Rear lamp problem - when is 12v not 12v ?

[franksm]

Hi folks

Summary: lamp has 12v with no bulb in; stick a bulb in, zero volts

Have a real oddball of a problem with the offside rear lamp cluster on my W124, specifically the tail-lamp. The brake/indicator/reverse lamps are fine, just the tail-lamp exhibits the problem:

For one thing it seems to get throught bulbs a bit more often than I'd like. It's possible that the bulb quality isn't great (Halfords brand) but... they work fine in other parts of the car.

The real problem is that the tail lamp now doesn't light up at all (even with known-good bulbs). The bulb-failure system lights up to confirm this. I take out the bulb, measure the voltage: a healthy 12 to 15 volts there. Put a bulb in, and it's zero volts. Totally repeatable, have never seen anything like it. The earth is all good there and like I say, the other bulbs work fine.

Could it be something screwy with the bulb monitoring system ?

I am using 10w bulbs there, as per the OEM application.

Thanks

Frank

 

[television]

The 12 means nothing if you have a poor ground take a wire from the out brass base of the bulb and take it too a good earth point and see if it lights

 

[jberks]

Alternatively there is 12v at the meter but not enough current to actually power the bulb. Once you put a load on it, the circuit gives up. Again, wiring or switch probably.

 

[wireman]

jberks is the best guess, I would check the plug connections on the loom in the boot. I(f the lamp on one side is OK the fault is in the wires after they seperate to each side.

 

[television]

When checking the wires leave the lamp in, then you can measure the voltage one end, then check the other, any difference the the cable is O/C

 

[David Nock]

Alternatively there is 12v at the meter but not enough current to actually power the bulb. Once you put a load on it, the circuit gives up. Again, wiring or switch probably.

Agree, suggests there is a high resistance somewhere in the feed to the lampholder from the battery. A bad connector perhaps as stated by Wireman. Inserting the bulb collapses the voltage across the high resistance point.

 

[wireman]

Only last night I got a bulb fail warning, replaced 10w tail lamp and the warning lamp stayed on. The 124 rear cluster has what appears to be galvanised steel "wiring" strips and the one to the tail lamp was white with zinc corrosion at the riveting of plug in on the wiring connector, wobbling has cured/made it work but I intend to solder the pin properly when I get some daylight.

 

[franksm]

Just to close this one... I had swapped bulbs in and out of the tail-light on each side, only to have spurious results.

Looking a the loom, the offside already had an 'extra' earth wire installed (to a bolt on the body). I fitted a similar one on the nearside loom.

A month later, and both bulbs are still working equally bright and fine.

Of course a week later, one of the H4 bulbs in the headlamps blew so I feel as if I am chasing bulb problems with this car, but all just coincidence touch wood heh heh

 

[television]

Oxidization can play havoc on bulbs and holders, always spray with a good non drying switch cleaner to stop running around in circles.

They should never be sand papered as the flash nickel coating on both is very thin

 

[beemerjay]

ACF 50 ------->>>>> LINKY <<<<<<----------

Get it into the loom connections and the bulbholder!!


Nuff Said I'll get me coat!!!