i just got done taking out my factory ECM harness from my 89 IROC-Z in preparation for the stand alone FAST system harness

one thing that stands out in my mind that was VERY weird when taking out the harness and cutting a few wires here and there (GM spliced many wires into the chassis harness)

On the plug that attaches to the alternator (the top one) there are two wires on that plug.

1) a big fat red one obviously power

2) a tan one

The WEIRD thing is that, that tan wire went all the way back to the ECM.....yes back to the ECM

why is that and what would the ECM need a wire from the alternator for?

Thanks a lot

ps just for reference...our alternators are 1-wire alternators correct?

 

i'm thinking it is for the warning light the computer flicks on?

maybe?

 

If it is a stock GM alt and a 1989 it is NOT a one wire system. A one wire alt, typically called a self-exciting alt is different. In 89(I have an 89 5.7TPI) GM used the CS series alternators. They were either two or three wire systems. The smaller tan wire you describe can be used for charge indicator light(depends on wiring) or used to trigger a SES light if an overvoltage condition exists. These alternators only need two wires to function, the line to the post on the rear of the alt, and the heavier wire going to the two pin connector on the top. This setup is basically referred to as an ignition triggered unit. You can convert these units to a one wire setup. Look at www.alternatorparts.com, they have a lot of mods and upgrades if you are so inclined.

 

thanks dano, i didn't think they were one wire but i was confused

ok, well i'm happy now that i didn't need that tan wire.

just the one to the post of the alternator and the plug will work for me

 

I am not sure if the ECM will trigger an SES if you leave it disconnected. You will have to try it. After everything is hooked up double check and make sure you are charging OK.

 

LOL, my FAST computer wont be triggering any SES lights...its a race computer

i just didn't know what it was for...but yes i'll make sure it is charging the right way when i get it up and running