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Alternator charge indicator

EFI Wiring Fundamentals

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Hello guys,

I´m planning to send signal to the PDM input from the alternator charge indicator. Once this signal get´s on, I will configurate it on my dash to send an alarm.

I´m in doubt about what configuration Pull-Up or Pull-Down should I get. Looking autodata, I don´t know if alternator switch closes circuit to gnd or to 12V. There is just one cable from alternator, I supose it is +12V signal but I dont really know how it works

Thank you

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Generally the lamp connection on an alternator serves two purposes. First, when illuminated it indicates the alternator isn't charging, second the current used to illuminate the lamp flows though the field windings in the alternator which is what enables it to produce a current.

So you generally need to have a lamp, or at least a source that can supply a current limited voltage.

What are the input capabilities on your PDM? Some can only accept a digital signal (switch) that they pull-up internally. The voltage on the alternator-side of the lamp with be low when the lamp is illuminated and high when the lamp is out (so no or very little current flows through the lamp. You might be able to splice this into your PDM.

My PDM has 32 power outputs. And then I have 8 inputs which can be configurated as digital or analogue, with pull up or pull down resistors. However, if you can tell me wich is is the easier way to set this alarm... maybe I could use a LED inserted next to my dash? Which way should I do this? I want to listen your recomendation! LOL


Personally, I would just have an alternator lamp on the dash. One side connected to ignition power, and one side connected to the Alternator D- connection. Simple, works like the alternator is expecting, and you get the alarm you wanted without wasting one of your PDM inputs.

If you do want to use the PDM. Then set the input to the analog input and connect it to the lamp as described above. You could probably use a 3W approx 50 ohm resistor instead of a lamp if you wished.

Thank you David, I will look on ebay a beautiful battery light ;)

Exactly what alternator are you using? There seems to be a minor contradiction in your description, and different designs/models have different wiring arrangements.

Gord, I´m using OEM Audi A4 1997 with 1.8T AEB Engine, should I test working type using a voltimeter touching on alternator output and grounding it? If I take a wire and conect GND with the alternator light output, the original light should become On?

... and the alternator? been a while but...

While my sister co-incidentally also has an Audi A4, I'm not familiar with that alternator and there can be anything from a single wire to 4 (5 with ground, if used) that might commonly be used.

As David said, a common configuration has a main output wire and a single lighter wire that is connected through a warning light to the switched ignition and that supplies a current to the field windings. With ignition on, but engine not running, the warning light is grounded through the field winding, when the engine is started, the windings are self energised from the running alternator to ~ battery volttage ans so there is no tpotential and so no current to illuminate the light - note, sometimes the field wind voltage will exceed the voltage from the switch, and there will be a slight glow.

David's suggestion should work well, but may not be quite you're looking for if you're wanting to integrate it with the ECU, so you could reference the field winding to the battery voltage. But if you have a battery voltage input, the easiest thing would be just to enable a warning if it were below, say, 13.5V?

Hello Gord,

Other option maybe could be using a digital input in my PDM? In this way, when my input see 12V, It wont send message to the dash, but when alternator is stopped, then that input will detect is no working?

Hello Ignacio, just wondering if you ended up doing what you said about running the charge light wire into a digital input on your PDM?