How alternator light works, a more detailed description Options

[type47]

...what is the battery voltage when the car is idling? 11.6V

Remove the gauge bucket with the GEN light. Start the car. Measure the voltage on both sides of the GEN light to ground.

On the double red/wht terminal, 11.6 V (assume battery voltage); on the blue wire side, 7.6V. At the 3 terminal plug, at the relay board connection for the red wire terminal (so this would be coming from the alternator) also 7.6 V

[Dave_Darling]

If that's the voltage across the battery posts (with a decent voltmeter--not the stock one in the center console!!) then your battery is not charging, and is about 3/4 dead. Re-charge it and keep troubleshooting the charging system, because it ain't workin!

--DD

[GregAmy]

So instead of starting a new thread, I'm going to necro this one and camp onto it because it's so useful.

Racecar: no need for a GEN light, I have a full LCD dash with voltage display and warnings. I removed the dash, relay plate (carbs) and mounted the voltage regulator directly to the sheet metal (easier in/out of drivetrain). Of course, the alternator won't start charging until I connect a test light between the battery and touch it to the red VR wire terminal.

Anyone have a rough idea what the resistance is on the dash bulbs to I can wire a resistor to the red terminal (don't have one handy)? Or, is any resistance at all required; can I just go with a fused jumper wire on it?

Thanks!

[Spoke]

I would take a guess at about 200 ohm should do. Being 14V maximum across this resistor, the resistor wattage should be greater than 14V*14V/200 = 1W. A 2 watt resistor should do.

[mikesmith]

After reading many accounts of how this circuit works, I felt compelled to investigate further as I did not understand how two positives would cause a light to operate. They won't. One must be somewhat negative to complete the circuit. Internet searches turned up the same basic explanation, still was not buying it. I think it was being oversimplified.

The functioning of the regulator is a bit sneaky and not necessarily obvious (though honestly you can say this of a lot of pieces of heavily cost-optimised technology).



Things to know:

- The alternator will only produce an output when current is already flowing through the field coil (marked G). This is the coil that spins when the alternator input shaft is turned by the engine.
- There are a couple of capacitors (close vertical lines) in the regulator drawing; I've covered some of their functions here but almost certainly not all of them.
- I only marked two of the resistors in the drawing, there's another that's blacked in - being blacked in almost certainly means something, but it's not clear what.
- Current takes time to start up / slow down, especially when it's flowing through coils like the field or stator coils in the alternator.
- It takes more current flowing through the coil to turn a relay on than it does to keep it on (so the 'turn off' current is lower than the 'turn on' current).
- Like most folks I'll use positive-to-negative when describing current flow; don't be upset that electrons actually move the the other way. 8)

When the engine is off, and immediately after it's started, current flows from the battery, through fuse #9, through the alternator warning lamp (K2), through the D+ terminal on the regulator, through the relay in the regulator and out the DF terminal, to the DF terminal on the alternator, then out the alternator ground back to the battery (current always flows in circles). This provides the necessary field coil current to bootstrap the alternator.

Once the engine has started the voltage across the stator coils begins to rise. Current will start to flow out the D+ terminal once the voltage rises above a certain point (tough to calculate due to the way the field coil behaves, but probably a few volts), and then out the B+ terminal once it rises above the current battery voltage.

While current is flowing out both terminals the voltage on each terminal will be nearly the same (since in both cases the current's coming from the stator coils). B+ will never fall below battery voltage; D+ will never go (much) higher than battery voltage / B+, but may be quite a bit lower at times.

As B+ is connected to the battery, and thus to the other side of the warning lamp (K2) this means there will be no current flow through the lamp and it will go out once D+ reaches battery voltage (no voltage difference -> no current flow).

The voltage on B+ and D+ will continue to rise (assuming the alternator is spinning fast enough) as the battery charges.

As the voltage on the D+ terminal continues to rise, the current flowing through the regulator relay coil and resistor R1 (circled in red) also rises.
Eventually this current is sufficient to energize the relay and it switches, disconnecting the field coil (and in fact shorting it to D- to ensure that no leakage current continues to energize it). The upper of the two capacitors in the diagram helps prevent damage to the relay contacts during this switching operation.

Removal of the field coil current causes the stator coil to stop generating (this isn't instantaneous; the field coil current falls, the magnetic field starts to collapse, the stator induced current falls...).

As the stator voltage falls the current out the B+ terminal falls, reducing charge current to the battery / rest of the vehicle.

At the same time the voltage on the D+ terminal and thus the current through the relay coil / R1 also falls; some current flows via K2 but this is insufficient to keep the relay energized. This causes the relay to switch back, returning the alternator / regulator to a state similar to the bootstrap state. At this point, there are several sources of current for the field coil; the bootstrap path through K2, the residual charge in the lower of the two capacitors, and the the stator via D+ and R2; one or more of these (probably all three contribute) cause the field coil current to rise and the cycle repeats.

The on/off process repeats very quickly; it can be several hundred times a second. During the off phase some current may flow through K2, but the average current will be low enough that the lamp won't light. The duty cycle (ratio of on time to off time) will vary based on the load on the system; more load will cause the B+ / D+ voltage to rise more slowly, delaying the time until the relay switches. With less load, it will rise more quickly. The time the charge circuit spends off will (tend to) be more constant, as it's related to the value of the lower capacitor and the characteristics of the relay coil, which don't vary with load.

HTH, and apologies for any errors or confusion this might cause.

[GregAmy]

That's some beautiful stuff right there (along with the prior posts).

So lemme ask two unrelated alternator questions.

- After posting, I took the racer out for a spin to check the drivetrain install (it's a caged racer but actually has plates and insurance). I was just cruising 5-10 minutes locally so didn't "excite" the alternator (not worried about battery level) but 5 minutes into the drive I glanced down and the dash was showing 13.5V.

How did this happen, and is it possible that this is a non-problem for a higher-revving race car?

- In the street car, I had a glowing GEN light that would be dim at idle and get brighter with RPM. I did some voltage checks and found that the B+ battery voltage was ~13.5V so it was charging fine, but the D+ voltage was full alternator output, from ~14.5V at idle to ~17+V at 3000 RPM. I drove it like that for a few months until I got motivated to replaced the alternator (the only thing the D+ was going to was the GEN light); a new voltage regulator did not resolve it, but a replacement alternator did.

Why is that? What failed in the alternator for that to happen?

[mikesmith]

- After posting, I took the racer out for a spin to check the drivetrain install (it's a caged racer but actually has plates and insurance). I was just cruising 5-10 minutes locally so didn't "excite" the alternator (not worried about battery level) but 5 minutes into the drive I glanced down and the dash was showing 13.5V.

How did this happen, and is it possible that this is a non-problem for a higher-revving race car?

Your description just begs more questions, sadly. Did the alternator self-excite? (residual magnetic field, current leakage into the field winding?). Is your battery just a monster (or are you running a 16V setup?), do you trust the dash's voltage reading?

- In the street car, I had a glowing GEN light that would be dim at idle and get brighter with RPM. I did some voltage checks and found that the B+ battery voltage was ~13.5V so it was charging fine, but the D+ voltage was full alternator output, from ~14.5V at idle to ~17+V at 3000 RPM. I drove it like that for a few months until I got motivated to replaced the alternator (the only thing the D+ was going to was the GEN light); a new voltage regulator did not resolve it, but a replacement alternator did.

Why is that? What failed in the alternator for that to happen?

If D+ is significantly higher than B+, then the high side of the main rectifier or something else between the stator winding and the battery (B+ terminal hardware, battery strap not tightly connected, etc.) is probably hosed.

Not sure about what you're doing with D+ only going to the GEN light... that's not how it's supposed to work. 8)

[GregAmy]

Your description just begs more questions, sadly. Did the alternator self-excite? (residual magnetic field, current leakage into the field winding?). Is your battery just a monster (or are you running a 16V setup?), do you trust the dash's voltage reading?

"Self-excite" is the only thing I can think of. But here's the rub: prior to my removing the replay plate and re-wiring everything, the VR was sitting on the relay plate, as normal. However, the stock dash was already removed and a Stack 8130 was stuffed in there, with no wires going to it for GEN/ALT light. I removed the Stack last year to install a Race Technologies DASH.

And yet...the alternator was charging. I *presumed* (but never verified) that the previous owner had stuff a resistor or bulb someplace in there, but was never curious enough to find it. However, today we know for a fact that since I removed the relay plate it's not connected.

And yet...?

And here's another rub. Some years ago I removed my VR from the replay plate on the street car; I described it in this thread here. At the time, I was not aware that the common knowledge was that the alternator would not "self-excite" without a GEN bulb..."and yet"...when I did that the voltmeter was showing charging. I only added that jumper wire in order to get a GEN light indication in case of failure.

So...I do wonder if it's actually needed.

Battery in the street car is a Miata AGM, and battery in the race car is a very small Deka ETX14 battery.

Not sure about what you're doing with D+ only going to the GEN light... that's not how it's supposed to work. 8)

Ok, you're correct. To clarify, the GEN light is the only D+ connection to the car. So I drove it like that, not worrying that it was going to fry any equipment in the car (the VR didn't seem to mind). I still have that alternator in a box in the garage, I'm likely to send it to an electrical repair shop to keep as a spare. But it wold be nice if it's a simple bench fix (while I'm handy with electtrical systems basics, I'm a mechanical guy, not an electrical guy).

[Spoke]

That's some beautiful stuff right there (along with the prior posts).

So lemme ask two unrelated alternator questions.

- After posting, I took the racer out for a spin to check the drivetrain install (it's a caged racer but actually has plates and insurance). I was just cruising 5-10 minutes locally so didn't "excite" the alternator (not worried about battery level) but 5 minutes into the drive I glanced down and the dash was showing 13.5V.

How did this happen, and is it possible that this is a non-problem for a higher-revving race car?

It is possible that the armature has some residual magnetism in it at rest such that without the current provided by the GEN light there is enough magnetism to get the alternator started. Once started, the GEN light is not in use thus the alternator is providing all the energy needed to drive the armature.

- In the street car, I had a glowing GEN light that would be dim at idle and get brighter with RPM. I did some voltage checks and found that the B+ battery voltage was ~13.5V so it was charging fine, but the D+ voltage was full alternator output, from ~14.5V at idle to ~17+V at 3000 RPM. I drove it like that for a few months until I got motivated to replaced the alternator (the only thing the D+ was going to was the GEN light); a new voltage regulator did not resolve it, but a replacement alternator did.

Why is that? What failed in the alternator for that to happen?

Sounds like a diode failed in the D+ circuit but it doesn't make sense to me. The D+ voltage drives the VR and armature and the VR keeps D+ in regulation around 14V. The VR has no idea what the battery voltage is doing. The VR only senses D+ to D- and keeps that voltage to 14V. In a fully functional alternator, the battery voltage will also be 14V.

[930cabman]

Another old, old thread

Generator light will not light engine off or engine on. When started alternator (with built in regulator) power at the battery reads about 12V. After running for a short time, 3 -4 minutes, power at the battery reads 13.8 or so. This is a /6 conversion so accessing the alternator is a PITA

[Spoke]

Another old, old thread

Generator light will not light engine off or engine on. When started alternator (with built in regulator) power at the battery reads about 12V. After running for a short time, 3 -4 minutes, power at the battery reads 13.8 or so. This is a /6 conversion so accessing the alternator is a PITA

So when you start the engine GEN light is not lit and the battery reads 12V. Then after running for a short time, you have 13.8V. Sounds like the VR is not making a connection to the armature so the GEN light is off. But after driving with higher RPMs, there's enough residual magnetism in the armature that the alternator comes alive. The voltage at the battery switches immediately from 12V to 13.8V.

The 911 charging system is similar to the 914 shown here for reference. The VR powers the armature through the GEN light causing it to light up and bootstrapping the alternator.

The culprit is likely either wiring or the VR. Hopefully it is just wiring.

[Lucky9146]

Glad this thread got bumped, the diagrams and info helped me understand what was happening with my alternator on a recent post on my thread.
(IMG:style_emoticons/default/driving.gif) (IMG:style_emoticons/default/white914.jpg)

[930cabman]

Another old, old thread

Generator light will not light engine off or engine on. When started alternator (with built in regulator) power at the battery reads about 12V. After running for a short time, 3 -4 minutes, power at the battery reads 13.8 or so. This is a /6 conversion so accessing the alternator is a PITA

So when you start the engine GEN light is not lit and the battery reads 12V. Then after running for a short time, you have 13.8V. Sounds like the VR is not making a connection to the armature so the GEN light is off. But after driving with higher RPMs, there's enough residual magnetism in the armature that the alternator comes alive. The voltage at the battery switches immediately from 12V to 13.8V.

The 911 charging system is similar to the 914 shown here for reference. The VR powers the armature through the GEN light causing it to light up and bootstrapping the alternator.

The culprit is likely either wiring or the VR. Hopefully it is just wiring.

Thank you Spoke, this issue is with a recent /6 conversion. Alternator (bench tested by local rebuilder) has a built in regulator. I was unable to get anything from it so the carbs, shroud, ... came off to access the alternator. Took into Phil and it tested ok on the bench. Now it's all back together and acting weird. After running a short (2-3 minutes) with a few blips, I will get 13 or 14 volts and it seems to remain until I shut the engine off. A restart will return the voltage to 11.8 V

[ClayPerrine]

This sounds like an issue with bad grounds.


Was the ground wire that connects the back of the alternator to the case present and in good condition?

[930cabman]

This sounds like an issue with bad grounds.


Was the ground wire that connects the back of the alternator to the case present and in good condition?

Yes, clean and tight

The GEN light does not come on in any scenario. I will dig into this as I sense the issue lies with "bootstrapping" whatever that is

[87m491]

Not for nothing, but unless I missed it you have not said the gen light bulb was present/connected/not burned out?

This sounds like an issue with bad grounds.


Was the ground wire that connects the back of the alternator to the case present and in good condition?

Yes, clean and tight

The GEN light does not come on in any scenario. I will dig into this as I sense the issue lies with "bootstrapping" whatever that is

[Spoke]

Thank you Spoke, this issue is with a recent /6 conversion. Alternator (bench tested by local rebuilder) has a built in regulator. I was unable to get anything from it so the carbs, shroud, ... came off to access the alternator. Took into Phil and it tested ok on the bench. Now it's all back together and acting weird. After running a short (2-3 minutes) with a few blips, I will get 13 or 14 volts and it seems to remain until I shut the engine off. A restart will return the voltage to 11.8 V

Sounds like you can drive the car just make sure to blip the throttle to get the alternator to bootstrap.
The GEN light off is the issue. Either the VR is not connecting the bulb to the armature or the wires don't connect.

Try testing the system by measuring the voltage of the blue wire from the alternator and both voltages at the bulb. Just turn the key to RUN w/o engine running. The GEN light should light.

[technicalninja]

Spoke and Dave Darling have their shit together!

Both members are freaking perfect!

Now, 930Cabman's car either has wiring issues or a bad diode in his alternator IMO.

Fast way to test is add ground to the D+ ignition in run, engine off.

Does the gen light glow?

Bad diode (95%) or maybe regulator.

Regulator would be a stretch but it's so much easier to swap that it's worth trying first.

Light doesn't glow?

Look to wiring for the light.

Another dead giveaway for bad diodes is an alternator that shows amp draw (any at all) with ignition off.

I've replaced more alternators than anything else for "If I let the car sit 3 days the battery is dead".

And I'll second Clay's recommendations. Take a set of jumper cables and connect neg bat term to engine case with the cables.
Does anything change?

[930cabman]

Once again, the 914 World Team has killed it.

Now I need to get to work and do a bit of digging.

Thanks all for your assistance

[930cabman]

Excited

Still having an issue, but seems as though the alternator comes into charging earlier. I am still unable to get the GEN light to light under any condition (Lamp will light with 12v applied). Red/white wire has switched battery voltage. Blue wire I cannot get anything, no voltage, no continuity to ground, nothing.

But, it seems as the system comes online quicker. Alternator was bench tested and was working according to my local rebuilder. Hate to source a new alternator, but kinda looking that might be the answer.