Alternators, Models & Design, Part 1, Delco 10DN

With this post, I'm starting a series of posts to help you identify the type and model of popular units and some advice on how to check them.  I however, will not be going into how to repair them.

I will start off with the Delco series of alternators and the earliest popular model, the 10DN.  The 10DN was the first widely used alternator on passenger cars and trucks, up until about 1971-72 and was an externally regulated unit, (no built in regulator)!

As you can see from the photos, it looks a lot like the next series of the most popular internally regulated units (next post) and though some of the parts are interchangeable, some are not.  The best way to identify this alternator is by looking at the back of the unit.

As you can see, the two wire plug in goes straight into the back of the unit and not around the edge like later units and is more squared.  You have, of course the 'BAT' or battery terminal on the left and a ground terminal 'GRD' on the right.  The ground is not often used as the alternator is internally grounded to the case.  There are however some applications where an isolated ground is used and that terminal is insulated from the case.

The two spade terminals where the wire plugs in, are the 'R' or stator terminal on the left and the 'F' or field Terminal on the right.  In most applications both of these wires go to the regulator.  The field terminal is where the regulator controls the output of the alternator.  Though it's not recommended, you can unplug these wires on the vehicle and get a short jumper wire and cross it over to the battery terminal, to check the alternator.  (Leave the battery wire connected!)  By doing this you are putting the alternator into full charge and run the risk of damaging a weak unit.  The battery terminal must have 12 volts going to it for the alternator to work.  If that wire does not show battery voltage then you have a wiring problem and need to trace it down!

To check to see if the brushes are functional, you can use an ohm meter to check for continuity from the field terminal to ground with the plug unplugged.  This won't tell you if the brushes are worn out or if the alternator is good.  It'll just tell you if that circuit is functional.

There are a couple of variations to this alternator, but are generally somewhat rare.  In a marine application, there are stud connections instead of spade terminals and it has an extension protruding from the rear bearing area.  There is also an agricultural application that has a mechanical tachometer drive, which is mounted on the back of the unit.  I've found these mostly on Allis Chalmers tractors, but have seen them on other applications.

There is also an optional external regulator available, that can be attached and bolted to the back of the unit, but it is generally more expensive than buying an internally regulated unit, so I don't do many of them.  The next series internally regulated unit is a better unit to work on anyway and is more economical too!