Alternators, Models & Design, Part 5, Delco 15 & 17 SI

The Delco models 15SI and 17SI are a larger version of the 10 and 12SI alternators.  Other than the part number, it is almost impossible to distinguish between the two, as practically all the parts are interchangeable between the two.  Pictured below is both models.  The 17SI is on the left and the 15SI on the right. 

You may notice some minor differences in the housings but both models could have any of the housings pictured.  Though they share the same basic design with the 10SI most of the parts will not interchange with it though.  The picture below shows how much larger the 15 and 17SI alternator is compared to the 10SI.

The 15 and 17SI units do not have the housing variations that the 10SI smaller units do, but they do have the terminal variations.  Testing this unit follows the same procedures as the 10 & 12SI units.  All terminals match the smaller units with the 'BAT' or battery terminal on the threaded post, the number one or spade terminal on the left is the indicator light and/or excitation terminal, and the number two spade terminal is battery sense.  The following paragraph for testing, is from the Delco 10SI post on 1/27/11.  For more detail information, refer to that older post.

Of course using a voltmeter across the battery terminals is the best way to check if it's working, reading a fully charged battery at 12.5 volts and then after starting the vehicle, it should read a volt higher.  Getting the alternator checked by a qualified technician is the best way to determine if it is good or not.  If the alternator is not charging, with the engine running, one test you can perform is to take a small screwdriver and poke it into the small 'D' shaped hole in the back of the unit (pictured illustration on the 10SI post) to bypass the regulator, which will put the alternator into full charge.  The grounding tab is about 3/4 of an inch from the back surface of the unit.  Don't poke it in too far or you can hit the spinning rotor, damaging the unit!  You do have at least an inch of space before hitting other components.  While grounding that tab, if the alternator charges, then the internal regulator is bad and will need replaced!