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delco1946 07-29-2021 11:58 PM

Generator light coming on

Hi all, I have a (different) generator light issue this time ( previously posted about the voltage reg sticking occasionally).

I’ve noticed on multiple occasions at night (lights on) I’ve noticed my generator idiot light start to dimly glow. The faster the engine revs, the brighter it will glow. It even tracks with the shifts - shifts up then the light dims a bit gradually getting brighter as I gain speed. If I add to the load, radio, fan, windows, etc the light gets brighter yet. The lights appears to go out at idle, at which time no amount of added load seems to turn it on. What gives?

I’ve heard that there is a bimetallic switch in the VR that gets affected in warm weather, but it still did it yesterday evening when it was only maybe 80ish. This evening it was mid 80s and doing it. Drove it this afternoon ( lights off) in 97 degrees and everything seemed fine. I have yet to notice this issue in the daytime with my lights off.

55, 6 volt system

rotorwrench 07-30-2021 08:10 AM

Re: Generator light coming on

The brushes may be worn down. The spring tension drops off as the brushes wear. Generators don't like high rpm. They tend to put out less the higher the rpm gets and worn brushes don't help much

A person can perform a running voltage check pretty easy but an unregulated output amperage test may tell more about generator function.

JimNNN 07-30-2021 12:24 PM

Re: Generator light coming on

Delco1946, my '54 wagon's 6 volt system and gen light does the exact same thing as yours. I completely rewired the entire car a few years ago (with a Ron Francis kit) including a rebuilt generator and new voltage regulator. It did what you describe both before and after the rewire. I'm lax about checking my gen belt, but I adjust it now and then, though. I'm going to do that in the next day or two. Two other things:

Do you ever drive your car at highway speeds? What I find is the light gets brighter and brighter up to about 55+ mph (about 2800 rpm) then the gen light goes out abruptly and usually doesn't come back any speed...until I turn off the car and start it again.

Besides the belt, I also suspect the voltage regulator, but I've put a few new ones on over the years. A new regulator seems to improve things for a short while, then the phenomenon seems to return. My existing regulator also has the issue of keeping the gen and oil lights on slightly after the ignition key is removed, which is remedied by lightly tapping the regulator. (That happens rarely, nowadays.) Very strange for a regulator with maybe 1500 miles on it. I suspect these new regulators are very poorly made. I do little about this issue because it has never resulted in my battery being drained or me being stranded.

delco1946 07-30-2021 01:21 PM

Re: Generator light coming on

Hi Jim,

I basically use my car as a daily driver in the summer, so i routinely am on the highway at 60-70 miles per hour. Those speeds are of course higher rpms, so at night (only) with my lights on, the gen light seems to be the brightest. Coming off the highway and stopping (such as to make a turn) the gen light is off. The car always starts, and both the generator and VR could be original for all I know (they're prolly decades old at least).

I have had the issue you mention of the gen light coming on once i turn the car off, but i sanded the VR contacts lightly and cleaned it. Since doing that, its only come on w car off once or twice and closing the hood always unsticks whatever is sticking :)

I'll check my belt, too.

rotorwrench 07-30-2021 01:25 PM

Re: Generator light coming on

The warning light indicates that the control unit is not allowing the generator on line. Residual magnetism in the field poles is enough to generate the current to close the cut out pole contractors and bring it on line. Flashing the field will restore magnetic gauss to the field pole shoes at the proper polarity.

The warning light gets power from the ignition switch. The ground comes through the armature pole terminal when the cut out contactors are still open. As soon as the generator comes on line the ground path for the warning light opens causing the light to go out. If it remains on at all then there is either not enough current being generated or the cut out is still open for some other reason.

JSeery 07-31-2021 07:21 PM

Re: Generator light coming on

Good explanation!

delco1946 08-02-2021 11:48 AM

Re: Generator light coming on

After pondering this and re-reading the shop manual, i think my problem might be simply that the voltage was adjusted (set lower) that is was supposed to be for the temp it was at the time (much cooler than it is now!). Thus with the bimetal switch acting normally, at the hotter temps with all the loads going, might be enough to prevent battery charging. The brighter light at highway speeds could simply be from the spark plugs sparking so much more frequently then at lower rpms (registers as extra load).

The bimetal switch seems pointless nowadays since batteries don't boil over (manuals description).

delco1946 08-04-2021 02:44 PM

Re: Generator light coming on

Ate my own words I think. This morning was in the mid 60s and before the sun rose, I was getting the gen light to glow just using the power windows (without driving lights on). I guess it was too dim to see in normal daylight.


Crankster 08-04-2021 04:06 PM

Re: Generator light coming on

The VR itself has temperature compensation, but the whole shootin' match is dependent on the voltage "setpoint" being adjusted correctly. Ordinarily shouldn't have to mess with it. Unless somebody previously really buggered it, I doubt that's the problem.

Charging voltage is temperature dependent, but it is automatic, because of the bimetallic spring or whatever. Generators have a notably higher charging voltage than alternators. The published set point or charge tables were also printed in the days of analog voltmeters. I don't know if that makes a huge difference or not, but generator charging systems are normally in the 14.x to 15.x volt range and even higher in subzero weather.

rotorwrench 08-04-2021 09:30 PM

Re: Generator light coming on

Most Ford type voltage regulators have a thermistor for temp compensation. Bi-metallic switches were used in the constant voltage regulators that power the King Sealey instruments that were powered on 12-volts.

KULTULZ 08-05-2021 04:19 AM

Re: Generator light coming on


Originally Posted by delco1946 (Post 2041939)

The bimetal switch seems pointless nowadays since batteries don't boil over (manuals description).

A WET FILL BATT won't boil and/or sulfate with a VR out of adjustment?

You need to re-think that one, especially with the common practice of buying the cheapest brand/make available.

JimNNN 08-05-2021 05:48 PM

Re: Generator light coming on


Originally Posted by delco1946 (Post 2042718)
Ate my own words I think. This morning was in the mid 60s and before the sun rose, I was getting the gen light to glow just using the power windows (without driving lights on). I guess it was too dim to see in normal daylight.


I was actually thinking of asking you about that. I've been deceived by ambient light, too. A good way to get around that is to take a take one of those long cardboard tubes - like what gift wrapping paper comes coiled around - and put one end up snugly to the gen light and sight through the other end to see whether your gen light is actually off, or just dim. Don't do this while driving, of course. :D

My 12 volt 1961 Mercury has a generator, too, but it gets none of the persistent gen light signals that my '54 gets (unless there's an actual problem with the generator or volt reg.) As I said earlier, however, my '54 light stays off consistently once I hit a certain speed/rpm, and at any speed after that point.

If your regulator is old, and you haven't replaced it yet, it might be time, but you might get a multimeter out first to see what's actually happening with your charging system.

rotorwrench 08-06-2021 01:40 PM

Re: Generator light coming on

An AGM or absorbed glass mat cell type battery is a bit different than the full wet cell type but not by a lot. The electrolyte is absorbed into the fiberglass mat between the plates. The mat will hold electrolyte against the plates and resolves some of the evaporation problems that happen with a wet cell. This is why they can go with no water replacement that is required for wet cells and it makes them service free.

The plates can still build sulfates so it's important to keep the battery working as much as possible to prevent that. Many folks use the battery tenders that cycle usage when the car is setting a lot. The round cell designs like the Optima has more room for plates the way it is rolled up but this battery is more expensive due to the expense of manufacture and the extra cell material that goes in there (near pure lead).

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