Join Date: Dec 2001
Location: Belleville, Illinois
Thanked 22 Times in 17 Posts
Re: Alternator hookup
The small white wire needs to have 12 volts applied to it when the engine is running. But you don't want it hot all the time because it will slowly drain the battery. That provides a little field current which the alternator needs to bootstrap itself up. Without that the alternator might wake up if you spin it really fast, or it might not.
The small red wire is the sense line. It tells the regulator what the voltage is so that the regulator can adjust the alternator output accordingly. If you hook it to the alternator output lug the regulator will regulate the voltage at the output lug.
If you do as RRich suggests and run that wire to the battery, the regulator will regulate the voltage at the battery.
Rich and I disagree on the significance of that. I say that the difference is too small to worry about if the vehicle doesn't use a lot of electricity, and have taken measurements on my (minimalist) Jeep that support that standpoint.
Rich says that the difference is significant, and I agree with him if it's a vehicle with a big constant electrical load, like an OTR truck or an ambulance.
But it's easy to run that sense line somewhere closer to the load, and it absolutely won't hurt anything. You can put it in the loom right along with the big output wire, run it to the solenoid and put it under the same terminal.