912 Electrical Issue

[reyn911]

Hello all, thanks for taking the time to help me diagnose this electrical issue I am having. I am new to classic 911 ownership and am looking for advice diagnosing an electrical issue. I have a 1968 Karmann 912 5 gauge car that was converted into a 911 with a 2.2L carbureted engine. I am having a multitude of electrical issues. I have recently replaced the alternator with an updated internal voltage regulator model. I have also removed the existing voltage regulator per the technical bulletin prescribed by Porsche. I am still awaiting parts to finish soldering in the new resistor and plan on replacing the ground strap that goes from the alternator casing to the engine casing.
The battery reads 12.4 volts with everything powered off and then if you turn the key to on everything fires up and lights up as it should (Red alternator light is on). If you then turn the key to engage the starter nothing happens. I tried this two sequential times and then rechecked the voltage on the battery and it read 11.2 Volts. Then about an hour later I went back out to the car with no charger on it and the voltage was again stable at 12.4. I had the battery tested at O’Riellys and it tested good. If I add external power ie a jump starter the car fires up perfectly and runs as it should. However when external power is removed the voltage drops and to 12 volts maximum then the battery slowly drains the red alternator light on the dash illuminates and the car ultimately begins to misfire.

Some other symptoms of the electrical issue are with external power connected and the car running the tachometer/indicators/lights work as they should. When external power is removed the tach works for a period of time unless you add an electrical load ie I turn on the blinker and the tach pins maxed out at 8k RPM while the blinker illuminates and then returns to normal when the light is no longer illuminated. Then eventually as the battery drains the tachometer pins itself at the top of the indication at 8k RPM and other accessories ie the blinker cease to function.

Lastly one stupid question does the red light inhibit the alternator from charging the battery or is just an indication. Sorry I am not an engineer. Thanks again for your time and willingness to help. Hope to see you at a Porsche Parade sometime down the road.

[PeteKz]

Since you mentioned the red alternator light, let me ask: is it an original style bulb,
Or did you replace it with an LED? That light provides excitation current to the alternator to get it generating when you first start the car. It must be a regular incandescent bulb.

What you have described sounds like the alternator is not working. However, it also sounds like you have a bad battery. The car should run on the battery alone for a couple hours, even if the alternator is not working. The testers that auto parts stores use do not identify all bad batteries. A good battery should be able to crank the engine for at least 30 seconds (crank it 10 seconds at a time and let the starter cool down for a few minutes in between). It sounds like your battery cannot do that, or even crank it at all. To crank the engine for 10 seconds without starting it, disconnect the primary wires to the coil, or unplug the CDI box.

First check all the battery cable connections at rhe battery and starter and ground straps from the battery to the body, and from the body to the transmission. Actually disconnect and clean them then put back together. See if that works.

Second, borrow a battery from another car and put it in this car. See if that works.

Report back what you find. If the battery is fully charged and known good, and it passes the cranking time test, then we will troubleshoot the alternator and its connections. You may have miswired something.

[911obgyn]

Check voltage while car is running, should be 14v ish. If alternator light is illuminated while running its not charging. The light is part of charging circuit when wired properly

[al lkosmal]

The alternator will not work, until you add the resistor, per the bulletin below....

[nickelplated5s]

Oh Vey! I've got a 67 K with a 2.2. Different but similar problem, check the ground straps at the battery and the wiring to the clamps. My guess is a bad battery. On the other hand a 2.2 swap means your rear harness has been modified. I bit the bullet and put a new, correct, rear harness in. I do get the red light with low RPM's. On the other hand probably Al's thought.

[reyn911]

Thanks for all of the wisdom. I will troubleshoot all of this and post my results on here once I am able to work on the car. I will be gone for work for the month of January. I will report back in Febuary. Thanks again.

[phunt]

Nickel said it with replacing engine harness. Either get a 911 harness that does not have the voltage regulator hook ups. New replacement harness is expensive. I have a 66 912 that was converted to six cylinder. I added efi so I just got a painless 21 circuit harness. Clean simple easy and cheaper. If your car is charged from original a harness with better wires and fuses seams obvious. $.02

[ChrisHoover]

The differences in the 912 and early 911 harnesses are few. The regulator panel changed but most of the wires remained.

My 912 with 2.7L had many electrical problems. Who ever converted the car was sloppy and miss wired the alternator and regulator. Grommets removed during painting and were never replaced. Wires became abraded. Splices were horrible. I think there may even been a wire nut. Anyway, I’ve reworked most if it. Once I cleared the ground faults and corrected the regulator all was well. The remaining work is mostly cosmetic. Long story to say the 912 harness had everything the 2.7L needed. This car is very simple without a lot of doodads much discussed in this forum. If you have a lot of doodads, they may need conductors the 912 did not have.

BTW, changing the complete harness is a pain in the backside. If the car is not stripped down to the shell, good luck. There is an excellent video showing replacing the entire harness on a 1968. It looks easy. Except he had the shell on a rotisserie. There was nothing but sheet metal harness. No seat shifter or e brake. No engine. No spare headlights fuel filler. Nothing to impede placing the harnes

[reyn911]

A quick update,

I was able to swap out the ground strap behind the alternator from the fan casing to the engine casing (It was beyond its useful life, image below). I also went ahead and replaced the battery. The car now cranks and fires right up! With the car running it still is not charging the battery. I suspect the culprit is the resistor per the Tech Bulletin. Can someone explain in plain language how a light inhibits the alternator from charging the battery, how the resistor will fix that issue, and how to wire it in? I have attached images of the part I ordered Per the Tech Bulletin as well as a picture of the back of the gague.

[gerakroo]

The charge light is connected between +15 (ignition on) and the regulator. This circuit provides a path from the battery to the field coil to provide a magnetic field to the rotor that will start the alternator charging when it is turning. Until the alternator is running, the electromagnet gets its power from the battery (trough the control lamp). When the lamp is glowing, it means that the alternator "field coil" (electromagnet) is being powered by the battery. Once the alternator kicks in, the lamp goes out because there no longer a voltage difference.

see also: https://www.kroonwireharnesses.com/other-items?product_id=1294

[spoke]

Originally Posted by reyn911

A quick update,
With the car running it still is not charging the battery. I suspect the culprit is the resistor per the Tech Bulletin. Can someone explain in plain language how a light inhibits the alternator from charging the battery, how the resistor will fix that issue, and how to wire it in? I have attached images of the part I ordered Per the Tech Bulletin as well as a picture of the back of the gague.

Question: Is the GEN light on always?

See the diagram for the 914 charging system. The 911 system is very similar. Alternators are pretty simple. Generally if you spin a magnet close to some windings a voltage is induced in the windings.

Alternators use an electromagnet in the armature which spins with the motor and generates ac voltages in the stator windings. The voltage regulator adjusts the current in the armature which adjusts the output voltage. Alternators use a second set of diodes (left side) to create a voltage independent of the battery and vehicle electrical loads (right side) for better stability.

If you remove the GEN light, notice there's nothing to power the VR when the engine is started and the alternator will not generate any voltage. With the GEN light in place, when starting the car, the GEN light will light and basically provide just enough current to get the power the electromagnet in the armature so the alternator can start up. Then the second set of diodes (left side) powers the VR; the GEN light goes out and the alternator runs ok. This is called bootstrapping as the alternator uses its own voltage to start up.

After reading that bulletin, it looks like the OEM 2W GEN light doesn't provide enough current to start the new alternator. They talk about needing a 4W GEN light or add a parallel resistor to the 2W GEN light to provide sufficient current to start the alternator.

[reyn911]

So I’ve updated everything per the technical bulletin and installed a new bulb and it still is not charging. The light comes on with the key on. Then it goes out once the car is idling. It’s only producing 12.4 volts. Also, the tachometer doesn’t indicate correctly when you turn on headlights, turn signals, etc.

[spoke]

Pull the instrument out with the GEN light. Start the car and measure the voltage on both sides of the bulb. Both should be 12V if the bulb is out when running.