light fault light on 240

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