Wednesday, August 8, 2012

Delco Alternator Rebuild Guide

Delco-Remy 12SI Alternator
Here's a DIY guide to rebuild your failed your Delco alternator requiring only basic mechanical skill, a few tools, a cheap rebuild kit, an hour or less, and these instructions.

For more information on identifying alternators, this article by Mark Hamilton is very well done.

Parts and Tools

Pictured below is a classic Delco (Delco-Remy) aka GM, Volvo, Saab, Jeep alternator, model 12SI, along with the tools you'll need for the rebuild. From left to right: hex key set (smaller hex key for holding brushes), needle nose pliers, combination wrenches, 1/4 drive extension, socket, and 1/4 drive wrench for removing housing bolts and internal bolts, flat head screwdriver to pry apart housing, and permanent marker.

Delco 12SI Alternator and tools for rebuild
Here's the alternator repair kit I bought from Autozone. Your rebuild kit should provide you with new brushes, a new voltage regulator, new diode package, a couple springs for the brushes, a thin metal wire, and a front bearing/bushing. Time to replace everything and reassemble.

The alternator kit I used from Autozone
You can pick up Delco-Remy 12SI alternator rebuild kits for under $15 at any chain auto parts store or even online. Here's a kit for 56A, 78A, and 85A alternators (P/N GMA01) from

Note: P/N GMA02 is for the 94A version. Another example is the Amazon alternator listed at the top right of this article (should work with the lower current 12SI).

Note: You might want to click the compatibility charts to make sure it will work with your vehicle.

Opening the Alternator

You'll start by marking the housing so you know how the two halves are supposed to go together. Called "clocking", these alternators can be put together so the mounting bracket on the front half and the field plug on the back half can be oriented at any position.

After marking, remove the four long hex head bolts that hold the two halves of the housing together using the right 1/4"-drive socket, extension, and ratchet wrench.

Remove hex bolts from rear of alternator to begin opening the case

Next, use a screw driver to pry the housing halves apart at the pry points shown and pull the case halves apart.

Pry alternator halves apart with a screwdriver

Now that you've got yourself two halves of an alternator take the back half and stare into it and you should see something very similar to the following.

Back half of the alternator with diodes, brushes, regulator identified
Below is a detail picture of the assembly that holds the alternator brushes. Brushes oftentimes are the main thing that wear out over time in an alternator.


Remove Regulator and Brushes

Start by taking out the the bolts holding the regulator and brush housing and remove both. Throw away the regulator but keep the brush housing.

Remove the 3 bolts holding the regulator and brush assembly
Note that there is something that looks like a threaded rod that is held down by two bolts that hold down the regulator and diode contact. Remember where this goes (refer to the pictures above). I think this is a resistor.

Old regulator, left; brush housing, right
Now remove the brushes but don't lose the springs, yet. Throw the old brushes away.

Match the old springs inside the housing to one of the new springs in your alternator repair kit. Install the springs in the housing and throw away the old ones.

Find the long thin piece of metal wire included in the repair kit. It holds the brushes in place until the alternator is reassembled.

There is a hole in the back of the alternator that you can use to pull this wire out, once you've reassembled both halves. If you don't do this, the brushes pop out of the housing and you can't insert the shaft.

Install the new brushes in the housing and force them into the housing until you can slide the thin metal wire through the hole in the housing and brushes.

Retaining brushes with thin metal wire provided in kit

Replace the Diode Rectifier

The diode rectifier pictured below is removed by taking out three additional bolts.

The diode rectifier on the left is held by three remaining bolts
Throw away the rectifier and replace it with the new one in your repair kit. Reinstall it in place and tighten down the three bolts on the left.

Replace the Regulator and Brushes

Now install the regulator and brush housing along with the threaded rod resistor thingy with the three bolts that hold it in place. One of those three bolts also holds down a leg of the diode.

Look at the back of the alternator and be sure that you can see and grab the thin wire holding the brushes.

Grease the Rear Bearing

The rear bearing on these alternators isn't sealed. Unless it's shot, throw some grease on it. I used axle bearing grease.

Pic 4. Brushes held by wire, greased rear bearing, final regulator bolt

Reassemble the Alternator Case

Put both alternator case halves together, aligned with the mark you made, and install the rear hex head bolts that hold the case halves together.

Replacing the Front Bushing

One thing I have not attempted is to replace the front bearing. However, I have had one of these go out before and they are included in the rebuild kit.

My educated guess is that the process is as follows: Use an impact wrench to remove the pulley and cooling fan, and slide the shaft out through the back of the housing. Use a bearing/seal driver to punch the bearing out. If I get around to doing it myself I'll post up definitive instructions.


Congratulations! You're all done and you saved yourself a pretty good chunk of change on a new alternator by spending a little time. This rebuild is pretty hard to get wrong so there's a good chance when you test it, everything will be a-ok.

If you do a lot of back country driving, the last thing you want is to have your alternator die on the trail so how about keeping a spare rebuild kit in your getcha-back-box? Or get a broken alternator from a friend for free and rebuild it as a spare.

Thanks, hope this article helps! If so, please share it with your pals.

1 comment:

  1. Just a note: take a small wire brush or sandpaper and make sure the housing is clean and shiny where the regulator bolts down to it, so that good electrical contact is made.