Lucas is not the only ‘Prince of Darkness’

[xvivalve]

Here’s a puzzler for those of you fluent in electrickery:

My Alfa 166, the stalwart tow car, failed its MoT recently, inter alia with a non illuminating offside fog light bulb - apparently you can have a non illuminating nearside bulb, but the offside is critical.

Investigation proved a blown bulb, same fitment as Dolomite rears, so sourced one from the shoebox full of spares I’ve accumulated over the last three plus decades (not so the pi55y little front sidelight bulb the size of my little finger which cost me a fiver!!). Replaced the bulb, but still not illuminating…tried another, same result. Tried the known good from the nearside, still no glow! Took the multi plug out of the nearside bulb holder and established which wire was the fog light feed and tested the offside: no feed to the plug. Wires both sides disappear into tightly wrapped loom, so no sign of damage. Checked fuse box as some os and ns functions on this car have separate fuses, but no, only the one for rear fog lights plural.

Needing the car to collect a couple of engines that Mad Mart has rebuilt for me I thought outside the box and wired in a shunt from the nearside feed across to the offside feed. Reassembled and tested…and neither side illuminated!!!

Took out the offside bulb again to check, and whilst I was holding it up to examine the element, the nearside suddenly illuminated. Oh, ok, I thought, so put the offside bulb back in and the nearside switches off again. Took out the offside again, and after a pause the nearside is back on! Repeated several times…

Both bulbs in, neither operating, I take out the nearside and lo and behold after a short pause the offside illuminated. Bingo: go for retest with the nearside bulb in my pocket and we get a pass!

Engines successfully collected yesterday, but WTF???

[Carledo]

The vehicle probably is running CanBus which means it's a sweet mystery to me how anything works!

Add in the fact that it's an Alfa, so electrical faults are kind of "de riguer" and you have your current situation! Sorry I can't be more help!

Steve

[soe8m]

Probably something wrong with your towbar socket or towbar wiring/module.

Normally when a trailer is attached the foglamps are switched off and only the trailer foglamps will work.

Faults in these can give some interesting vehicles rear lamp configurations.

The modern equivalent of the dimmer relay.

Jeroen

[matt of the vivas]

Agree with Jerome. Look at the tow bar wiring...
Most of the 7 pin sockets have a little pressure pin that disconnects a wiring loop for the fog light when the trailer is plugged in. These corrode and stick. New sockets are pennies.