Bizarre rear light problem

Post by **mat_fenwick** » Mon Mar 07, 2011 8:06 pm

Plugged in a trailer tonight (which has a 21w interior light run from the LH sidelight circuit) and instantly the LH sidelight and number plate lamps went out. Ahhh, I thought, that'll be the 5A fuse (no. 12) blown. But no - changed the fuse and still the same. Swapped fuses with 11 (RH sidelight) and proved that fuse intact.

Just got back in and having a bite to eat - wiring diagram shows no other fuse. Towbar wiring is genuine Citroen (plug in). Going to have a poke with a meter shortly but wondered if anyone had any ideas?

Way2go · Post by **Way2go** » Mon Mar 07, 2011 8:38 pm

If RH sidelight is working then as the power is routed through LH sidelight does that not suggest the bulb may have blown? Otherwise it is a bad connection in the vicinity of that LH cluster.

rmattila · Post by **rmattila** » Mon Mar 07, 2011 8:45 pm

I'd first check that fuse 12 gets 12 V at its input terminal. If not, the the problem is in the fuse box connections. If it does get the voltage, then proceed to the input plug at the LH taillight terminal and see if it reaches the destination.

Not many possible explanations to such a failure - even the earthing point of the number plate lights and the rear lights should be different.

(This all presuming that the car has not been previously owned by a similar mad scientist as my Wife's break, where hardly any electrical wiring even remotely corresponds to the spirit of the original design.)

Post by **mat_fenwick** » Mon Mar 07, 2011 10:38 pm

Just got back in from the cold (0.3 degrees C - no comments abut how lucky we are over here needed

) and it's working again!

Way2go wrote:If RH sidelight is working then as the power is routed through LH sidelight does that not suggest the bulb may have blown?

RH sidelight is routed via the LH cluster but with a seperate fuse (11). I knew it must be something simple but had ruled out bulbs as it would be unlikely for 3 to blow at once!

First checked the fusebox as Riku suggested, 12V at both fuses. Checked the resistance between fuse 12 output and earth - about 200 ohms(!) Looking like a poor connection somewhere so I checked the voltage at pin 6 of the multiplug at the LH cluster and it showed 0V. I then checked the resistance between pin 6 and the fusebox and it was close to 0 ohms - very strange.

Put the fuse back in and getting nearly 12V at pin 6, LH sidelight now works and nothing I do replicates the fault. It must have been a connection I disturbed to 'repair' it! Anyway, I cleaned everything up (although they all looked fairly clean) and coated with silicone grease to protect. Time will tell if the problem returns...

citronut · Post by **citronut** » Tue Mar 08, 2011 8:16 am

did you also tighten up the jaws in the plug conectors at the rear lamps,

regards malcolm

Post by **mat_fenwick** » Tue Mar 08, 2011 5:41 pm

I didn't - they seemed pretty tight as they were so left them alone apart from the silicone grease. I'm not convinced I've actually fixed a problem, more that it's just gone away for now.

It was bizarre how they suddenly went out completely as soon as I plugged in the trailer - I would have expected them to simply dim if there was a high resistance connection somewhere (and to be less than full brightness beforehand).

Way2go · Post by **Way2go** » Wed Mar 09, 2011 1:18 am

mat_fenwick wrote:
It was bizarre how they suddenly went out completely as soon as I plugged in the trailer - I would have expected them to simply dim if there was a high resistance connection somewhere (and to be less than full brightness beforehand).

Not necessarily, the connection problem could be because given the age of our BX's, wires may have oxidised inside the crimps progressively reducing the connection area as oxides are good insulators.
Pulling your higher current can cause the last bit in contact to burn/oxidise so until you move the wires mechanically a connection is not remade. Sadly it's then only likely to fail again unless the offending connection is stripped, cleaned and recrimped or soldered.

citronut · Post by **citronut** » Wed Mar 09, 2011 8:19 am

well i had a simmilar thing yesterday on a freinds VAU***LL,

on fitting a new stop tail bulb when pressing the brake pedle with side lights on, one side went out,

pulled the multiplug of that light unit cleaned the pins which wernt very dirty at all, put it back together and all worked normaly,

regards malcolm

Post by **mat_fenwick** » Wed Mar 09, 2011 12:53 pm

Way2go wrote:the connection problem could be because given the age of our BX's, wires may have oxidised inside the crimps progressively reducing the connection area as oxides are good insulators.
Pulling your higher current can cause the last bit in contact to burn/oxidise so until you move the wires mechanically a connection is not remade.

Now that sounds plausible, as although the terminal contacts looked OK, who knows what the inside of the crimps looked like. That said, it did pass the wiggle test afterwards. I'll try plugging into the trailer and see what happens!