I am apparently too much of an idiot to understand what my idiot light is trying to tell me. My ignition light will randomly come on when the car is running and the volt meter shows 15 volts or so. If I put a load on the system, such as turning on the head lights, or the radiator fans (I have them on a switch), the ignition light will go off. Any suggestions on what the problem might be? If the ignition light comes on, can I continue to drive the car without frying the electrical system? I am in Idaho, so just pulling over to the side of the road is not always an option unless you want to sit there for an eternity.
What car? Generator? If so, then it’s coming on because the rpms aren’t high enough to provide enough current for your load. So long as it goes off once you get above 800 rpm or so it’s all good.
It is a 1969 Series 2 with the original alternator. The light will stay on well above idle, even at 2500 to 3200 RPM. As I stated, the volt meter shows 15 or so volts the whole time, and adding a load drawing more current will make the ignition light go off. And it only happens intermittently.
I’ve been to this dance, on my 1970 S2 that has just been thru a bolt level restoration last year. Exactly this symptom. It had a new Voltage Regulator, new 3AW, the Alternator was rebuilt as part of the restoration. Good news is that I am a Systems Integration and Control Engineer (Mechanical) so out of desperation, I took one of my dual trace scopes and mapped the Voltage regulator to alternator current and the 3AW input from the Alternator. Here’s the scoop:
The Regulator (at least by 1970) manages the field coil on/off such that the battery side voltage is around 14.5 +/-. It’s not an analog variable circuit, rather it is full digital pulses to the field that get integrated a bit by the inductance and probably hysteresis of the iron. But not much. If you have a modern AGM battery, it has a different absorption profile for the current inrush than the old wet cells. So the regulator sees a slight decay of the battery side voltage and hits the field with digital pulses on/off (from full on to many times a second in some cases, depending on the load, or lack of). So far so good.
There is the AL lead off the Alternator that outputs raw AC from the 3 Stator coils over to the 3AW relay. In Analog form, the AC signal gets run through a rube Goldberg wire that heats up (a kind of mechanical integration of the AC with a physical time constant, t) and makes a set of contacts that will ground the ignition light if there is no output for that time t) and indicate that the alternator ain’t producing output.
For my car, freshly rebuilt, the 60A alternator is really on vacation during summer, since I have efficient electronic ignition, LED lighting, excellent fresh cooling system that rarely triggers the Otter switch and loads the fans. Further, the car starts instantly in less than one turn, so the battery is leaving the property essentially at full charge with nothing to do. Like yours, as the alternator checks in on the slightest drop, it hits the AGM battery for a few milliseconds, but pushes the voltage up near 15V simply because it’s a 60A Alternator hitting a fully charged battery so that the pulse inertia drives it above the setpoint, where the regulator shuts down the field coil in excess of the time t delay allowing the wire to close the WL circuit to ground momentarily, until the next time the field coil lights up off the regulator and passes some AC down the AL wire.
That’s why when you turn your fans on, or headlights, it behaves. It loads the alternator enough to keep enough pulses coming down the pipe to keep the 3AW’s contacts open.
Here’s what I did. I bought an electronic 3AW unit from SNG, thinking that it would be the fix. Initially, it wasn’t. It did the same thing, only faster. So I took it apart and reverse engineered it. I then changed the 0.22uf capacitor on it’s board to a tantalum 4.7uf capacitor to extend the time constant t past the worst case when the AGM battery was at 14.8v and no load with the engine running. I took the old 3AW apart and put the new SNG tabs and board into the original can. It has and is working perfectly. Should fool any judge.
This is a very interesting writeup. Thanks!
I have a 69 OTS with a rebuilt alternator whose stator was rewound as it got an internal short.
The regulator and the 3AW are both the new tech electronic types from one of the usuals.
I have had no charging problems since and the car starts within about 4 revolutions of the engine when cold, full choke and after letting the float bowls fill !
Interestingly, the ignition light is quite dim when it is on. But the strange phenomenon is that sometimes, the ignition light will come on and off about every two seconds (also dim). It does this regardless of load and I can see the voltage is varying as the headlights and dash gauges are also slightly pulsing with their intensity
I did put a new transistor in the regulator when it went a few years ago but I suspect the regulator is not happy as the lights are pulsing when this happens. But the battery is happy so I have not been too concerned about it.
If you have any thoughts about this I am grateful. I suspect both the 3AW and the regulator are not quite 100%
Dennis
69 OTS
cool cat has a great article on the series 2 alternator anight d charging system, easy to understand and why things work the way they do. once you understand how the system functions, its easier to fault find. and the article explains what the yellow light means. read it and take the mystery out of the system.
Why do I suddenly have a picture in my mind of the “Taxi” episode with Jim Ignatowski taking his drivers test and asking, “What does a yellow light mean?
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When the Battery is being charged after cranking, it is more or less normal for things to flicker a bit, as the regulator “On” step is long enough to be of a frequency that you would notice, up to say, 20 cycles per second. The regulator is capable of switching much faster than that, and as you get closer to fully charged, it will look smooth as a movie would, but at that point there could be secondary harmonics over top of the individual regulator step on-offs that you might be able to sense, depending on your particular vision sensitivities.
How old is your battery? Do you use a trickle charger? Is it fully charged when you get back from a ride on the basis of it not taking much charge with a smart charger? If all OK, that should eliminate the battery and broadly, the alternator as an issue. Batteries as they get older can protest taking the charge in a smooth fashion, and cause a rise and fall in the voltage as the alternator tries to stuff the step current inputs from the regulator field.
A dim ignition light likely means that the path to ground is flakey somewhere in that connection. That’s a pretty simple test to do. Unhook the WL black/brown wire from the 3AW and turn the ignition on. There should be nothing. using an alligator patch wire, connect the loose WL wire to ground. The battery is right there, so touch the clip to the negative post, just to be sure. The ignition light should be full bright. If so, the 3AW is suspect.
I’ve found that the blade connectors between the regulator and the alternator can slack off, so a loose blade/socket could contribute to steady state cruising intermittent flickering. Worth checking those.
One other intermittent culprit in the charging system can be the 6RA relay, which can have burned or dirty contacts. It powers both the regulator and the + side of the alternator field when you turn the key on.
I’d look at the above items before springing for a new regulator tho.
Dana, your explanations have been really helpful. My battery is getting toward the end of its useful life, but still turns the engine over easily so I have not thought it to be a problem. I do not have it on a trickle charger, and don’t believe it is an AGM battery. From my simplistic reading of your explanation, it would not appear that the ignition warning light coming on will not mean that I will be left stranded by the side of the road or at risk of cooking my electrical system, but the charging system needs to be evaluated and perhaps a new battery is needed.
Dana,
Thanks for the troubleshooting tips!
Dennis
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Nothing is for sure, but that would be my take on things. Watching the voltmeter as you have been should do it.
No problem. You should sleep better now 
Applause. +…20
dana our ign light does not come on at all I have tested bulb and its ok. this is a 1964 3.8
does that mean the ignition lite is connected to the oil pressure switch on this model???
Derek/jim