W124: bulb failures, when is 12v not 12v

[franksm]

Hey folks

One of the front foglamps on my W124 is getting a healthy 12V, the other shows between 6 and 9 volts – upon plugging a good bulb the voltage goes down to zero. It’s as if any load on the circuit brings it down. Bulb-failure warning is lit up the whole time. The earths there are sound, I tested it with a new earth lead to a test bulb; the cabling is sound too. Problem is not the bulb, I have swapped in known-good bulbs

I need to trace things back to the ‘lights control unit’ - must be under the dash ? Fuses are good too – the number-4 fuse servers front fogs as well as the single rear fog.

Smells like a bad earth, but apparently not !

BTW I reported something similar in the past - with the rear bulbs that time. These things seem to come and go with the rears on my car, even after I organised some new earth cables there (bolted to metalwork) http://forums.mercedesclub.org.uk/showthread.php?t=33276

Any ideas ?

Cheers

Frank

 

[Xtractorfan]

Just wondering if your fog lamp comes into to service when the side lamp bulb blows, this would feed a lower voltage to the lamp to avoid battery run down..try removing the side lamp bulb on the 'good' side and see if the fog lamp lights up in dimmed mode.

 

[kth286]

franksm

There is a resistance in the circuit which is causing the voltage to drop.

You need to measure the resistance from the bulbholder back as far as you can.

There should be NO resistance showing on ohm meter.

Keep measuring back and back until you get to the feed or a good 12 volts.

 

[wireman]

The lamp sensing device is in the black box behind the fuses, RHS under bonnet near the brake servo.
Power for the fogs comes from the switch to the sensing thingy and then via two wires one for each fog lamp to the front. The fault you describe will be in one of these two connecting wires.
There may be a plug break in the wiring loom (its behind the RH head lamp on a 190) which has gotten its self all full of water and gone a bit grotty, it is likely to be one of the contacts on any wiring connectors in the circuit, I do not have my wiring diagrms to hand and cannot tell you which wires to examine but they will be the same colour(s) as the ones that go to the lamp unit. Sufficient data is in the haynes book of lies to get you sorted. Any brown wires are connected to the body shell and hence battery negative, so check this at the lamp unit first.
Beware that the high resistance connection will disipate power and warm up perhaps to a temperature which will damage your wiring loom, so fix it soon before a fire fixes it for good.

 

[trowell]

hi i had the same sort of fault with my w124 300te it turned out to be the solded joints inside the control unit, resolded the joints and all is working fine now, mine was the o/s rear side light best of luck