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Alternator D+ on PDM output

PDM Installation & Configuration

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I have BMW alternator with B+ and D+ terminals and would like some guidance on the D+ wiring. The B+ is obvious but the D+ is where I don't fully understand the functionality. Currently it is connect to the Run/Start positions on the key via the alternator charge light + parallel resistor. I understand how this light illuminates if there is no voltage on D+. I assume that this also supplies field current before the alternator starts spinning.

My questions are:

- given this is generates a current and voltage will I get weird stuff happening on the PDM putting a voltage onto an output pin (AIM PDM32)? Do i need the resistor if I do.

- should I connect it directly to the battery and if so will I need a resistor to replace the bulb/resistor? I'm not sure what function this serves other than the fault indication I no longer need/want. I have an isolator but I guess this means the field winding is always on if the isolator is closed even when the ignition is off.

OEM wiring attached.

Attached Files

The D+ terminal's feed from the ignition is, as you say, to provide the initial current to the circuit for the magnetic field. The warning bulb would normally be the only resistance to limit the current but if it blows there would be no current passing to the field windings, and that's why there is a resistor in parallel with it - to ensure there's still a current.

Once it's operating, the back EMF from the rotor winding's magnetic field will provide the current and voltage for it. Usually the voltage now at the D+ terminal to the bulb is approximately the same as from the ignition to the bulb, but depending on application, sometimes it can be higher and the small current in the reverse direction can cause the warning light to glow - some of you may have seen that on older vehicles, at night or under other high load condition - and that's why there is a diode in the circuit, to block that reverse current. You probable need neither, but safest thing is to simply replicate the diode and resistor.

If you are using the PDM to switch the circuit, it shouldn't be affected by the 'reverse' voltage - but using a diode will remove any possible concern or issues with other devices on the output pin. What is a concern, though, is that directly supplying 12V "may" blow the windings because there is no resistance reducing the voltage and hence current - I say "may" just in case, because testing the alternator will often mean simply using a jumper between the D+ and output terminal when it's spun up on the test bench.

You could tap the D+ to the output, but as you say it will be always live with a trickle current passing through the field windings - not an issue if it's a track/race car as you can simply make a practice of turning the electrics off with the isolator when the vehicle isn't running.

As a side note, some alternators are mounted using isolating mounts, these will need an external ground with a current rating matching the output.

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