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Discussion Starter #1
sorry, long winded post, bare with me :)

So as long as I’ve owned my 76 spider, the red gen light has been glowing on the tach.
I did put a new alternator in last year and I’m getting charge at the battery, just a hare over 14v. I also cleaned and replaced all fuses in the fuse box.

Looking at the wiring diagram, I see the pink wire from the alternator light also feeds the circuit for the fuel pressure and low oil pressure lights. Neither of these lights have ever come on as long as I’ve owned the car, and to my understand both should flash on briefly when I start it.

Now here’s my question, would a bad bulb or faulty wiring to either of these lights cause an open circuit and trigger the alternator warning?
 

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No there should be no way the indicator lights should affect the alt. Light as the indicator lights are further down the pink wire. As for the alt. Light I would have a shop check the regulator as that's what the ground for the alt. Light is controlled by. This is just my opinion though as I am only familiar with the 73 spider.
 

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Discussion Starter #3
Thanks for the reply, okay that was my first think to check.

The alternator has been replaced with a new Bosch al16x with an internal regulator

Ive also been reading some older posts on here that say replacing the bulb with one of an incorrect wattage will cause it to light. Any truth to this?
 

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My understanding is that the bulb lights when there is a difference between the battery voltage and the green wire from the alternator. So when you turn on the ignition but do not start the engine it glows brightly. When you start the engine and the voltage on the green wire increases to match the battery voltage it goes out. If it glows dimly then the alternator is putting out less than it should. The bulb should definitely be of the correct wattage.
 

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Thanks for the reply, okay that was my first think to check.

The alternator has been replaced with a new Bosch al16x with an internal regulator

Ive also been reading some older posts on here that say replacing the bulb with one of an incorrect wattage will cause it to light. Any truth to this?
Yes I've had the opposite happen with the indicator lights inside the car not flashing when the flasher was replaced. I would dig into the specs of the alternator to figure out the correct wattage of the bulb.
 

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Supposedly LED lights won't create enough resistance and the light will glow. Not my experience but others have mentioned that. Could apply to the wrong bulb as well. I don't have a wiring diagram for a 76 but is the pink wire feeding the gauge lights, or the gauges themselves? The gauges are fed with off the same circuit as the gen warning light on my 71 so if there's an issue on that circuit, those gauges don't work.
 

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I just looked it up in "Automotive Electric/Electronic Systems" by Bosch.
On starting, the pre-excitement current to the alternator excitation circuit is fed through the bulb. It supplies enough current for the Alternator to generate voltage. After that the excitation circuit is fed from the stator and there should be no voltage across the bulb and it should not light. The normal rating for the light is 2 W.
 

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Supposedly LED lights won't create enough resistance and the light will glow.
No. The resistance is too high and it will not supply the required current to the excitation circuit.
 

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The 2 w bulb is no doubt specified for a worst case condition. If the rotor has sufficient residual magnetism then it will start up without any pre-excitation current.
 

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... mine works normally with an LED bulb.
As Ed wrote, alternators will self-excite even with no current coming through the warning light. But in that mode, the alternator usually doesn't begin charging until the engine is revved to a few thousand RPM. With an incandescent light in the circuit, they'll start charging immediately. So sure, the charging system will work with an LED - you just have to watch the LED glow until you get underway.
 

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Discussion Starter #13
So first off, there’s a 4w bulb in there so I will replace that with a correct 2w, not sure if that that’s a major difference but it’s something to try. Tomorrow I’ll get the old multimeter out and see if I’m getting current at the green wire at start up.

I’m not sure if the green wire is not energizing, or something is back feeding from the pink wire.

all the gauges work, the oil pressure one is very erratic, but the sender for that one is pooched. The only other issue is, as I mentioned, the oil pressure and fuel pressure lights don’t work, but I can imagine that’s a bad bulb for each. The low fuel warning light on the fuel gauge itself does light up if it gets low
 

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would a bad bulb or faulty wiring to either of these lights cause an open circuit and trigger the alternator warning?
Sorry, I just read your original question. My answer is yes - faulty wiring to the GEN warning bulb would cause it to glow when the charging system is working OK. As Ed wrote in post #4, the GEN should go out when the alternator is charging because there is +12V to both sides of the bulb. But if your pink wires are not getting power, then one side of the GEN bulb gets current from the regulator and its other side dissipates that current through the other bulbs connected to the pink wires. Those other bulbs don't glow (visibly) because several of them are sharing the current.

I'm guessing that your GEN light doesn't come on after your have turned on the key, but before you start the engine. Yes? I'm also guessing that the reason your fuel and oil pressure lights don't work is because the pink wires that power them are dead; it would be quite a coincidence for both of those bulbs to have blown.

Changing the wattage of the GEN bulb won't fix this. You'll need to trace why the pink wires aren't getting power.
 

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Discussion Starter #15
Results are in
Battery is at 12.6 volts nice and healthy, alternator is charging at roughly 700rpm idle, but only at 13.8v

Green wire to alt light is getting voltage

so is 13.8 low enough to trigger the light? If so what’s next, check for bad ground?
 

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so is 13.8 low enough to trigger the light? If so what’s next, check for bad ground?
Hmm, I get the sense that you don't understand how the GEN warning light works. Two points that might help:

- The light is never "triggered". It's analog not digital. The bulb is simply wired between the alternator's output and the battery "+". As long as the alternator is putting out voltage about equal to the battery (like 13.8 versus the battery's 12.6) that little voltage difference ( .8 V in this example) won't be enough to light the bulb (at least not so you could notice). Think of the bulb as a gross indicator - not a precise one. If the alternator was putting out 0 V, it would light. But it can't show the difference between the alternator putting out 13.8 V versus 14.2 V.

- The GEN warning light isn't grounded the way other bulbs are, so there's no need to check for bad grounds. With the key on, but the engine not running (or the alternator not working), it's grounded through the regulator/alternator. But since your alternator appears to be working, current is flowing through your bulb in the opposite direction. That is from the alternator, through the bulb, and to ground through the other devices on the pink wire circuit (since I'm guessing that circuit isn't getting 12 volts with the key on).

It would help to know the answer to the question I asked in post #14: I'm guessing that your GEN light doesn't come on after your have turned on the key, but before you start the engine. Yes?
 

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Discussion Starter #17
Sorry, no, the light comes on as usual when the key is turned to on and does not go out when the engine is started
 

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Does your car have a separate voltage regulator that connects to the alternator through a multi pin plug or does it have a built in regulator with just a heavy gauge red wire and a small green wire connected to it?
 

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Sorry, no, the light comes on as usual when the key is turned to on and does not go out when the engine is started
OK, well there goes my theory about the pink wire being dead.

Ed's question about the voltage regulator - is it internal or external - is another good one. A '76 spider would have come with an external regulator, mounted near the left upper corner of the radiator. But someone may have put in a later, internally-regulated alternator. Either way, as Ed notes, the green wire operates the GEN light. With both types of alternator, there is a connector on the green wire at the regulator (if external) or alternator (if internal). Try breaking that connection, and see if the light still stays on with the engine running.
 

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Discussion Starter #20
Okay thanks for that lead. So the cars been converted to an internal regulator , the external one is still fitted and the plug for it is stuffed way down in the fender. I’m beginning to think that someone just didn’t connect the green wire to the alternator properly when they converted it.

Would anyone be kind enough to share a wiring diagram for ‘76 spider if the that have one ? I’ve been working off of one from a 78 and I think a few of the colours might be different
 
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