Originally Posted by pavelvoivoda

Thanks guys I was thinking the same thing. But since toyota puts arrows in their schematics indicating the direction of current travel between systems (not the diode arrow symbol), I was confused it might control the alternator somehow.

Like in this diagram of the 1KD motor which also has a 4 pin alternator connector, you can see that the arrow points towards the alt and has a description "From engine ECU", while current travels from the alternator to "To Daytime Running light relay" - arrow points away.

I did read that the M terminal on alternators mean "Monitor", but only in general pdfs and sites around the web, not official Toyota description.

I think that it is in fact a monitor function, but it acts differently. If the alternator fails due to a belt breaking, it will stop rotating and all lights on the dash will flash. That's how you tell that it's specifically an alternator problem. All those lights that turn ON at the same time have a diode that prevents them from getting current when the alternator is running. But when it stops, current stops pushing on the diodes and has less voltage than the battery the battery's current coming from the bulbs and goes to the alternator's regulator that leads them to ground (when not energized) and that's how all those bulbs turn ON.

I might think that it might be the same thing with the M terminal. If the alt fails, the ECU, sending current to the alternator, will also get to ground and will know that the alt failed, and i think it might switch to limp mode, especially if the alternator belt is also running the water pump.

But if that's the case, why would they put a separate pin on the alternator instead of just connecting it to the same circuit that the bulbs are connected to? I dont know...

What do you think?

Thanks
Pavel