Number plate light

[ChrisHC]

The O/S number plate light on my 1987 325i stopped working and started smoking, which is never a good sign. When I investigated I found it broken and corroded so replaced both light units. The feed for the O/S light was dead however, and was all the way round the inside of the boot. I presume the cable had burnt out somewhere inside the loom, as a result of a fuse of too high a value being fitted, as I later discovered. To get round this problem, I ran a wire from the N/S light to the O/S and had two working lights. What I discovered when I drove the car with the lights on is that the check control then showed a "licence plate fault" as a result of the O/S wiring now being open circuit.

What is the best way forward now? Where should I next check the original loom or where to start going the other way? Alternatively, can the check control be fooled somehow?

[paultv]

Yes you can fool it:

The N/S output from the rear lamp check unit is now disconnected so it shows a fault, ie. no bulb. The system works by passing the voltage and current through a coil to each lamp - inside the coil is a reed switch, this closes when the current flows to the bulb as the coil creates a small magnetic field.

You can open the check unit and short out the pertinent reed switch, soldering iron and small skinny cable needed for this, but it might be better to fix the loom as it could cause another short somewhere.

Check control is in the boot behind the aerial on a saloon I think, and in the left (NS) rear window cavity on a cabby, seat out and trim/speaker removal to get at it.

I have pics of the innards if you are interested.

Oh yes, if the lamp was "smoking" it's very likely that the printed circuit track responsible for this feed has burnt out, again it is easy to bridge and solder to repair once you've checked that the wiring has no obvious faults.

Paul

[ChrisHC]

Thanks very much, Paul. I would certainly welcome any pictures of the innards of the check control unit. I take it that the printed circuit track you mention is part of the check control unit?

[Brianmoooore]

Would be just as easy to run a replacement wire from the bulb monitoring unit to the number plate light and do the job properly.

[aimlessrock]

Would be just as easy to run a replacement wire from the bulb monitoring unit to the number plate light and do the job properly.

+1- bodges/work arounds will only cause further problems further down the line..

[ChrisHC]

Paul was right, the printed circuit board in the unit had burnt out. A blob of solder repaired it, I checked the continuity of the loom which was fine, so I undid my bridge from the N/S light and restored the original connection. Everything is now fine, the number plate lights work and the check control accepts that they do. Many thanks to all who contributed, I had no idea that unit was there, there seems to be no mention of it elsewhere that I could see.

[paultv]

How Impertinent!

I was under no circumstances suggesting a bodge - I was answering the OP's question, and helping to diagnose his problem.

Damn cheek

Glad I could help you Chris.

Paul