bulb and the holder. So I tested it again as I did yesterday. With the bulb in the holder and the holder not connected to the wiring I supplied a direct earth and 12v to the holder terminals where the wiring plugs into it. The bulb works fine. I also tested it for continuity while wiggling the bulb around in the holder, perfect, no loss of continuity at all. So that rules out a bad connection between the bulb and holder.

Every test and check I do rules out each possible cause. The only other thing I can think of would be if the bulb had a high resistance internally, but it can't be that either. That 'could' explain why a bulb works when powered directly, and not when connected to the car wiring. I've come across that before with relays and other components, but never with a bulb. And the odds of have 3 bulbs, 2 of which are new, all having a high resistance are ridiculous.

I can't believe how something as simple as this has got me stumped. It's one bulb on a very simple circuit !!!

[manga_blue]

Does that say you've got just enough power going to the lamp holder to enable a 12V reading on a multimeter but not enough to drive a bulb? I.e. the circuitry fails somewhere when it's drawing more than a couple of watts. It could be interesting to put a pot and a multimeter across the lamp holder.

You could be working at the wrong end of the car and the brake switch or the fuse or the fuse holder are still worth checking.

Edit: correction, just read about the other brake light working. It must be weakness somewhere in the centre light loom. I'd try temporarily bypassing it with jumpers to see what happens.

[Mr Morlock]

I reckon its the bulb holder. The b/holder is usually 2 wires with a terminal in the b/h. Either there is a break or a bad connection which does not pass current. I would substitute the b/holder with another.Some bublholders have a terminal pushed into them which locates in the moulding and held by a sprag on the terminal. The connection with the bulb is made with the terminals and the elements on the bulb- a visual examination might be in order as well.