Spurious bulb failure warnings

[Raistlin]

I wonder if anybody can give me a brief explanation as to how the bulb failure monitor works please.

Almost every time I use my headlights now, when I switch them off I get a spurious bulb failure warning indication the O/S dipped beam.

Then sometimes as a result of switching the lights back on and then off again, or by pushing on the O/S light unit connector, or by going over a bump, the warning will cease.

I've had the connector off the light unit and cannot see anything amiss.

Any ideas please?

[stocktake]

Sounds like the chaffing of the wiring that is spoken about on the forum, quite common seemingly on the offside as the harness passes over the bodywork behind the bumper

[Raistlin]

Originally Posted by stocktake

Sounds like the chaffing of the wiring that is spoken about on the forum, quite common seemingly on the offside as the harness passes over the bodywork behind the bumper

Wiring to both headlamps showed no signs of damage a few weeks back when I did the I/C "O" ring though, and I beefed up the loom with spyrowrap.

[stocktake]

Originally Posted by Raistlin

Wiring to both headlamps showed no signs of damage a few weeks back when I did the I/C "O" ring though, and I beefed up the loom with spyrowrap.

Sorry about that Paul. All I know is that the bulb failure system seems to measure resistance, that is why you cannot use LED's for sidelights unless you install a resistor. Other than that in the words of the dragons

"I'm out"

[Christopher]

The bulb failure system measures current drawn from the circuit to assess whether a bulb is working or not - hence an open-circuit or LED lamp (which is high resistance) indicates a blown bulb as no current drawn. I think that the problem as you describe it is a "flaky" connection issue. I once had such an intermittent fault in the rear o/s brake-light - I took the bulb out and cleaned the contact on the bulb and it sorted it out.

HTH

[Raistlin]

Originally Posted by stocktake

Sorry about that Paul.

Not at all. It's where I would have started.

Originally Posted by Christopher

The bulb failure system measures current drawn from the circuit to assess whether a bulb is working or not - hence an open-circuit or LED lamp (which is high resistance) indicates a blown bulb as no current drawn. I think that the problem as you describe it is a "flaky" connection issue. I once had such an intermittent fault in the rear o/s brake-light - I took the bulb out and cleaned the contact on the bulb and it sorted it out.

HTH

So, the fact that disturbing the multiplug sometimes cures it suggests a dirty or corroded contact at that position then.

I thought it was a current comparator, except that it seems to be able to detect a failed bulb even when the lights are off - unless it sends a small current round the circuit continuously.

[Christopher]

Originally Posted by Raistlin

I thought it was a current comparator, except that it seems to be able to detect a failed bulb even when the lights are off - unless it sends a small current round the circuit continuously.

Exactly - probably using a Parametric Measurement Unit as used in testing leakage and o/c in semiconductors - injecting a small current into the circuit and measuring the load.

Corrosion as you explain is what I would suspect too yes - good luck!

[JohnDotCom]

The light leads can get damaged over time.
They can wear badly as shown in this photo.
Cloth reinforced tape was placed over seam join to prevent rubbing and wires re sealed with tape.

[Raistlin]

SOLVED

Disconnected the plug from the light unit and applied a judicious spray of Servisol contact cleaner / lube to the contacts. Plugged and un-plugged the connector a few times to get the stuff into the depths of the contacts.

No more spurious bulb failure.

[tigerchubs]

That was gonna be my advice, It would appear connectors are a bit of a weakness on the 75.