I'm sorry I know I've been asking a lot of questions, I think this should be my last one for a while. Ever since I bought my car ('68, I6) a year and a half ago, the alternator has always fluctated constantly up and down by one volt (I think thats the right unit? we measured it once) all of the time. It's enough to see your dome lights dim and brighten at night, and you can see the head lights fluctuate when they're shining on a wall too. I noticed the other day, that it doesnt do this when the car is in reverse. The alternator gauge simply stays at one volt, or at least the highest point it goes to when it's fluctuating. I guess my question is why does it do this? can or should it be fixed?

 

It could be a faulty voltage regulator or fluctuating voltage output from the alternator. A simpler cause would be an intermittent connection or harness problem with the lights
Check the alternator and charging system. Take the alternator to an auto supply place and have them run a test on it. If it turns out to be a faulty alternator, be sure you also replace the voltage regulator too. If the alternator turns out to be good, check the instrument cluster wiring and harness for a poor connection. If you don't find one, don't worry too much. I really don't see this issue as ending the "life" of your fine old Mustang.

 

Drive the car down to autozone and they will test the cars charging system for free. They have a machine that hooks up to your car and can diagnose problems. A replacement voltage regulator which could be your problem is about $10 at most auto parts stores and takes 5 minutes to change.

 

Dano670 gave some good advise. The one thing they won't check for is Voltage drops on the positive or negative side.

I've got the same issue with my car. has the old style point type regulator. When i opened it up I found the coil (for the regulating point set) getting very hot. After a thorough check of the whole system it seems that this car has had an issue with grounds. Someone repainted the engine compartment and engine. They didn't clean any of the surfaces where the grounds attached to. I've since clean up all the issues (no more voltage drops on the negative side), but the damage has all ready been done to the regulator. At this point I'm not too exited. My voltage does not go above 15V nor below 13.8V. I can live with the fluctuating headlights and interior lights, until I do my engine swap. Brings back memories of my first British car ;-)

The fluctuations will reduce bulb life and possibly switch life if the voltage drops low enough though, so you really should get it fixed.
Use some emery cloth on all the grounding cables and the points at which they attach so that you get a good clean metal to metal connection. Do not rely on the nuts and bolts to be your "connector"). Make sure there is a ground strap from the engine to the chassis, and that there is no voltage drop through any of the positive or negative cables.

Rule of thumb is no more than 0.20V through any cable or switch. In reality I don't get too exited as long as it's under 0.5V total in any given circuit.

I'm often amazed at what these old cars will let you get away with. I have a neighbor who had a 6V drop to his coil (+3V through resistor). The issue never arose until he added electronic ignition and it wouldn't start. His gas mileage, drivability and power increased substantially when he fixed all the wiring problems.

 

All good points Scott. All the grounds must be soilid metal to metal. Grounding bolts get loose, rust gets under them., etc. But after you get them all cleaned up an working, paint them well. Three coats.

 

Reading through the service manual I found a good section on adjusting the Voltage regulator. Thought I'd try it with mine to see if one in poor shape could be fudged closer to spec.
Just got back in from this experiment. Thought I'd post my findings.
BTW: my regulator has seen better days and should be replaced, so your results may vary.

After cleaning the points with some worn out 400 grit emery cloth, I was able to close the point gap down to ~ 0.011" on the Voltage limiter coil. This reduced the voltage spread to ~ 0.4V just outside of what the book calls for (0.3V). I had to readjust the the spring support to get the voltage not to exceed 14.8V (which IMO is as high as I want a new generation battery to go). I went ahead and adjusted the Field relay contact points as well, but it really wasn't necessary, and had no effect on the voltage regulation.

Bottom line for this worn out regulator is that I still have fluctuation (albeit much less) Won't know until tonight how this effects the lights. However, more important to me is that I am able to keep my maximum voltage to 14.8V. I don't mind replacing bulbs and switches, but I absolutely do not want to deal with the battery gassing and causing corrosion.

Hopefully your mileage will vary to the positive side.
as a foot note, I could have done the whole thing in the car, but it was easier to do the initial point gap adjustments on the bench. From there I got everything up to temp (as per instructions in S.M.) and tweeked it in the car. It was kind of fun to mess around with.
If I was going to keep this engine/charging system I'd definitely go ahead and get a new electronic voltage regulator though.

 

Ah, OK Scott. Bottom line, replace the point system to electronic.