A “minor tweak” here.
I was trying to work out a neat way to get the signal from the alternator’s D+ terminal to the instrument cluster without running a cable directly all the way between alternator (C) and instrument cluster, so I went back to the basics.
When the engine is not running, the alternator isn’t turning and cannot provide its own excitation current to establish a magnetic field between the rotor and the stator windings, so even if the rotor were turning, there’d be no significant electricity produced. In a normal car circuit, initial excitation current is provided from the battery to the D+ terminal via the ignition switch and a light bulb. That bulb also then indicates if the alternator is generating. The current is small, about 150 mA (not yet measured) which is only drawn while the alternator isn’t generating.
Once the alternator is turning and generating, it can self-excite by using a set of diodes off its stator windings to provide the small current needed by the integrated regulator to keep the rotor excited.
J-Front can provide the initial excitation current. And provide a status to the network so that the D+ signal proxied by J-InstAux lights up the corresponding LED on the instrument cluster at pin 16 of that cluster. The dashboard has only an LED as an indicator but several resistors in parallel with it and a diode in series to provide excitation and protection against reverse current or excessive voltage across the LED. J-InstAux has to draw significant current from pin 16 to light up the LED.
J-Front can be smarter than an LED as well, taking into account the engine starting process that it is controlling. It can delay excitation until the engine has fired up which not only reduces the direct battery current draw slightly, but also prevents the alternator from trying to generate during the cranking process; presenting a larger load on the battery via the starter motor (B). After all, the starter motor is trying to turn not just the engine, but all of its auxiliaries driven by the belts. The lower the load, the quicker the start.
The D+ terminal provides one of the most hazardous voltages to the electrical system in the car, apart from the high-tension ignition side. Under normal conditions, there is little to worry about. It’s the load dump condition, where the battery is suddenly disconnected from the alternator while the latter is generating that can easily produce voltages higher than 40 volts. One must protect against 60V to be reasonably sure that it’s not going to force the magic smoke out of expensive electronics expecting only about 14V.
All car electronics should protect themselves against such voltages, or at least, deal with them without permanent damage.