Alternators, Models & Design, Part 3, Delco 12 SI

The Delco 12 SI became the later, ramped up version of the 10SI.  This unit was used from the early 1980's to the late 80's.  Notice, I don't use specific years because some models used several different units during the crossover years. 

Though all the parts are somewhat interchangeable with the 10SI and a few with the 10DN, there were differences.  I've seen all the parts interchanged as some point or another and though they will work, they don't fit properly.  The 10SI's maximum designed output was 63 amps, but has been rebuilt to much higher outputs.  A big majority of the 12SI's were at 78 amps output and some were designed to give 94 amps.

Externally the main difference was the enclosed fan.  Behind the fan, the drive end housing had a raised ring that the fan meshed with.  This gave it more efficient cooling and I should mention that most all external fan alternators, cool by drawing air in from the rear of the unit, through the unit and out the fan, instead of pushing it through from the front!   There is also a larger cooling area on the back of the unit over the rectifier.

The 94 amp unit has cutouts on the drive end housing around the stator windings and it also has double angled cooling fins on the rectifier.  This makes it easier to identify when working on the unit.  Sorry, I have no pictures of the 94 amp-er.

Internally, the rotor and stator are thicker than the 10SI units and the rectifier has an extra grounding and mounting slot or hole.  I've had problems swapping a 12SI stator, putting it in a 10SI unit because the stator leads are not quite long enough and must be stretched and put in a bind to get connected.  They have broken in service, so I seldom do that anymore and only if necessary!  Hookups and testing are the same as on the 10 SI, so refer to that section for that information.