What is the BROWN Alternator Wire for?

[Gregski]

Originally Posted by greg64

Okay, here's how it works. These alternators we're using are Delco 10SI and 12SI. These were the first Delco alts with built in regulators (that were solid state too). They replaced the 10DN that uses the external mechanical regulator from the 60s. The SI is really good, and only tends to fry if it overheats.

The SI has 3 wires. The heavy red wire, that comes off the threaded terminal is positive power. Obviously the housing is negative. The other two wires are part of a common spade lug connector. These wires are red and brown. The red is smaller in gauge compared to the main power red.

The red wire is "voltage sense". Wherever this wire is terminated will be held by the regulator at 14.4V. This is a great feature, because this can overcome any voltage drop in the main power wire from the alt. This wire should make it back to a junction block on the firewall.

The brown wire is "energize". See, the problem is that when the engine first starts, the alternator is spinning, but the rotor is not yet energized, so the alternator puts out nothing. The heavy red wire can't be used for this job, because it is connected to the battery always, so the rotor would drain the battery when the engine isn't running. If brown isn't connected, like a "one wire alternator" (dumb, I might add), the alt will eventually self-energize if it is spun fast enough. Instead, the brown wire is hot when the ignition is on, which feeds a bit of power to the rotor and energizes the magnetic field. Brown also passes through the Alt idiot light, which causes the Alt light to be lit when the alternator isn't producing any output. This works because juice is flowing through brown to energize the rotor, but the alt isn't putting out any power.

Ask any questions you like about the SI alternators.

Greg that was some serious know how you dropped on us, thank you very much.

Couple pesky things to follow up on.

My spade lug connector does not have a red wire it has a yellow wire and a brown wire. But sure enuff about three feet later the yellow wire splices into a red wire and runs to the bulk head.

Now back to that Alternator Idiot light, if there is one on my truck I sure would love to know where?