Alternator Lamp on C127

[adrian]

I don't know the specifics of the EVO9 alternator but typically an alternator requires the lamp to provide excitation current for the rotor. If you remove it then there is no field and the alternator doesn't work.

A Google search for 'Alternator Lamp' will show you the circuit and why it is required.

You can replace it with a resistor if you don't want an actual bulb in the car. You will have to select an appropriate value that provides enough current. I have found this to be in the hundreds of ohms range on a few that I have done before.

[Loqy]

Hi Adrian,

Thank you, that's why I'm looking for, not using the standard bulb.
Really appreciate it.

[gtihk]

Last time I did it, the resistance was below 100 ohm.
Need to look out for wattage rating if it gets too warm.

[LouDen Motorsport]

OK, most modern vehicles use something like a 3W bulb for the alt. warning light. If my calculations are correct, and using V=12 I get P=250mA. That gives me R=48, so let's call it a 50Ω resistor. (I'm happy to be corrected)

Here's my other thought. How long does the alternator need to see the excitation current, before it gets excited?

I'm in a similar situation to the OP. We're using a PDM30, a C127 and an M130. In one of those "apple on the head" moments I considered using the starter solenoid signal wire, (via the resistor) to run to the alternator D+ terminal. We're lucky in that we have a two wire (B+ and D+) alternator. Obviously I would need to also use a diode to stop the alternator feeding back to the starter.

Will it work, or am I barking up the wrong tree?

[LouDen Motorsport]

For anyone following this, I overlooked the obvious. Instead of using the starter signal, take ignition feed ( via a resistor ) from either the coilpack or injector line. The resistor will need to be something robust enough to handle a bit of current, of course.

Much simpler, no need to worry about a diode to stop back-feed.