What's the "Gen" light trigger

You don't . The only thing that the "gen" light reliably detects (if the alternator is working) is if the alternator output goes below the battery voltage (because the belt breaks). The gen light will often come "on" if something goes wrong inside the altenator, but not always.

One side of the gen bulb filament is connected to the alternator output diode array and the +12V post on the battery (call this terminal X). The other side of the gen bulb filament is connected to another (identical) output diode array inside the alternator (call this terminal Y).

At low current load from the alternator, terminal X = terminal Y = 13~14V = no light from bulb, and your battery gets charged.

At high current load from the alternator (as your in your example), the voltage might droop down to ~12V, but terminal X still = terminal Y = no light from bulb, but you may not be charging the battery.

If the alternator belt breaks (or with key "on" but the engine not running), terminal X drops to the battery voltage ~12V, but terminal Y goes to 0V = the bulb lights brightly.

If something goes wrong inside the alternator gizmology where terminal Y is just always the same voltage as terminal X = the gen light never comes "on", but your battery still goes flat . This is the case where an ammeter gives you better information than an idiot light (but ammeters are troublesome because the current is so high so manufacturers have good reason to not use them -- ask the 246 guys ).

 

Good write-up, Steve. There is one additional note: for the 328 alternator light circuit, there is a diode in line with the light that is forward biased when the alternator is not running and reverse biased when it is. Thus with the alternator (actually a generator since the VR is part of the alternator assy) off, current will flow through the diode and the light, illuminating it; with the alternator on, the reverse biased diode prevents current flow through the light, keeping it off. To address the Voltage level required to turn it off, that would be when the alternator is providing at least 0.7 V more than the battery; as an example, if your battery Voltage is 12.3Vdc, then the alternator must supply about 13Vdc (actually, a diode is not a go-no go device; with the diode 'on' a Voltage of about 0.3Vdc may be enough to turn it 'off' while the 0.7Vdc will ensure no current flow).

Also, the actual battery charging is done through the same larger-guage line used for the B+ for the starter (the starter only turns on when the solenoid is energized from the ignition switch). The alternator light is an auxiliary circuit.

 

My comments were directed to the OP's vehicle's configuration -- '77 308.

Can't agree that you need a certain voltage difference to turn a diode "off", but do agree that the diode is there on a 328 to prevent the gen light from glowing slightly when things are actually OK. The route from the D+ terminal (terminal Y in the prior example) of the alternator to the gen light is fairly direct; while the route from the B+ terminal (terminal X in the prior example) of the alternator to the gen light is much more convoluted (and current is tapped off at various places along the way). I.e., at the alternator, the D+ and B+ voltages are exactly equal (say 14V) , but, at the gen light, the B+ value might be slightly lower than the D+ value causing the gen light to glow a little even though the alternator output is just fine -- which would needlessly worry the Driver without the diode

 

in my 78 308, the generator light is now alway on (glowing at idle and at higher rpms, it gets brighter) when driving the car. this has been happening steady for about 30 minutes of driving, today. its back in the garage now.

i measured the battery voltage at its terminals and have:
12.00 volts with the car off
13.65 volts with engine idling

ground cable was replaced and attaching points cleaned.
the positive cable was atleast cleaned, i cant remember if i ever replaced that one. battery is an optima - about 2 years old.

alternator belt is relatively new with good tension.


what can be triggering my gen light to stay on if everything appears healthy.??

 

Did you measure the AC voltage as well?

 

no, you mean a/c across the battery terminals? carreaper suggests it is a dead cel in the battery.

 

Yes. The difficulty is that the ONLY thing that the alternator warning light is really designed to reliably report is if the alternator belt breaks when the engine is running. It will often misbehave with alternator or battery problems, but all you can really do is test both independently to know for sure. I go from easiest to hardest:

1. Measure DC voltage at battery with engine off, measure DC voltage at battery with engine running, measure AC voltage at battery with engine running; if inconclusive:

2. Have battery tested; if OK:

3. Have alternator tested; if OK:

4. You have a Gremlin somewhere deep in the wiring ...

JMO

 

sounds like a good plan Steve (i thought you were still up in boulder).
what should the ac voltage read across the battery terminals for a green light condition?

 

Very low -- like ~0.1V or less.

No, not for a few years now. It's just starting to get to the time of year when it's much better to be in Boulder rather than TX, but that changes again in ~mid Oct

 

Regulators control alternator output voltage by modifying the dc current to a coil which controls the magnetic flux between windings in the alternator. The regulator is powered by voltage FROM the idiot light.

Idiot light: A 'well' alternator needs a very small input current (through the idiot light) for the dc magnetic flux winding to function correctly. High battery loads will drop its voltage slightly requesting a small increase in magnetic flux to bring the voltage back up, as will higher rpm also increase voltage output.

A 'soft' system malfunction such as a bad diode will call for very much more input current, and will be outside the ability of the alternator at lower rpms. As a higher input current is demanded the higher input current can make the light glow dimly.

The idiot light limits the amount of input current which is available. With a 'hard' failure such as a broken belt or several bad diodes, the regulator is beyond its control range and the resultant great demand for input current lights the idiot light full on.

Note a bad or disconnected idiot light means no output because the idiot light FEEDs the magnetic flux winding. When first turning the key on, the idiot lights because the regulator is feeding the alternator maximum current, through the light. Once the engine starts and voltage output comes up, the required control current is reduced and the light extinguishes.

 

Incidently, measuring ac ripple voltage in order to find a bad diode is futile with an ac voltmeter but can be done with a scope, hence is not a DIY test method but only used in well equipped shops, and then only rarely.

There is a large problem in measuring ac voltage which rears it's ugly head here. Voltage wave shape is as important as magnitude and digital voltmeters are all different. With the same odd waveform they will produce quite different readings. An open diode doesn't increase the ac ripple voltage but leaves a gap where the peak should be and CAN give a lower than normal reading. A shorted diode OTOH, with it's short condition, also can give a reduced ripple reading with it's skewed waveform. And there are horrendous spikes which alter the reading.

Digital AC voltmeters have so vastly different conversion parameters mostly in rms conversion, which although a reading is obtained, varies so much from meter to meter as to be virtually useless here. Plus the difference in auto wiring distance from alternator to battery greatly alters one's assessment via voltmeter.

Rather than trust your own devices, go to an Autozone etc and they will use their accurate dc meter expressly calibrated to weed out good from bad alternators. Or just change your regulator...that 13.6v says bad diode.

 

Kragen Auto Parts (gradually being renamed O'Reilly Auto as they bought and are in the act of ruining Kragen/Shuck's/Checker) have testers that work pretty well at detecting diode ripple.

Has anyone just ditched the old design alternator and made use of a late model GM design? They're light years ahead in terms of performance and reliability. They come in such a variety that I'm sure you could come pretty close to finding one that would work with minimal bracket fabrication.

 

Here's where Ferrari owner personalities (and wallets) comes to fore:
308, 328 owners tend to take their 10 year old alternators to a rebuild shop for $110
348, 355 owners face annual replacement a few times before researching a reliability upgrade (I would too facing a dealer service of 5k for a new battery and alternator.)

If you are serious, start your quest here and take lots of photos for us.
http://www.ferrarichat.com/forum/showthread.php?t=273799
or
http://ferrarichat.com/forum/showthread.php?t=124504

If you wish more, search is your friend, do an advanced search in 'Technical Q&A forum' for 'alternator updates'

 

thanks guys, i have the battery with me and will get it tested today.
i measured A/c voltage with my digital fluke meter and i read around 40mVolts. maybe i do have a bad alternator.

 

Did you mis-type here? 40mV AC is a good result for your alternator (it's less than 0.1V AC).

 

You are correct Steve, .04 volt is less than .1 volt.
so, perhaps the rectifier circuit is doing its job. i will be getting the battery tested in about 5 hours.

 

the battery tested out fine, 799 amps out of a 800 amp battery.

is there any good music to yank an alternator by??

 

I have a somewhat weird problem with my 81 308GTSI. I recently replaced the battery and found that the "gen" light goes off while driving, but when I shut the car off, it remains on as a bright red light and I have had to disconnect the new battery in order to prevent its drain. Any suggestions on why this would happen now that the car has a new battery?

Thanks.
James

 

Almost certainly has to be a bad electrical component inside the alternator (the battery has no relationship to the problem).

 

Thanks Steve - I will have the alternator tested.

 

One thing I have always wondered with my 328...why does the warning light stay on at start up if the RPM doesn't initially rise to 1800-2000 RPM. I have to tap the accelerator to get sufficient RPM to "activate" charging/turn off the light. After that it stays off/charges normally at idle and above.

 

I wouldn't call that normal or expected behavior -- although some alternators do develop that behavior.

 

Yeah - I've never seen that in any other vehicle but my 328's done that since I bought it in '08. It's like a "switch" turns on once the RPM get to a certain point and then it stays on until the engine is turned off. I've checked the batt voltage and the meter shows exactly the same thing - no charge at, say, idle after startup if the RPM hasn't been 'blipped' to around 2k. As soon as blipped, then normal charging at idle/all RPM. I thought it was some sort of "feature"!