One day I had a big electrical load on and I noticed the alternator light on with the voltmeter down under 12.

After I reduced the electrical load the voltage came back up to the 14-15 range where it usually is and the alternator light is still on, although very dim.

The car starts and runs fine and the battery is charged (put a charger on for a while just to make sure).

Do I have an alternator problem?

Thanks for the help, Bill

First, make sure the belt is tight. It should be tight enough that you can't turn the alt. pulley (fan) by hand. Then make sure all the connections are tight at the alt. and batt. Put a voltmeter on the batt. and check voltage there, under load, and also at the alt. and compare the readings. If it's low at both places, then the alt. is probably the prob. There are other checks you can do, but this is the basics for start.

A slipping belt is a good possibility.  But it also sounds like you may have one of the diodes in the rectifier bridge failing.  The alternator is three phase A/C and is rectified into D/C current.  If a diode fails you lose 1/3 of the charging capacity.  It will charge, but under heavy loads or low speeds it won't keep up, and voltage drops.

If the belt is good condition and tight, connect an A/C volt meter to the battery with the engine running  If you see more than about .2V A/C, you have some diode leakage, and need to replace the rectifier bridge.  If there is not A/C voltage, shut off the car, disconnect the battery, disconnect the battery lead at the back of the alternator, and connect an ohm meter to the battery lead and the alternator case.  Take reading and reverse the meter leads to the post and case, take another reading.  One way should be an open circut, the other way should be a moderate reading.

You could also just go to AutoZone, Murrys, etc and have them check the alternator amperage output.  Have them connect the amp inductive probe to the alt battery cable from the back of the alternator.  If the output is down about a 1/3 to 1/2, you have diode problems.

Note I didn't say the rectifier bridge this time.  There is also a light minder diode that could fail, and cause similar problems.  

If you fiind the out put is low, and the belt is tight and not glazed, and want to dig into the alternator and save a lot of $, let me know and I will give you directions on how to do this.  You will need an ohm meter with diode check to do the test.  This info is in a past post somewhere, but I can type it in again.

I'm pretty sure Ken is talking about the information he shared with me when my voltage light was coming on.  The advice he gave in that instance saved me the cost of a new alternator, and for the cost of about a $10 part my alternator was back to normal and no more voltage light.

Don't know how far back the archives go, so Ken may have to crack his knuckles and sit down at the keyboard again, but I would recommend doing a search on "alternator" or "voltage light" in the electrical forum.

And remember, I took my alternator to an alternator shop that said the alternator was shot or needed to be rebuilt, before I asked for advice from the group.  Don't trust everything you hear from these types of shops unless you happen to be in Cuyahoga Falls and get Ken behind the counter of the AutoZone there!

Repairing an alt. yourself saves quite a bit on $$$$$, and you don't really need all that much in the way of tools. Toothpicks come in real handy for holding the brushes in, along with a good volt/ohm meter with the capacity to do diodes, as Ken said, and a few nutdrivers.(don't say it )

I wonder how many altenators Ken sells at the store, Ha Ha !

Thanks for all the ideas.

The battery was 12.27-volts before start and read 13.34v with the engine running and everything electrical on, at the alternator it was 13.72v, unloaded it read 15.02v.

It looks like it fails the AC test though as there were 27.9v loaded and 32.9v unloaded. What has me puzzled though is that it was zero with the leads reversed?

I did a search for the previous post and could not get any hits so it looks like I would like the information repeated so I can replace the diode(s).

Thanks for the help, Bill

Do you mean 27.9 and 32.9 AMPS? not volts?

Another thing I don't like.  At the alternator terminal you have13.7 v and at the battery 13.3 v under the same conditions.  That is a .5 drop.  That is enough to create a noticable difference in the system performance.  I don't think that's your only problem, but it indicates a high resistance somewhere between the alternator and the battery.  While this is not rare in a 30 year old car, it is something that needs checking into.

I will type up the directions to open the alternator , test and replace all of the diodes.  Give me a couple of hours and I will have it up.

To inspect and replace the alternator diodes.

 

This applies to the 1974 and newer alternators with the internal regulator.

 

Once the alternator is off the car, place it with the pulley down.  You will see four bolts around the outer diameter of the case.  Remove the bolts.

Now the back half of the alternator case can be lifted off of the other sections.  You will see a thin center section between the alternator halves.  Make sure this part stays with the back half of the case (opposite side from the pulley).  As you lift, you will hear a light snap.  This is the alternator brushes dropping out of place.  Two springs may fall out.  These springs will look like a spring in a ball point pen.

 

Inside the back half you just removed you will see the brushes hanging by a ceramic holding fixture.  Looking at the outside, you will see a small hole that lines up with holes in the ceramic brush holder.  Place the springs back in the section where they were in the holder, and where the brushes slide in place.  Now line up the brushes and slide them back in place.  Insert a piece of wire, toothpick etc, into the hole from the outside of the housing, and into the hole in the brush holder.  Push the brushes backing far enough that your pin will hold the brushes back in place.  Insert the pin far enough to go through the top hole on the brush holder.  This the only way to hold the brushes back in place when reassembling the alternator later.  If the brushes are worn get a new set.   They will come installed in a new holder.

 

Now look down in the alternator back half.  You will see the center section is bunch of wires in a wound bunch.  This is the alternator stator.  You will see three wires from the stator held in place with three nuts.  Remove the nuts and lift the stator out of the case.  Use an ohm meter and make sure all three wires in the stator have contact with each other.  If not, replace the stator.

 

Where you removed the nuts you will see three taps going to a rectangle block with another tab going to the regulator and brush holder.  The same screws hold the regulator as the brush holder.  There are three of them.  Two of the three have insulators under the screw heads.  Don’t mix them up.   Remove the screw holding the last tab on the block and lift the block out of the alternator.  This block is the light minder diode, also known as the diode trio.

 

Use an ohm meter set to diode check.  Touch one lead to the single tab on the trio, and touch the other meter lead to each of the other three tabs, one at a time.  Read the meter.  Now reverse the leads and check all three again.  One combination of the leads will show an open circuit on all three of the other tabs.  When reversed you should read an almost equal reading at all three.  If it fails either way, replace the diode trio.

 

Now back down inside the case. And back to where you first removed the three nuts.  They are located in the rectifier bridge.  This contains six more diodes.  These affect the charging rate, and control changing the three phase A/C into D/C current.  Rotate the case so the rectifier bridge is at the top of the case.  This is just for reference and description.  With the case in this position, you will see cooling fins on both the bottom and top side of the bridge.  Connect your ohm meter lead to the bottom set of fins, and check the three lugs where the nuts were.  Now reverse the leads, just like you did with the trio, and take another set of readings.  Again, there should be an open circuit one direction, and a constant even reading with the leads reversed.   This checks three of the six diodes.

Now put one of the meter leads on the top set of fins, and repeat the checks to test the other three diodes.   If any of the diodes flow both ways, or are open both ways, you will need to replace the rectifier bridge.   It is held in place with a screw into the case on the right side, and by the nut holding the terminal on the battery connection on the outside of the case.  Remove those and the wire from the condenser and the bridge lifts out.  Be sure to reinstall the insulator on the battery terminal, or the electrical system will short out, and blow a fusible link by the starter.  If all test were good but the alternator does not charge, replace the voltage regulator.

 

Before reassembling the alternator, look at the two copper rings on the alternator rotor in the front half of the case.  If they are rough and burned gently sand them to a smoother surface.  Don't sand too much or you will need a new rotor.  You can't put the copper back.  It's best to hold the rotor upright, and wrap a piece of very fine sandpaper around it, and pull back and forth like Chubby Checker doing the twist.  This way you don't create low spots. Use your ohm meter again, and make sure there is a connection between the two copper rings.  If not, you will need a rotor.  This is fairly rare.

 

Be sure to remove the pin, toothpick, etc after assembling the alternator.  If you forget, the brushes won’t make contact and the alternator won’t charge.

 

I have drawings, but they are copywrited, so I can’t post them.  But I can e-mail them if you need to see them.

 

Once the alternator is off the car I can do all of this in about 10 minutes.  It's not that tough. Your first time will take quite a bit longer.  Just take your time.

Ken,

Thanks for all the info - I may get to it tomorrow and do some more checking. On the AC readings - I was told in the post to look for volts - I didn't check amps!

Bill