If a heavy electrical load is dumped on the alternator at idle, the engine RPM drops until feedback from the PCM can direct the idle air control (IAC) to compensate. But if the PCM knows
a heavy load coming it can feedforward a signal to the IAC in anticipation of the RPM drop. The tail light relay (above) turns on a bunch of lights at once. The idle-up signal from the tail light
relay increases engine RPM at the same time the alternator is trying to bog down the engine. The results is a barely perceptible change in engine RPM under changing electrical loads.


There are five other electrical circuits that also send an idle-up command to the PCM.