Electrical Wizard Help Needed, Alternator Light

[RSBob]

So there is no confusion about cars, this is about my 914-6 with a 3.0. When originally had the car converted to a 3.0 from a 2.0, I installed an aftermarket combo gauge which provides Volts, oil Temp, oil Pressure and Fuel. All the “bulbs” including idiot lights are LED <- key point. When starting, the red G (alternator light) would come on as expected, but would not go out until I hit 3000 RPM. Below 3000 the volts would read 12. When I hit 3000 then the volts read 14.5 regardless of RPM there after.

So the alternator died last month and was replaced with a new one. Same issue with the G, but now I must hit 4000 RPM for the light to go off and to maintain 14.5 volts. I believe the original resistor is in place on the wire to the alternator LED, but won’t swear to it. I obtained another from Porsche and placed it in-line to the LED and the G spot won’t light up at all, regardless if placed on the hot lead going into the LED or the ground coming out. With the resistor in place the max volts produced are 12.5 regardless of hitting 3000 or 4000 RPM. So I removed it.

My theory is that the LED is not creating the correct resistance to send feedback to the alternator/ voltage regulator to produce the 14.5 until 4000 is hit. For fun I temporarily wired in a higher voltage light (not a gauge light) instead of the LED and the engine wouldn’t turn off even with the key until the bulb was disconnected. If I felt like it, this would be a good place to wire in a kill switch, but I don’t.

Now keep in mind I am an electrical idiot, (and idiot in many other rights) own a volt meter but don’t know how to use it. So my questions are:

1. what is the correct resistance needed to turn off the G light at start up and
2. where exactly do you, and
3. HOW, do you measure it? Is it on the red wire going into the LED or the black wire/ground coming out? Remember this is a non standard gauge.
4. What settings on the Voltmeter to read?
5. Why Is putting the new resistor in-line creating too much resistance thus preventing the alternator from correctly functioning?

When I learn the correct amount of resistance I need from you guys and then can learn to read mine, then I would think it would be a simple matter of subtracting my readings from the correct resistance to buy the correct resistor. That is my best guess as an idiot. Please feel free to correct me.

I don’t have access to a standard gauge.

Until then, HAYULP!

[76FJ55]

I am not familiar with the resistor you are referring to, but here is my understanding of the system.

Hello, don't know if you've made any progress on your charging issue yet or not so thought I'd try to address a few of your questions.
Colors are based on original 3.0 installed in the 911.
The Red/Black wire should be switched ignition power and supplies power to the Alt charge light. You should get 12V at the Red/Black wire with the key in the run position. This should supply power to the bulb socket for the Alternator charge indicator light.
The other terminal on the bulb socket should be attached to a Blue wire. This wire should then run back to the D+ terminal of the voltage regulator. Without the engine running the blue wires should ground through the regulator to illuminate the Alt charge light as the initial test indication for the bulbs.

The alternator needs to have the field excited to begin charging. This is accomplished through the power supplied by the Red/Black wire through the indicator bulb then through the Blue wire to the voltage regulator. If the indicator light does not pass enough current when illuminated (likely your problem since you are using an LED) with the key turned to run position the alternator field will be too weak to induce charging. I would suggest replacing the LED with a standard incandescent ALt charge bulb. adding a resistor to the circuit for the ALt charge light will only decrease the available current for creating the field, which is why it increased the necessary RPM to get the alt to start charging.

when you get the RPM high enough the Alt will start charging even with a weak field. Once the Alt starts to generate power it can then use the generated power to supply the field windings. This is why once you hit the necessary RPM the Alt will start working and stay working till you shut the car back off.

Let me know what you find and I'll see if I can help further.

Good luck,

[Norm01]

Do you have the proper regulator matched to your alternator

[RSBob]

Thanks 76FJ55, how about if I just call you Bud? I really appreciate your thoughtful response. Independently I came to the same conclusion about deleting the LED, but without the insight of your knowledge, and substituting a standard bulb. I have a bulb and may just solder on a couple of wires and see what happens.

Norm, my alternator came from the manufacturer with a built in VR. My mechanic says he is only aware of two types of these for Porsche’s.

[al lkosmal]

rick,
here is the tech bulletin regarding the resistor.

also, i have recently had two instances of run-on, after installing engine/alternators/ignitions in projects and there is a diode that you can install that eliminates that problem if it persists, after installing the right resistor.








I needed to install this diode on a few cars.........Andrew (Rasant) provided the illustration.......


I'm sure i have a diode or two, if you need one.

regards,
al

[Duc Hunter]

No wonder why, when I switched to an LED indicator like in my M64 power '75, the light would not go out until the car went over 2,800 RPM. With a traditional bulb it went out at about 1,800 RPM, and usually did this on startup. Hmmm

[RSBob]

Al, I missed your response. Thank you for your research.

[al lkosmal]

Rick,
No worries mate (can you tell I've been working on some Aussie EFI projects, these days?)
al

[RSBob]

Hey Al, Bernie provided me with diode 91161710801 https://www.porsche.com/all/media/flash/classic/book/en-us/index.html#22

Which should work. Keep in mind there is a dedicated hot lead going to the LED at 1:00 (red) and black wire going to the blue wire using the light blue connector. I have tried inserting the resistor above on the red and white and then on the black and the alternator will only put out 12.5v with it installed at idle. As you can see, I did not hack into the thin red wire to insert the resistor but may need to. But again it only connects to the LED and not a bulb so that is probably part of the issue. Don’t have a bulb holder for the old bulbs I have. What is the proper orientation for the resistor, asks the electrical idiot? On the female side of the resistor, there is an interesting occult pictogram with a circle with a triangle with a T at its top which would suggest orientation to a lessor idiot. Question one, on which color wire should the resistor go, and 2. What is the orientation of the resistor on said wire? 3. What to do about the difference in resistance between the current LED versus the standard bulb or.... 4. Should I first try wiring in a bulb and bypass the LED, but how without a bulb holder?



Just a tidbit, as an experiment, I added a way to high voltage bulb because it had a bulb holder which stayed illuminated with the car running. When I turned the ignition off, the car continued to run until I disconnected the bulb.

[al lkosmal]

Rick,
The part that Bernie provided is a diode, not a resistor.. this diode should be installed to correct the run-on condition, per my previous email.

However, first you need to fix the Alt light, alt charge indication/alt excitation issue....that is still a resistance/current problem methinks.

1, get the proper resistor, per the previous email (91 Ohms, 5 Watts or close)
2. attach it in parallel with the LED wires

( parallel........I.E. across the LED wires.)


once you get the proper resistance across the LED, the current will be enough tp trigger the alternator excitation.

After that is complete, if you have a run-on problem, you can apply the diode to fix that too.

[al lkosmal]

option for steps 1 & 2......

Next sunny day, haul your 914/6 up to the North county for alternator excitation day.

al

[RSBob]

Alternator excitation day? No wonder the 914 is wet, besides driving it in the rain and snow flurries today.

Would this chunky monkey fit the bill? https://vetco.net/products/91-ohm-5-watt-resistor/nte-5wd91?gclid=EAIaIQobChMI6Mz1-MGB2gIVAwNpCh2xxAtyEAQYByABEgKmEfD_BwE

And this would need to be soldered to/between the very thin black and red wires. Doing this insitu would require a surgeon’s skills. Looks like it would be spaghetti-fest removing all the connections. I may just pay you a visit because there are usually gremlins hiding in the weeds. But I will order the resistor if that is what works.

[RSBob]

Resolution recap. Per Al, a resistor (10 Ohm, 5 Volts) and the above Porsche diode were added in parallel with the LED. (Original suggestion was 90 Ohm, but was able to precisely measure required resistance using a device which allowed me to dial in resistance and then measure it.

Now at start up the G Alternator LED immediately goes off and the Volts are at 14 with no throttle input and 14.5 with. All is good. Thanks to everyone for their input.

[jjeffries]

Reading this thread, I just learned a few things, one of which was the answer to a question I had been trying to figure out for about fifteen years! Thanks to all, John in CT.