Thread: Seeing PWM Data
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Old 01-11-2019, 05:07 PM
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wpoll wpoll is offline
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Just been looking at the wiring diagrams (via for the front signals on the pre-LCI... jeepers, they are somewhat confusing. There doesn't seem to be a single diagram that shows the entire assembly - it's all spread across several diagrams. And then there's the year variations!

I think part of the problem is terminology - signals, side lights, turn signals, parking lights - they use all these terms, and then we throw in angel-eyes (AEs) and halos to the mix!

To come back to the OP's question, anytime PWM is involved, the system is designed around incandescent lamps (bulbs), using the slow reaction time of a glowing filament to "average" the PWM pulses into a consistent brightness level. The lower the PWM duty cycle, the lower the average (but not actual) voltage on the filament.

When you put LEDs in the circuit, replacing the incandescent lamp, a couple of things happen. Hot and cold monitoring get screwed up - and they are different things - and can cause warnings about lamp failures on the dash. The fixes included coding out hot and cold monitoring or inserting resistors in parallel with each LED device to provide a sufficient load for the monitoring systems.

These resistors negate part of the advantage of LEDS - lower power consumption. Putting resistors in circuit to fix the hot and cold monitoring brings the power consumption back up to that of incandescent lamps.

Also, on lamps that run continuously, the resistors can get VERY hot, so watch out for that - place them carefully!

Some LEDs are advertised a being "CAN bus" friendly, which is technically incorrect, as they probably just ship with resistors.

The second problem LEDs have in PWM lighting systems is that they switch on and off VERY fast - so they do not provide the pulse averaging that the incandescent lamps do. The resistors mentioned above (if fitted) can act like a pulse averaging circuit and help somewhat, as can capacitors, but the level of dimming on an LED is not the same as on a PWM-modulated incandescent lamp, so it may or may not work as intended. This isn't an issue on a lamp that is either ON or OFF but it may be a issue on a lamp designed to run at different brightness levels in normal operation - e.g. the tails on LCI cars are low brightness as tails (low duty-cycle PWM) and brighten when the brakes are applied (high duty-cycle PWM).

All of which is to say, LEDs can be made to work, and work well, but they aren't ALWAYS a better option and you may need to modify a bunch of things to get them to work correctly, especially in a system designed for (and that relies on) incandescent lamps.

And as I said, getting back to the OPs question, you are likely going to need to add resistors to the LEDs in question to deal with the issues you are seeing... I doubt goofing with the PWM duty-cycle will sort it
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