generator light

[sp53]

I have never understood the relationship of the generator light and the generator. The problem I am having is the system shows a charge on the amp gauge, but the red light will glow dimly when I start turning the lights and the heater on. I just replaced the generator with a rebuilt one and perhaps the generator is weak. However, the generator and the system itself worked fine at first. Any suggestions?

 

[karls59tr]

Make sure the fan belt is tightened to spec. Sometimes if the idle is a bit too low the light will come on as well.

 

[CJD]

My gen light glows dim when I turn on all the lights too. I discovered it's just the ambient light coming from the instrument bulbs, so it has nothing to do with the generator. I am thinking about painting the outside of the generator (and turn signal) plastic housing to "seal out" the ambient.

John

 

[sp53]

I will check the fan belt and I know what you mean John and this one is different. I can turn the heater on and watch it glow a little brighter. I am thinking I will do the polarity deal again and perhaps the battery is too old. It is 10 years old, but very high quality.

 

[Geo Hahn]

I have never understood the relationship of the generator light and the generator...

Perhaps you already know this much, but I think the bulb is 'comparing' the voltage from the charging system with the voltage of the battery. A hot wire is offered from each source to each side of the bulb (bulb is not grounded).

If the voltages are equal (0=0 when ignition off, 12.6V=12.6V when engine is running) the bulb is unlit. If the voltages differ (such as when you switch on the ignition but the engine is not yet running) the bulb glows. The bigger the difference the brighter it glows.

 

[TR3driver]

Perhaps you already know this much, but I think the bulb is 'comparing' the voltage from the charging system with the voltage of the battery.

More accurately, it compares the voltage from the generator at the control box, to the voltage from the ignition switch. Normally those two voltages are the same whenever the cutout relay (inside the control box) is closed, since the cutout relay links the generator output (D terminal) to the wire that runs to the headlight switch and ignition switch (A1 terminal). But, if there is a bad connection along the way, or the ignition switch has a high internal resistance, then there might be some voltage difference between the two points.

At least in theory, if the ammeter is showing charge, then the light should be off, regardless of loose fan belt, weak generator, etc. If you are seeing both charge on the ammeter, and the light glowing at the same time, then there must be a problem in the wiring or switch.

I would start by trying to duplicate the situation, and check the voltage at the control box between the 'D' and 'A' terminals, then again between 'D' and 'A1'. There should be no more than 0.3 volts difference there (which obviously isn't enough to light the lamp). If that looks OK, connect a clip lead to the output terminal of the ignition switch under the dash (or the hot terminal of the starter pushbutton will do if it is easier) and compare that to the voltage at the A1 terminal. Again, should be no more than a few tenths of a volt (less than 0.5).

If all that looks good, then likely the light is coming from somewhere else (as John suggested), or there is a problem with the wiring to the lamp itself.

Note that, under the dash, the NU wire should go first to the headlight switch and then have a fairly heavy gauge jumper over to the ignition switch. In the past, I've had a lot of trouble with that connection at the headlight switch; apparently the wires vibrate and over time the individual strands work-harden and break.

PS, I'm assuming this is a TR3/A/B, and you have already verified that the ammeter is reading the proper direction. It's a good idea to mention which car you are working on whenever you start a new thread, since they aren't all the same (and I find it hard to keep track of who owns what). Or just put it in your forum signature.

 

[Roger]

If your dynamo (or alternator on later cars) is not well regulated, it can overcharge and that will turn the light on too - when charging voltage is significantly greater than battery. Other symptoms of this are fluctuating excess brightness of lights, and battery maybe overheating.

 

[TR3driver]

I disagree, at least for cars equipped with Lucas DC generators/dynamos. Since alternators don't use cutout relays (the output is always connected directly to the battery), they work quite a bit differently.

 

[Member 10617]

Having followed the BCF for about three years, I have to say that I think the topic of the ignition light comes up more often than almost any other topic (or... at least very often). I can relate to this because the ignition light is a source of great frustration to me.

The answers to the questions are always detailed and very helpful, but they are never quite the same. The variations, for me at least, raise as many questions as they answer. For those "in the know," this is not a problem, but for people like me the mystery remains.


Someone who understands the workings of the ignition light (battery, generator system) could give the rest of us a great Chrismas present by setting this all out (again) in logical, step by step detail, including how to check the voltage (with photos of the connections at the regulator, of the voltage meter, etc., etc.), what the readings should be, and at what rpm (more or less) the light should come on. The Lucas manual describes much of this, but it is written for trained mechanics and assumes much more knowledge than many of us have.


For example, I have understood that the light comes on when the draw is from the battery (the generator not producing enough), but in reading some of the postings above, it appears that the light can come on when things are "out of balance," including when the generator is producing too much power...

See.... I'm confused...

 

[Geo Hahn]

Another time when the light may come on is after you have switched off the ignition. If that occurs I suggest you immediately pull the ground cable off the battery.

I had this occur intemittently and found I could usually get it to go off by switching the ignition off then on again -- but finally that did not work and I looked under the bonnet. I found that a contact in the control box had stuck and was sending battery current back to the generator. This turned the generator into a motor and the little guy was valiantly trying to turn the engine! I keep the belt a bit on the loose side so I could see the generator (now a motor) pulley slipping against the belt.

Cleaning the contacts in the control box resolved this.

 

[sp53]

Thanks much you guys this is something I would like to learn. Randall I assume that the engine should be warm and an idling at perhaps 900 to 1000 when I take the readings from D to A1 and D to A. Moreover, and just to be sure I take a voltage reading from the ignition to A1 and compare those findings?

 

[TR3driver]

Wait a minute, I am confused here. You are looking for a problem that causes the light to come on at the same time the ammeter is showing charge, correct? Or do you just mean that sometimes the light comes on and sometimes the ammeter shows charge, not both at the same time?

Idling at 900-1000 rpm is not going to produce any appreciable amount of charge, and the light may or may not be on at that speed without necessarily indicating a problem. These things are not precision devices, there is definitely room for variation between generators (and between control box adjustments). Not to mention variations in the dash tachometer, it is not unusual for them to read high by several hundred rpm.

 

[sp53]

Sorry for the confusion Randall. I reread your post and realized that for some reason I thought you mentioned 12 volts in the testing, but now I see that you were taking about relative relationships in voltage when the problem is present. Yes they both are working at the same time--- the ammeter and the generator light. The light comes on very dimly at about 2000 with the lights on and when I turn the heater on it glows just a fuzz brighter, but again very dim. Perhaps I am making a big deal out of it because the brightening of the light is difficult to see. If I lived somewhere where the sun shines, I probably would not see it. I am going to read up on setting the regulator and do the test the way you described it.

 

[TR3driver]

Oh, Ok, just wanted to be sure we are on the same page here. Personally, I would still want to investigate the light glowing, even dimly, under those conditions. Not because you have a charging problem, but because that much voltage drop in the wiring, switches and/or control box probably means that something is getting very hot and will likely let the smoke out in the future. It's much better to find such problems in the garage than out on the road on a cold, wet night.

And you are right, you want to be looking at the voltage difference that is powering the light, rather than checking from each point to ground. It is much easier to see 0.5 volts on the meter, than the difference between 12.6 volts and 12.1 volts, especially since the actual battery voltage tends to wander around during the testing.

If it is easier for you, you could do a preliminary check with the engine not running. Turn on the key, heater and headlights, then look at the voltage differential between A1 on the control box, and the load side of the ignition switch. A few tenths of a volt is normal, but I'm guessing you will see several volts, meaning there is a (relatively) high resistance somewhere in the circuit. Or possibly even several of them. The ignition switch itself is a prime suspect, especially if there have been any extra loads added (like a stereo).

 

[TR3driver]

For example, I have understood that the light comes on when the draw is from the battery (the generator not producing enough), but in reading some of the postings above, it appears that the light can come on when things are "out of balance," including when the generator is producing too much power...
See.... I'm confused...

Well, it is a confusing topic. Part of the problem is the primitive nature of these old charging systems; the various indicators don't give a simple "go or no-go" indication but something that has to be interpreted in context with other conditions. Then you have all the possible failure modes, which (as in this case I believe) may not even be within the charging system. The only way to get a simple explanation is to overlook some possible conditions, which then have to be considered separately. There is also room for differences of opinion, on the best way to simplify the description.

It is also easy to get confused between various setups (like alternators) that may appear similar on the surface but work very differently otherwise. With a generator, the light operation depends on the cutout contacts; but alternators have no cutout contacts! So even though the result is similar, the workings under the hood are completely different.

And of course, don't believe everything you read on the Internet (including what I write).

That said, I believe that both of your statements are incorrect (for Lucas DC generators). The light does not directly sense whether the battery is being charged or discharged, or even over charged. The light merely indicates whether the logic in the control box 'thinks' the generator is producing enough output to be connected to the battery. Thus the generator might only be producing 5 amps, while the total draw is 20 amps, meaning the battery is being discharged at 15 amps; and the light would be out since the generator is obviously connected to the circuit. Or the generator might be supplying it's full rated output (either 19 or 22 amps, depending on which TR generator you have) into a battery that is already fully charged and the light would still be out.

'Normally' (in the absence of other problems and the ignition switch is on), the light will come on only when the control box thinks the generator is not capable of producing any output current at 12 volts. The Lucas specification (for the early 19 amp model) is that the cutout should close (putting out the light) when the generator is turning between 1050 and 1200 rpm. I don't know the exact drive ratio from the crankshaft offhand, but assuming it is around 2:3, that would be between roughly 700 and 800 engine rpm. But as I mentioned before, with equipment this old there is a lot of room for variation without anything necessarily being "wrong". And you have already mentioned that your control box is not set to Lucas specifications, so it would not be at all surprising to find that it does not meet Lucas specifications.

 

[Member 10617]

Randall,

It's becoming clear and clearer. Many thanks for taking the time to lead me (and others) through this.

I'm glad to have a better idea of the relationship of the light to the battery... and to the output of the generator. As for the 700 to 800 rpm point when the cutout might close, my light (adjusted by Moss), comes on when rpms drop to 900 or 1000 rpms or below. I understand from you, however, that there can be a great deal of variation in rpms depending on many factors, including the adjustment of the tach. Mine was recently rebuilt, but it still could be off some.


In addition to running a new control box (cleaned, adjusted, and tested by Moss), I am also running a new generator (which I believe was purchased from Moss by the shop that rebuilt my motor). When I first purchase my car, it had the original generator and the original control box in it, and the light didn't seem to come on very often. After all the changes I made, the light now comes on as I slow down and reach 900-1000 rpm or, of course, if I idle at a stop. I just didn't know why there would have been a change... but perhaps you put your finger on it by pointing out that Moss doesn't use Lucas specifications.

The good news (also assuming there is no "bad" new here), is that the light immediately goes out and remains out when I am moving, at 900-1000 rpms and above.

The ammeter has been critical in alerting me to problems from time to time, and I check it often. But I don't think I get very useful information from the light... (probably because I haven't yet learned to interpret it).

I would like to test the control box for volts but don't know how to connect and test the terminals, what gauge wire to use with the aligator clips, or how to use the voltmeter, etc., etc. I'll keep exploring this until I learn.

In the meanwhile, many warm thanks for your guidance.... and the guidance of others.

Unusually warm today.... the open road is calling. More fun than addressing Christmas cards...

 

[TR3driver]

But I don't think I get very useful information from the light... (probably because I haven't yet learned to interpret it).

I certainly consider the ammeter much more informative. As I see it, the main advantage of the red light is to hopefully get your attention sooner when the fan belt breaks, giving you a chance at stopping before the engine overheats. And maybe as an indication that the ignition switch is on, when the engine isn't running. The book does describe it as the "Ignition warning" light, after all (which also adds to the confusion).

 

[sp53]

Like I said I do not understand the generator stuff, so I read up on it and that if I take A1 and A and hook them together and put the other end on D I should get a voltage reading somewhere in the area of 15.5- 16 volts depending on temp. So I do that and get a reading of 5 volts. Could that be possible? The battery charges up and the car runs great. The book said to adjust the unit slowly by turning the (B) screw down. But will it do that much or could I be doing something wrong.

 

[TR3driver]

Nope, definitely something wrong with this picture. And as it isn't broken (from your previous comments) I strongly advise against trying to fix it !

Do you have the voltmeter set to 20 volts DC, and connected from terminal 'D' to ground (terminal 'E' or any good solid ground) ?

 

[KVH]

Just adding to the discussion, my ignition light shows a very faint glow all the time, almost invisible in daylight, but brightens at below 800 rpms. Everything works great and seems normal.

I decided to leave it alone, though it's awfully temping to tinker with something.