| What you measured there is actually correct. The 550 ohm resistor is a current limiting resistor which is also limiting the voltage across the LED. You aren't going to see it blink as the base pulse frequency is much higher than what your eyes will detect, but it is pulsing. You will however see the intensity of the LED change as you swing through the AAC duty cycle, which it appears you are seeing. The voltage is changing because the duty cycle is changing. Your multimeter is taking an average voltage from the terminals so even though the signal is switching from 0v to 12V, the multimeter is only going to see the a percentage of 12V dependent on the duty cycle. If you remove the LED entirely and put your multimeter terminals directly into the harness connector, at 50% duty cycle your multimeter will report ~6V. This is the beauty of pulse width modulation (PWM) as when it operates at really high frequency, it behaves as a voltage controller. :) So, with that said, it does not appear your problem is in the ECU or wiring harness. That's a good thing, but doesn't provide the answer you are looking for.
Enthusiasts soon understand each other. --W. Irving.
Are you an enthusiast? If you are out to describe the truth, leave elegance to the
tailor.
Albert Einstein
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