My 72 has the stock charging set up. It has a 61 amp alternator that is original, but the bridge rectifier had been changed a few years ago. The external voltage regulator is a NOS Delco Remy installed a few years ago. The settings were not touched when it was installed. The battery is 3 years old. I did open up the alternator to test the bridge rectifier, and it tested normal. The brushes looked OK.

Recently my battery would not start the car after a 20 minute drive. I charged the battery and was able to start up without issue. Prior to this, the car started with no issues except for a couple of months ago the battery would also not start the engine after sitting for a week. I charged the battery and it started the engine. The car is driven 2 or 3 times per month for about 20 minutes each time.

With no accessory load, I measured the voltage at idle and it was the same voltage as with the engine off. I increased the RPM to about 2000 and the voltage increased. It would run up to above 15 volts then drop down to 14+ volts or sometimes into the 13s. It tends to fluctuate. I seem to remember when I first installed the regulator a few years ago that the voltage was higher at idle. Definitely higher than the engine-off voltage. I tried using a different battery that I know is good and the voltage increased but not above 14 volts.

The GEN light is on when the engine is off. The GEN light goes off with engine running. There is no dim illumination at any time.

Should the voltage at idle by higher than the battery voltage? I know there is an adjustment for voltage in the regulator, but I don't want to tweak things off the factory setting if it is not necessary.

 

Test the battery, sounds like its junk from here, and yes, I saw it's three years old.

 

Is the GEN warning lit with ignition key OFF? That's usually a diode failure. Have the alternator tested.

 

The light is off with the ignition off. The warning light is operating like normal.

 

So, is it normal to be at battery voltage during idle? I always thought that alternators are good at charging when at idle and generators are not so good at idle.

 

I'd first clean the cables and battery terminals. Then check the voltage at the battery with the engine off, then with the engine idling. A fully charged battery is 12.6 volts and with the engine running at idle you should be around 14.2ish. Auto parts stores can check the alternator and regulator.

 

I agree with Jetstar. It looks like a new battery is in order. Retest after with a new battery. So far it seems the alt and reg are working, but retest with the new battery.

 

If you're exceeding battery voltage at rpm above idle, the alternator is probably fine.

 

Voltage with engine off is about 12.6 volts. Then with engine on at idle it is the same. Increasing the RPM to 2000 increases the voltage.

 

I did check it again with a known good battery from another car and it pretty much behaved the same way.

 

You leave out important information. Increasing RPM to 2000 increases voltage - to what? The DC Voltage should never exceed 15VDC. If so, you have a faulting VR.

 

With no accessory load, I measured the voltage at idle and it was the same voltage as with the engine off. I increased the RPM to about 2000 and the voltage increased. It would run up to above 15 volts then drop down to 14+ volts or sometimes into the 13s. It tends to fluctuate. I seem to remember when I first installed the regulator a few years ago that the voltage was higher at idle. Definitely higher than the engine-off voltage. I tried using a different battery that I know is good and the voltage increased but not above 14 volts.

 

The VR is designed to “maintain” a “steady state” delivery of DC voltage w/in the range of ~14.2VDC +\-0.3VDC. Both undercharging and overcharging a DC battery will shorten its lifespan and bring it to its knees. It’s called a voltage regulator because its function is to regulate voltage. Yes you are correct about a GEN vs a ALT at idle. An ALT should always be able to provide charging DC voltage to the battery at idle. You might likely need a new battery, but I’d be suspect of the VR. There should be no fluctuations or swings in voltage, it should never exceed 15VDC and should always maintain a charging state greater than the battery at idle and/or above.

 

Be mindful my post does not discount what has already been stated. It is imperative the cables, cable terminals, battery posts and connection points to the block, and horn relay distribution block are free of corrosion/oxidation. Remove each wire brush sand etc (or replace) to maintain the most optimal charging performance. This is basic wiring maintenance 101.

 

Resistance (electrical) is futile. 😂

 

If it turns out to be the VR, what should I look for internally? Is this an adjustment issue?

 

I have attempted this in the past on various vehicles & I have failed miserably in making any adjustments to the VR, I am sorry to state. I'll provide guidance best I can; yet, their are others with far more experience at making adjustments to a non-faulting VR. I say non-faulting because it's unclear if an "adjustment" would be in the best interest if a VR component has failed. I have replaced the VR in all vehicles I have dealt with. I think it's a hit or miss proposition on making adjustements to the VR, but you stated it was replaced several years ago, I think. It's not uncommon I should say to find a new VR to be faulty, as well. I suspect it's worth a try, but I'm not the person to assist - I (personally) think it's a tricky proposition.

Here's a decent enough thread (while there are many other VR threads, as well):

Voltage Regulator adjustment

 

Go to Autozone and get a VR715 solid state regulator in place of the one you have. You can swap covers if you want the original look.

Duralast Voltage Regulator VR715 | AutoZone