Gen light on/off with engine rpm?
 

1970 GTO just put in new battery and new regulator, the alternator was new last year. With the engine running and under light throttle the gen light comes on, but with more throttle the light goes out. Where do I start looking?

Make sure the battery is fully charged, as the battery if it's low at idle will make the alt. try to charge it & over work the alt. Make sure it has a complete full charge. Ater that look into the brushes in the alt.

Yes, the battery just came off the charger before I installed the new regulator. I know the battery is 100% fully charged. Tested the alternator and at all rpm's(idle up to about 3000), the voltage maintained 14.5volts. Although during cold start(choke on) engine rpm at about 1200, the voltage at battery was fluctuating and got to as high as 18.5volts?
This happened with the old regulator(factory original), which is why I changed it out and went with new. But as I said the new one showed 18volts on cold start, then calmed down after engine warm up. The gen light issue only showed up with the new regulator!
Is an alternator bench test at the parts store going to show anything?

Is it one of those new transistorized regulator? Constantly seeing problem on here about those. In my opinion gotta use old style regulator. Especially if this started happening after regulator was replaced.

X2, What he said!

Sounds like a bad connection (high resistance in line going to) or low voltage at number 3 terminal at regulator or possibly ground side. Most other causes would have voltage remain high. Check grounds and voltage at #3 reg terminal when this happens. I would look for burnt or corroded connections as it may be burning back to a good connection to return to proper voltage. Problem will get worse and not return to proper voltage but continue to overcharge.

High voltage would cretainly indicate the reg is not doing it's job.

If the wiring is correct, the reg is your current problem.

george

I would check the wires at the reg. connection, if the wires are loose or are corroded it will cause a bad ground concern. Make sure the wires are not loose or burnt through or even have a few of the wire strands broken that will cause a high resistance & cause the reg. to keep in a high charge pattern. Also check the wires on the alt. to make sure you don't have the same problem there. If you have any loose wires it will cause you to have the problems your describing. Or again it could be the reg. itself. I went through 3 of them before I got a good one that worked on my 68 bird. I forgot ,, the brushes can cause that too, so check the brushes out when you get it off if your going to take it to the parts store to get checked out. It only takes a few min's. to pull it apart to check them & another to replace them if there bad. Bad brushes will ground out & bounce on the armature when there spinning at normal speed. Also as they get hot they will make better connection. Check them out just to be on the safe side.

The brand new regulator is the old style contacts made by Standard Blue Streak, not transistorized new style. I bought it to replace the factory original one that I thought was not working, but did not completely test it out. The test I performed was with engine running, battery voltage showed up to about 18.5volts. That was at first startup, when the electric choke was on and engine idling at about 1100rpm. Thought I needed a new regulator since it was overcharging, but the new regulator did the same thing with the added bonus of the gen light coming on.
With the old regulator the gen light did not come on during engine running so I presume the alternator and brushes are fine. I did also notice a new humming noise and am now starting to think it is the new regulator. Also, the wiring and contacts at the reg and the alt looked good, but I removed each one from the plastic connector and cleaned them anyway. I installed a ground strap from the reg body to the ground strap connection at the firewall to be sure.
I will run the car tonight and take some more readings then report back.

The hum is definitely coming from the new regulator, and is present when the gen light is on. With the car running at operating temp here is what I measured:
Battery voltage 14.0(14.5 during warm up).
Terminal 3 reg 16.0(14.5 during warm up).
Also noticed that the gen light is on bright at idle now and goes out with some rpm. Does this mean the new regulator is no good?

If you put the old regulator back in do you get same readings and light on or off?

Yeah just did that... Gen light off.
Readings: battery 18.5, term 3 reg 18.5
Voltage at term 3 should be between 13.5 to 16.0 correct?, maybe should try to adjust old one and stick with it?
Seems the new reg is proper battery charging voltage but not enough voltage to turn off the gen light, is that from reg term 4(7.3volts)? The old reg has 18.5 on term 4.

I don't know enough about regulators, hardly ever messed with them. George will be able to tell you what you should be getting at what terminals. Or some others also.

Gen light on/off with engine rpm?
 

I agree with George, if all else is good VR is a primary suspect. Not sure if you have a GM shop manual for your car and how much work that you like to do on your vehicle. I had a similar issue when I rewired my car. My 65 manual has an excellent trouble shooting repair guide that covers the charging system. All you need is a good voltage meter. As I was doing a complete restoration I installed all new electrical components: harness, alternator, battery, regulator, etc only to find that some of the new style regulators do not function properly. I located some older Delco units, picked the best one cleaned the points etc and installed. All is working as it should.