Electrical help - lighti ng!

[gdunc]

Chappies, my ns headlamp (dipped only) started 'blinking' on and off then finally just off on the way home from work. I thought this unusual as a lamp would normally just go.

Upon investigation i've found that there is voltage across the plug for the lamp when removed. However, when I plug the lamp into the plug the voltage disappears?!

So, it seems there is no lamp problem as i've swapped across the working os lamp to try and it didn't work. There is voltage up until the point of making the circuit with the lamp.

Any ideas, i'm kinda lost!

[That Daddy]

what car?

[Le TVR]

As soon as the bulb draws any current the voltage falls.
So there is a poor connection or earth return somewhere.

Try all the connectors and see if any are corroded or loose.

[gdunc]

Cheers guys

TD - car is a '97 A3. The dipped beam has its own individual plug.

Le TVR - earth fault sounds like the thing but unsure as to where it could be. After the plug there is about 100mm of cables which then join the main light loom and disappear into the bodywork. Guess i'll just have to check around the visible bits.

Dunc

Edited by gdunc on Thursday 4th January 16:40

[steve_d]

Confirm the earth fault first by running a new earth wire from the bulb holder to a good earth like part of the engine.
The fault is more likely to be at the light connector than anywhere else.

Steve

[gdunc]

Ok, really confused now. The earth resistance from the brown -ve wire is the same as on the working os light. I have a potential of 12v at the connecting crimps until i plug in the bulb. The bulb work perfectly on the os power.

The crimps are intact, the wiring continuity is correct, the bulb is intact, the power is there up until i plug in the bulb. The earth is common to the main beam which is working correctly.

Really stumped!

[Le TVR]

Connect the voltmeter -ve directly to the battery -ve terminal.
Voltmeter +ve to +ve terminal on headlight.
Fit bulb.
If volts drop to zero then the fault is in the supply wire to the bulb.
If volts dont drop then voltmeter +ve to the earth terminal on headlight. If voltage is not zero then the fault is the earth return.

[r1ot]

Make sure that when you push the lamp into the plug that you are not pushing the crimp back into the plug. Sometimes the crimps come loose inside the plug and are no longer retained so when the lamp is pushed in it can force the crimp away from the terminal on the lamp. This might explain the lamp flicking on and off, as the crimp might just be making contact.

[GreenV8S]

The fact that the voltage changes when you plug the bulb in proves that you're getting a good circuit through the bulb. The problem is in the wiring/switch etc. The symptoms suggest you have a high impedance on the supply or return, and you could work out which by seeing which side the voltage changes on when you apply the load (i.e. plug the bulb in). You don't have any sort of bulb failure detection system on that car, do you?