Alternators, Models and Design, Part 2, Delco 10SI

Next in the Delco series is probably the most popular alternator of all time, the Delco 10 SI.  This alternator was the simplest in design, easiest and cheapest to repair and very versatile in it's applications.  GM started using this unit in all their vehicles in about 1972 and has been widely used in all sorts of vehicles, trucks, tractors both agricultural and industrial, and marine applications, up until the early 80's.  This unit is still in wide use today and is being rebuilt and/or reproduced even now.  I've built or rebuilt and sold a lot of these units myself, over the years!

Lots of you might recognize this unit, as the one they built a lot of 'one wire' units out of.  Many have used this unit as a replacement for equipment with bad wiring, on street rods, replacing generators on both vehicles and tractors, or any application that doesn't have all the proper wiring to operate wired units.  It's great for people who don't understand wiring and need something simple to hook up.  All that is needed is a wire connected to the battery post on the back of the alternator to the positive post on the battery.  That's it!  It's just that simple!  The unit grounds through the case, so if external grounding is necessary, there is a threaded hole in the back for a bolted connection.  I have never seen an isolated ground on this unit but that doesn't mean one hasn't been produced!

Converting one of these units to a one wire application is fairly simple, but sometimes not easy.  The only component that needs to be changed to make it a one wire, is the regulator.  The problems you can encounter is the properties of the stator and rotor.  There must be a tight clearance between these two components or it will have trouble self starting.  Another factor is the ability for the rotor to hold residual magnetism.  Some alternators will still work with poorer components, but you have to race the engine up to get it to energize.  In my experience, it's about a 50-50 shot, that changing the regulator will make one work as a one wire unit, without changing other components!  Usually if the unit is a one wire, there will be a black rubber plug capping off the two spade terminals. leaving only the post to hook the battery wire to.

In most applications, the terminal connections are easy to identify and are always the same.  The only exception is if the unit is built for an external regulator, which I will cover in the next post on variations.  The 'BAT' or battery terminal is of course the insulated post.  The two spade or regulator terminals are as follows, the number 1 terminal is the exciting and indicator light terminal.  This terminal must be connected to a switched source, meaning hot when the key is on and dead when the key is off.  It generally is also connected through an indicator light (or dummy light as it has been called), or usually some sort of resistor needs to be inline with this terminal.  It should not receive a full 12 volts as it was not designed that way.  The light bulb serves as a resistor which is inline with this terminal so sometimes if the bulb is burned out, the alternator won't charge!

Warning; If you connect this wire to the ignition coil, the alternator will feed back into the coil and you won't be able to turn off the vehicle, without grounding out this terminal or unplugging the harness plug from the alternator!

The number 2 terminal is a battery sense terminal, which senses the amount of charge that's in the battery and tells the alternator whether to charge more or less, controlling the output of the unit.  If you are wiring this unit up to something that doesn't have the proper harness, you can route this wire back to the battery terminal on the unit.  Though it's a viable solution, this is not the best way to hook it up because if there is any resistance in the battery wire, it may tend to run a bit hotter and overcharge a bit.

Testing the unit doesn't leave you many options.  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 (Illustrated below) to bypass the regulator, which will put the alternator into full charge.  The grounding tab is about 5/8 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!