Alternator wiring when using standalone ECU

[Hobby Racer]

So I am using a standalone ECU and when I run the engine the alternator is not charging the battery. I am wondering if I need to do something with the wire from the alternator that used to go to the charge light on the dash. Since I am not using the Subaru ECU I assumed I did not need it. Does that wire have an effect on the alternator's function or is it only for the factory ECU's use?

[phil1734]

I've heard that occurs when the odometer is unplugged to prevent racking up miles on the car without them being recorded.

I don't know for sure that's your issue, or how to fix it, but might be a start. I'm sure somebody has figured out what signal that wire is looking for to engage the charging circuit. You do definitely need that wire connected to something.

[aquillen]

Pretty sure Subaru alternators use the same setup as pretty much all the rest. Typically the charge light circuit provides the initial trickle current to start the magnetic field up in the alternator. If it is disconnected, the field can't build = no output. Sometimes an alternator has enough residual magnetic field to get going but that old timer's claim may be a wives tale, not sure.

[Wayne Presley]

On the plug, apply battery voltage to the blue/red wire and switched voltage to the black/white wire through the alternator light in the gauge cluster.

[Hobby Racer]

On the plug, apply battery voltage to the blue/red wire and switched voltage to the black/white wire through the alternator light in the gauge cluster.

I am not using a Subaru dash, so no light to connect through. Can I just connect it to 12V switched voltage or does there need to be some current draw in the circuit?

[Bob_n_Cincy]

No, You need a light bulb or resistor in the circuit.

Here is a nice discussion on alternator warning lamps.

http://www.lotuselan.net/forums/elan-archive-f16/alternator-warning-lamps-etc-t10387.html

They refer to the bulb being 2 watt 12 volt.
so if you want to put a resistor in that line instead.

V^2 / P = R ------> 12^2 / 2 = 72 ohms

So I would something around 72 ohms + or - 10.
You need 2 watts.
I don't like things that get hot, so I would use at least a 10 watt resistor.
Bob

[Hobby Racer]

Thanks Bob, you the man ;) I'll try that this weekend.

Although, once the alternator self excites and the voltage on both sides of the resistor are equal, there would no longer be any current flowing and hence no heat right? So the resistor does not need to be so beefy as its only has current flowing for a brief moment until the alternator self excites right?

[Bob_n_Cincy]

Thanks Bob, you the man ;) I'll try that this weekend.

Although, once the alternator self excites and the voltage on both sides of the resistor are equal, there would no longer be any current flowing and hence no heat right? So the resistor does not need to be so beefy as its only has current flowing for a brief moment until the alternator self excites right?

Yes, BUT
if you have an alternator failure, belt failure, or just forget to turn off the key. the resistor or light stays on.
Bob

[Hobby Racer]

Yes, BUT
if you have an alternator failure, belt failure, or just forget to turn off the key. the resistor or light stays on.
Bob

Good point. better safe than on fire. ;)