Wiring 10Si, 12Si, 15Si, 17Si, and 27Si Series Alternators
General Motors has only had 4 different series alternators since it first replaced generators with alternators in the 1960's. The very first alternator was the 10DN externally regulated alternator. The first internal regulated alternator was the 10Si series starting in the early 70's and used till the mid 80's. These alternators were quite popular for auto, truck, industrial, marine, farm and adaptive applications. Since the introduction of the 10Si, GM's Delco-Remy line has offered the 12Si, 15Si, 17Si, 27Si then the CS and AD series alternators for cars and light trucks.
This first section will cover wiring information for the SI series only, we will go over the CS and AD series wiring later. Although they may vary in size and output, the wiring is the same for all the Si series. Wiring these alternators is quite simple. All the Si alternators can be considered both 2 and 3 wire systems. To activate these alternators you are only required to supply the main battery wire to the (BAT) terminal which must have power on and an ignition wire to the #1 terminal. Most all the Si series alternators should have two spade terminals, but some that have three terminals, the third is for a tachometer connection and senses the alternator RPM.
Near the spade terminals, the rear housing should be marked #1 and #2. Some aftermarket housings are not marked and others may be worn off. So if your alternator housing is unmarked, look from the rear of the alternator: the #1 terminal is on the left and the #2 on the right. You only need an ignition wire to the #1 terminal to make an Si series alternator work. The #2 terminal is for voltage sensing, and is optional. The #2 voltage sensing terminal allows the voltage regulator to sense the battery voltage so it knows when to turn the alternator on and off. The #2 terminal, if it is not used, causes the regulator to revert to internal sensing and pick up the battery voltage at the main battery wire on the back of the alternator. Some alternators are wired with a jumper from the #2 terminal directly to the battery connection at the back of the alternators, but this is not needed.
The ignition wire to the #1 terminal can be supplied in two ways, either a direct ignition wire from the key switch or through a light bulb. Running the ignition wire through a light bulb before connection to the alternator will give you a warning if your alternator fails to start charging or if it quits working while the engine is running, this is commonly known as the idiot light. Another thing that should be mentioned is the #1 terminal must be ignition switched. I have heard of situations where a wire was run to the #1 terminal that had power on at all times. What happens is when the engine is turned off the #1 terminal switches to ground which over time can burn up the voltage regulator or the wire if power is not cut to the #1 terminal. Another note for special installations of the Si series alternator is that in some cases when you try to shut the vehicle off, power to the #1 terminal from ground to positive power, the positive power feeds up to the coil and keeps the vehicle running. To cure this you must install a diode in the ignition wire that activates the #1 terminal. The diode will stop the power from feeding back up the ignition line.