What to do with blue wire on 2-pin 944 alternator/starter plug when ECU controls alternator?

My swap has been up and running 5 years. I'm switching my engine management over the winter to a different ECU and harness. With the new setup the ECU controls the alternator. Previously I had the blue wire on the 2-pin 944 alternator/starter plug wired into the L terminal on the alternator plug. Where do I send this blue wire now if it's not needed to excite the alternator? How do I make my volt guage and volt light work correctly?

Thanks for any help.

Chrenan,

Happy Thanksgiving!

I’m curious to see the forum input on this, being at a similar place in my build.

I’m using an MS3-Pro ECU adapted to a factory 2002 LS-1 wiring harness by my nephew who was employed by MegaSquirt at the time.  The LS-1 harness has an alternator trigger wire which I have connected.  I’m confident the ECU will be managing the alternator, but I won’t know for sure until I fire the motor.

I have several light gauge wires coming from the harness trunk line that do not have connections identified, but I suspect some of them provide data from the ECU to dash gauges (including the voltmeter) while others are leads to my purge valve, water temp, oil temp, and oil pressure sending units.

At some point my nephew will hopefully enlighten me …but he’s busy getting married on Monday.

For your project, the wiring harness diagram you posted does seem to have an alternator trigger wire, but I understand you may not need one at all depending on the alternator and ECU you’re using.

I’m sure the answer you need is out there but IMHO you may have to identify the specific ECU and harness that you’re installing before the forum experts can weigh in.

UPDATE

I did a little digging around on the forum regarding alternator hook-ups etc. and found something you may be interested in. Check out the following post:
by Porsche951toLS3
on Mon Jul 08, 2019 11:48 pm

At the end is a comment on the "blue wire."

The blue wire is just 12v from the light and resistor in the dash to excite the alternator. The "L/Light" terminal on a "dumb" alternator is a ground until the alternator starts outputting. Once it no longer becomes a ground the light turns off. There is really no reason to move this to an ECU unless you ditch the factory dash.

If you have a 2-pin PWM alternator then your ECU will need to generate a 5v 128hz PWM signal to control voltage. In order for the dash light to turn on/off you would need to connect it to an output on the ECU and have it provide a ground below a certain voltage(i.e. less than 13v).

The volt gauge operates independently from the blue wire.

Sixline,

Thanks for your input! I'm going to save your directions for future reference...hopefully in the next several weeks I'll be finalizing my electrical system.

I'm sure others will appreciate your directions as well!

Almost every stand alone harness made, has the correct wire to excite the alternator. I have never had to use the blue wire in the Porsche harness.