light fault light on 240

Anyone have any ideas why the orange "light out" indicator is on, even
though all the lights are operating? I had a lamp go out in the rear
light cluster, replaced it and no joy....

ed

 

I had a similar problem due to a false contact on the rear lamps....try
to push the brake pedal an watch what happens. If the brightness of the
lamps change when you push the pedal there is a bad contact.

Andrea..

 

The "bad lamp" indicator works by detecting the difference in current
being drawn between "left" and "right" side bulbs (brake, headlamps,
etc). If a (single) bulb is replaced with one which draws a bit more (or
less) current than the one on the opposite side, the indicator will come
on. Best solution: replace in pairs. (It's also possible, but less
likely, that the device which performs the current-sensing and drives
the indicator is faulty, but if the problem appeared immediately after a
bulb replacement this is unlikely.)

 

The previous post was an excellent explanation. One way to troubleshoot
this is to remove wiring harnesses from pairs of lamp holders. The
problem can either be a mismatch in bulbs or a mismatch in socket
resistance or both

First do the obvious,

1) if on in headlight position or off/running light position and not
parking light mode then it is a headlight mismatch.

2) otherwise it is a parking light mismatch, either a front or rear
parking light mismatch.

You can troubleshoot this by unplugging the connectors to both of the
two front parking light assemblies.

3) if it goes out then it is a front parking light mismatch.

4) if not it is a rear parking light mismatch.

Once you zero in on the mismatch, then you have to determine if it is
the sockets or the lamps. Usually it is the lamps but on older cars,
especially some 240s it was the sockets. They were not designed right
and there was a recall. They got too hot and corroded.

Once you fix one mismatch, there still may be another. When putting the
connectors back on check each step of the way to make sure the problem
doesn't come back.

 

Or just remove the bulbs, that's usually easier than unplugging wiring,
aside from the headlamps. Most of the time this problem is caused by
either mismatched lamps, or corrosion in the socket.

The sensor is actually a very clever bit of engineering. It consists of
a number of reed switches which consist of a pair of tiny flexible steel
contacts sealed within a glass tube. When exposed to a magnetic field,
the steel reeds attract one another and close the circuit to the lamp
failure indicator. Around each reed switch are two coils wound together,
one in series with each set of lamps, with one wired so current flows in
one direction while the other is opposite. So long as the current in
each coil is equal, the magnetic field cancels out, but an imbalance
allows a magnetic field which activates the reed switch. They are really
a bit too sensitive, I suspect with a turn or two removed from each coil
they would work better but I haven't attempted to modify one. Still, an
ingenious bit of engineering, something that could have been built with
technology of the 1800s that still works well.

 

Is that why lamps come in "two packs"? I replaced the opposite number
lamp with it's mate from the pack and presto-- no warning lamp. That's
interesting how they engineered that switch with the coils and the
reed switch. It reminds of what someone might say of Tesla's
continuing relevance in electric fields, a classically trained
electrical engineer must have come up with that one. So thanks all,
problem solved!

Ed