bulb failure control unit.

[Frank O' Phile]

One minor niggle with the new car (w124 300E) is the bulb failure lamp comes on when lights are turned on.

All the lamps are working so I thought that changing, cleaning bulbs and fittings would cure it, but no. Further investigation shows that the rear tail bulbs have been 'rewired' so that the o\s feed runs from the n\s. That would cause the failure to show up.

So I wanted to investigate why it had been done and, after undoing the wire mod, found no voltage supply on the wire at all. I have tested the circuit from the cluster back to pin10 on the control unit and it appears ok with no shorts.

I have searched the forum and the general consensus is that the control units are pretty reliable. Is it possible for the unit fail and not supply a feed to just the one line?

Have I overlooked any other possibility?

I will be a the S.Yorks GTG if anyone can let me try a known good one in mine to see if that is the fault.

I have scanned fleaBay and found some that look the same but the part numbers do vary. Will any other model fit or does it need to be a identical match?

Any help, advice or useless witty comment appreciated.

[Dieselman]

Can you get the unit open and meter further back for power. It's possibly a dry solder joint at the output stage.

IIRC these are fairly simple devices that use a resistance much higher than the exterior bulb and wired in parallel with it.
If the bulb is intact then the power flows so no power gets to the indicator lamp, if the bulb goes open circuit the power diverts through the resistor to the indicator lamp.

There might be an internal link you could cut to disable monitoring of the lamp in question if you can't fix it.

[Frank O' Phile]

Thanks DM, I did open the unit and give a quick visual check and nothing looked wrong. I could do as you suggest and fit the unit in without the cover on and see where (if at all) the power starts and stops.

It's a bu66er of an area to work in as the bonnet edge is there and the bulkhead. And the pin is question is the one at the back. Might have a go later.