Repair & Rebuild your Alternator
by Lars Grimsrud
Musclecar, Collector & Exotic Auto Repair &
This tech paper will discuss the
disassembly, diagnostics, and repair of GM Delco alternators used after
1971, and specifically discuss the alternators used from 1971 to 1988.
Alternators used after 1988 are similar, and the same techniques and
principles apply (called the “CS”
Alternators fail frequently, and good rebuilt units
are pricey (typically in the $150+ range). But virtually any alternator
can be easily repaired for less than $50. You can do it yourself in your
garage or driveway using ordinary hand tools and a little
History & Principles
Electricity is produced by
moving coils of wire through a magnetic field, thus producing a current
flow in the coils of wire. Two different devices have been used on cars
to produce electricity: Generators and Alternators.
Early GM cars,
up through 1962, used a Generator to produce electricity. Generators, for
those of you too young to have seen one, are about the size, weight and
shape of a GM starter. They use permanent magnets to produce Direct
Current (DC). The magnets are located stationary around the case, and the
current-producing coils are spun on a shaft in the center of the
generator. Generators are neat in that they do not need any external
source of power (a battery) to begin producing electricity: all you have
to do is to spin them, and they produce a DC output. But they are heavy,
and they do not produce much output at low rpm: you’ll typically
see the headlights on older cars with generators go noticeably dim at
In 1963, GM introduced Alternators on its cars. Alternators
do not have permanent magnets, but rather send a small current through a
series of coils to produce an electrically-induced magnetic field. In an
alternator, the magnetic field is created by spinning the
electrically-induced magnetic field in the center of the alternator,
producing current in the stationary, case-mounted coil. This makes an
alternator much smaller and lighter, and its output at low rpm can be
maintained by increasing the strength of the magnetic field. An
alternator, however, does not produce DC output: Due to its design, an
alternator, as the name implies, produces Alternating Current (AC). This
AC must be changed to DC before it can be utilized in an automotive
Alternating current, if visualized, is like a
wave moving up and down: it cycles from positive to negative. It the mid
point between positive and negative, there is no current flow at all.
Obviously, then, if we only had a single coil producing AC power at low
rpm, this cycling and “dead spot” would make our lights and
electrical system blink on and off very quickly. Not good for our
application. An alternator, then, typically has three separate coils,
each producing its own “wave.” These waves are set as far
opposite each other as possible, so by the time they
“overlap,” they are producing a steady stream of AC power.
But now we must convert it to DC.
Each of the three current
producing coils is attached to two diodes. A diode is an electrical
component that allows current to flow one way, but not the other.
It’s like a one-way door. One of the “one-way doors” is
set to “open” in one direction, while the other one
“opens” in the opposite direction. Thus, when the alternating
current is flowing in the “positive” direction, the positive
output is shuttled out of the one diode. When the current shifts to the
“negative” direction, it is allowed to go only out of the
other diode. Thus we have separated out the two elements of the
alternating current into a positive and negative DC power output. With
all three of the coils doing this at staggered times, a steady stream of
DC power is realized.
Pretty simply, huh?
Parts & Systems
The alternator consists of 4 basic
Current Producing Parts
Each of these can fail, and each can be
repaired at a very nominal cost.
The housing serves to
contain all the parts in one place, provides a bearing surface for the
Rotor to spin within the Stator, and acts as a heat sink to dissipate the
heat generated by the internal parts and components.
modes (listed in order of frequency):
1. Bearing failures. Most
common failure is the housing front bearing. The front bearing takes most
of the load imposed by the fan belt. In spite of this, the bearings are
surprisingly durable, and will easily give 150,000 miles of service
provided the belt is tensioned correctly. Front bearing failures were
much more common in the days of the V-belt drive: “real” men
would use a crow bar on the side of the alternator and tighten the V-belt
up so tight that it could be played like a violin string. Alternator
bearing failure then occurred within a few miles of operation. Now, the
serpentine belts have automatic belt tensioners that provide correct belt
tension at al times. Bearing failure is evidenced by a growling or
howling from the alternator.
2. Dirt, Oil & Grease. Since the
housing serves to dissipate the heat generated by the internal components,
dirt and grease on the housing will impede this process and lead to early
component failures. Keep the alternator reasonably clean at all times to
help increase its life.
Current Producing Parts
The parts which
actually produce the current are the Rotor, Stator, and Brush
The Rotor spins in the center of the alternator. It is
charged with a variable current to produce a variable magnetic field, thus
producing variable output of the alternator.
The Stator is the
three-field coil mounted stationary circumferentially in the case. It
produces the actual power output.
The Brush Assembly provides
electrical contact to the spinning rotor. The brushes “feed”
the current to the Rotor to produce, and alter, its electro-magnetic
Failure modes (listed in order of frequency):
Brush failure. The brushes, since they contact the rotating Rotor, are
subject to wear. Typically, brushes will last over 100,000 miles. Once
worn out, they will no longer provide a good electrical connection to the
Rotor, and alternator output will fail.
2. Open or Ground
(“short”) failures in the Rotor or Stator. These failures are
extremely rare. They would occur if one or more of the wires in these
parts burned, broke, and shorted to ground due to an insulation failure.
In all my years of fixing cars, I’ve never seen a Rotor or Stator
To change the AC to DC, the early
alternators used 6 separate diodes that were pressed into the alternator
case and into a diode “bridge.” The next generation
alternators (starting in ’71) used a finned Rectifier Bridge and a
transistorized Diode Trio. The Rectifier Bridge is attached to, and
grounded to, the alternator case. It allows the “negative”
element of the AC power to go to ground through its three terminals hooked
up to the three current-producing stator fields. The Diode Trio, also
attached to the same three stator fields, allows the
“positive” AC element to go to the “+” side of our
DC system and to the regulator.
Failure modes (listed in order of
1. Any of the three “one-way doors” on
either the Rectifier Bridge or the Diode Trio can fail. Failures occur
when the “one-way doors” either allow current to flow both
ways (“leak”), or allow no current through at all. This will
either produce a lower-than-normal output of the alternator, or produce no
output at all. It can also allow current to slowly “leak”
through the alternator while the car is sitting, producing a slow drain on
the battery (typical “dead battery in the morning” symptom).
The Diode Trio is one of the most common failed parts in these
To control the output of the
alternator, a regulator varies the flow of power to the Rotor, thus
changing the strength of its magnetic field. From 1963 through 1970, this
was done by an externally-mounted, mechanical voltage regulator, normally
mounted on the firewall. Starting in ’71, GM used a small
transistorized, internal regulator in the alternator.
modes (listed in order of frequency):
1. The most common of all
alternator failures is the failure of this regulator. It is simple to
diagnose and replace.
Ohm Meter (For this
process, I prefer one of the really cheap analog meters (the kind with a
needle and scale. This type of meter will provide instant, easily
understandable information about circuit continuity. For this purpose, I
find these quicker and easier to use than the more expensive digital
meters. You can get a cheap analog ohm meter at Radio Shack or your
hardware store for about $12.)
�” drive impact gun
�”-drive socket set
with ratchet, extension, and nut driver. For pre-”CS”
alternators, socket sizes 5/16″, 11/32″ and �” are
Plastic or rubber
Toothpick (obtain a toothpick by mixing a good, dry Martini
with two Jalepeno-stuffed Olives on a toothpick before starting this
alternator procedure. By the time you need to use the toothpick, you’ll
have just finished the Martini)
you have an understanding of the operation of the alternator, and know
the component parts and their typical failures, you’re ready to
start your alternator repair. I like to lay out a clean towel on my
workbench. As I disassemble the alternator, I carefully lay all the parts
out on my towel in the right sequence and order: there are several
insulating washers inside your alternator, and it is imperative that they
all end up back in all the right places. So lay your parts out in a nice,
1. Disconnect battery negative terminal.
Remove the serpentine belt (simply release tension on the idler tensioner
on the passenger side of the block and remove the belt).
Disconnect the wires from the alternator and unbolt it from its
4. With a felt marker, draw a straight line across the
alternator case where the front and rear case halves bolt together. The
alternator case can be reassembled and “clocked” in any way to
customize the alternator to various bracket end engine configurations, so
you want to be sure you “clock” the cases correctly when you
reassemble your alternator. If you intend to paint your case, use a
scribe to make a line instead of a marker.
5. Spin the alternator
by hand to check the bearings. It should spin smoothly and freely with no
jerkiness or noise. A slight “swishing” sound from the
brushes riding on the Rotor is normal. Any roughness indicates bad
6. You will need to use a 15/16″ socket on an
impact gun to get the pulley and fan retaining nut off. Before I owned my
own compressor and impact gun, I used to just take the alternator down to
any local shop and have someone with a gun zip the nut off at no charge.
To do this, wear a glove to hold the pulley & fan, or wrap a rag around
the pulley and fan, hold on tight, and zap the nut with the impact gun.
It’ll take about 2 seconds. If you don’t wear a glove, or use
a rag, the fan will rip your hand up when you hit the nut with the
7. Pull the parts off and lay them out carefully in
sequence on your towel: first the nut itself, then a lockwasher, pulley,
fan, and finally a little shaft spacer.
8. Using a 5/16″
socket on an extension with a �” drive ratchet, break loose the four
case through-bolts. I prefer a 6-point socket for this, as a 12-point will
sometimes round off the bolts. These case bolts can sometimes be in
pretty tight. Once you’ve broken them loose, switch over to your
nut driver and pull all the bolts out of the case.
9. Split the
case. The front half of the case should now come off of the alternator.
Make sure the centrally-mounted Stator (sandwiched between the front and
rear case halves) stays with the rear case and does not want to come off
with the front case. The Rotor, if the shaft is dirty, may come out with
the front case. Use your plastic hammer or rubber mallet to tap things if
they don’t want to come apart. Use the mallet on the Rotor to tap
it out of the case front bearing if it doesn’t want to slide out.
When the case comes apart, and the Rotor comes out, you’ll hear two
little “snaps,”and you’ll see loose springs and parts in
the bottom of your alternator. Don’t worry: it’s normal.
Pick the two springs out of the alternator and lay them on your towel
with the other parts.
10. Remove the three nuts that attach the
three Stator wires to the Rectifier Bridge. These are 11/32″ and
have lockwashers under them. Lay the nuts and lockwashers side-by-side on
your towel. Now, lift the Stator out of the rear case.
Diode Trio is the small component attached to the three studs on the
Rectifier Bridge (that you just pulled the nuts off of). It has a single
strap connecting it to a Philips screw on the Regulator. Note that the
screw has an insulating washer on it. Remove this screw and remove the
12. Remove the other two Philips screws holding the
Regulator in the case. Make sure you note where the insulating washers
go. Remove the regulator.
You do not
need to do further disassembly in order to troubleshoot the alternator and
its components. Do the following tests:
1. Test the Rotor.
Using you Cheap-O ohm meter, set it to the 1000 ohm scale (or any other
fairly high scale). Touch the leads together to make sure the needle pegs
out on the scale, and make sure it returns to the left when you disconnect
the leads. Put one lead on the one copper-colored slip ring on the Rotor,
and the other lead on the other copper ring. The needle should peg to the
right (continuity). Now leave one lead on the copper ring and touch the
other lead to the central shaft of the Rotor. The needle should show no
continuity. If you get any other reading, the Rotor is bad.
Test the Stator. There are three wires (terminals) coming off the Stator.
Place the ohm meter lead on any one of the three wires and verify
continuity to the other two. There should be continuity between all three
of the wires (ohm meter should “peg out”). Now leave the lead
connected to any one of the three wires and touch the other lead to the
metal frame of the Stator (the part that gets sandwiched between the case
halves). There should be no continuity. If you get any other reading, the
Stator is bad.
3. Test the Diode Trio. Place one of the ohm
meter leads on the single connector on the Trio (the strap that was
connected to the Regulator). Touch the other lead to each of the other
three connectors, one at a time. The meter needle should either show
continuity or no continuity, but should be the same for each of the three
connectors. Now reverse the ohm meter leads and do the test again. The
meter reading should now be reverse of the reading achieved on the first
test (if the needle “pegged” on the first test, it should do
nothing with the leads reversed). All three connectors should be the
same. This test verifies that current can only flow in one direction, and
the same direction, through all three of the diode connectors. If any of
these readings is not correct, the Diode Trio is bad.
4. Test the
Bridge Rectifier. This will be tested in the same way as the Diode Trio:
place one of the ohmmeter leads against the cast, finned body of the
Rectifier. Touch the other lead to each of the three terminals, one at a
time. The meter should be either pegged or have no reading, but all three
readings should be the same. Reverse the ohm meter leads and do it again.
The reading should be opposite, and all three should be the same. If not,
the Bridge is bad. The Rectifier for a “CS” alternator (the
most expensive part you can buy to repair an alternator) is $47.
Visually examine the brushes. If the brushes are shorter than �”,
they’re worn out and should be replaced. The brushes are available
complete as a brush and brush holder assembly.
6. If everything
so far checks out good, you have a bad regulator. You can take the
regulator down to your local NAPA store, or other properly equipped parts
store, and they can test it for you on a dedicated machine. If it tests
bad, buy a new one. The regulators for the pre-’CS”
alternators cost about $30. “CS” regulators cost about
You have now identified your problem and procured a
replacement part. There is no part in an alternator that normally fails
that costs more than $50.
replacing bad electronic components, you may need to replace your
bearings. If your bearings felt rough, or made noise, when you spun the
alternator by hand earlier, they need to be replaced. If not, leave them
alone: they really do last for a long time.
The front bearing is
held into the front case by a retainer plate. Remove the three screws
holding the retainer plate into the case. Find a socket that’s
bigger than the diameter of the bearing and use this socket to back up the
case on the inside. Use a socket that’s slightly smaller than the
bearing on the outside (front side) of the case. Now, use a hammer to
lightly tap the bearing out of the case. I’ve seen guys do this
without using the backup socket on the back side, and I’ve also seen
some of these guys crack their alternator case when they’ve beat the
bearing out. The new bearing simply taps in from the back side. Then
install the retaining plate.
The rear bearing very seldom fails. It
is pressed into the case. I remove these by placing the alternator rear
case in a vice, with a large socket providing backing on the inside of the
case, and a smaller socket pushing the bearing in from the outside.
Tighten the vice, and the bearing is pushed out. Press the new one in the
To do a nice job, I always paint my alternators.
If you glass bead blast the cases, you have to remove the bearings to
prevent contamination. If you clean up the case and paint it, you can
leave the bearings in place. The Eastwood Company makes a really good
“Cast Aluminum” paint in a spray can. If you don’t want
to mail order, VHT, carried by most independent parts stores, also makes a
great “Cast Aluminum” paint. If you clean up your alternator
case and give it a light coating of this specialty paint, it will look
better than new. VHT and Eastwood also offer a “Cast Iron”
color. This color is almost identical to the dark, phosphate coating used
on the pulleys.
Now that you’ve procured your
new parts and cleaned everything up, the assembly process is very straight
If you have procured a new brush and holder assembly, it
comes pre-assembled with a retainer stick in the holder. If you’re
re-using your old brushes, and did not pull the brush holder out of the
case, you need to install the springs and brushes into the holder as
Drop one of the springs into the brush holder in the rear
slot (closest to the rear case). Slip the brush into the holder on top of
the spring and depress it with your finger. Now, eat the last olive from
your Martini to liberate the toothpick, and slip the toothpick in through
the little hole in the back of the alternator case, across the top of the
brush, and into the center “toothpick hole” in the brush
holder. The toothpick will hold the brush in the holder against the
spring pressure. Drop the second spring into the forward brush holder
slot, and push the brush into the holder. Depress it with your finger,
and push the toothpick over the top of the second brush. This will hold
the brushes in place until you get the alternator
Assemble the rest of the components in the same sequence
as you disassembled them. Once you have the regulator and rectifier/diode
parts installed, with all of the correct insulating washers in all the
right places, drop the Stator into the case and install the nuts securing
the Stator terminals to the Rectifier. Carefully drop the Rotor straight
down into the case until it drops into the rear bearing. Place the front
case onto the assembly, assuring that your marks align for the case
“clocking.” Slip the 4 case bolts in, and tighten it up.
Pull your toothpick out of the back side of the case to release the
brushes, and re-install the fan spacer, fan, pulley, lockwasher, and fan
nut. Give it a one-second zap of the impact gun, and it’s ready to
If you’re still not up for doing
a rebuild yourself, but want to keep your numbers-matching stock
alternator on your ‘Vette, you can send me your alternator for
rebuild. I will tear it down, replace any bad components, clean it up,
make it look nice, test it, and send it back to you. I charge $30 for my
time, plus my cost on the parts and shipping. If you’re interested,
drop me an e-mail at: