911S 2.7L Charging Problem...Please Help

[jkshadow2]

Hello fellow Pelicans,

I need some serious assistance with a charging problem. I have a 1975 911S with a 2.7L engine (stock set-up). During normal start-up the “generator” dash light would illuminate, but then after a few revs, the light would turn off. A few months ago I was driving and noticed that the “generator” dash light would occasionally illuminate while I was driving at speed. It would stay illuminated for a bit, then turn off. After happening a few times, I decided to check the charging voltage at the battery because I did not want to get stranded. I found that the voltage would vary from between 13-17 volts at a steady RPM. I immediately thought my voltage regulator went bad. I purchased one from our host and installed it without issue. I checked the voltage and it would be within spec (13-14 volts). Resting battery voltage (car off) is 13 volts.

A few weeks later I was driving and saw the “generator” light illuminate again. This time, the light would vary from dim to bright. The light would eventually turn out, but then it would do it again. I had one of those cigarette lighter voltage meters attached and saw that the car was charging at 11 volts.

I decided as a preventative measure to have the alternator rebuilt. When I removed the alternator, I found that it was an internally regulated Valeo brand. On the rear of the alternator, the red (B+) wire was attached, the blue (D+) wire was attached, and the brown wires (4 of them) were NOT attached via a spade bit. I was not sure if these wires were originally disconnected, or the wires pulled off the spade when I was removing the air horn from the back of the alternator. The black (D-) wire was unattached.

I took the alternator to a quality rebuilder. The diodes, brushes, and slip ring were replaced. The alternator was bench tested and its output was 14.3 volts.

I cleaned all the alternator wire contacts with steel wool to remove any corrosion. I re-installed the alternator, disconnected the external voltage regulator connector, and started the car. The “generator” light illuminated, but did not turn off with an increase in RPMs. I cleaned and retightened the battery cables. Unfortunately, the car was still not charging.

I removed the alternator and checked all the wires again. Using a continuity tester, I confirmed that the ground strap to the alternator was grounding properly, the red B+ wire showed continuity to the battery, and the blue (D+) wire showed continuity to the “generator” dash bulb socket. I tried the alternator with the brown wires connected and disconnected. None of this worked and the car was still not charging.
I also took the alternator back to the rebuilder and bench tested it again. It was working perfectly.

Since my car originally came with an external voltage regulator and had been converted to an internal one, I read the Porsche Technical Bulletin 1 regarding the conversion that our host has posted. This Bulletin verified that the alternator black colored D- wire was not used anymore, as I had seen when I first removed the alternator. I also verified that the external voltage regulator is supposed to be disconnected.

On page 2 of the Bulletin, it directed that a 91 ohm 5W resistor (Porsche part# 911 641 981 00) be installed. Basically, a resistor is added in series to the “generator” dash bulb socket because the regular light bulb is insufficient to cause the exciter current to activate the alternator. My car did not have this addition, but it did not stop it before from charging.

I figured if the Bulletin said that it was needed, then I would install it. I checked with our host and found that this part is unavailable. I looked on this forum and found a basic work-around. I purchased the necessary parts to build a homemade version of the required addition. I installed it and no luck. The car still was not charging, though the resistor did get warm to the touch, meaning it was working. The overall resistance of my set-up was 102 ohms.

While the car was running, I removed the entire dash light bulb socket. I verified that there was no current at the blue D+ wire at the socket. I was getting 12 volts at the other wires (see photo) that attached to the socket. I even installed just the resistor into the wires at the “generator” dash socket, just to see if the increased resistance would do the trick, but again it was not charging.

I have checked the wiring diagram for the car to see what else may be an issue, but I am not very good at figuring it out. I did see that the warm-up regulator is connected to the brown wires that connect to the alternator, but since I do not have a fuel problem, I did want to mess with that and cause another issue.

I am totally at a loss for what to do/check next.
I would appreciate any ideas you have or areas for me to investigate.
Thank you
Jeff K.

[ed mayo]

Use ohm meter to check continuity of blue wire from bulb socket to alternator.

[mysocal911]

Originally Posted by jkshadow2

Hello fellow Pelicans,

I need some serious assistance with a charging problem. I have a 1975 911S with a 2.7L engine (stock set-up). During normal start-up the “generator” dash light would illuminate, but then after a few revs, the light would turn off. A few months ago I was driving and noticed that the “generator” dash light would occasionally illuminate while I was driving at speed. It would stay illuminated for a bit, then turn off. After happening a few times, I decided to check the charging voltage at the battery because I did not want to get stranded. I found that the voltage would vary from between 13-17 volts at a steady RPM. I immediately thought my voltage regulator went bad. I purchased one from our host and installed it without issue. I checked the voltage and it would be within spec (13-14 volts). Resting battery voltage (car off) is 13 volts.

A few weeks later I was driving and saw the “generator” light illuminate again. This time, the light would vary from dim to bright. The light would eventually turn out, but then it would do it again. I had one of those cigarette lighter voltage meters attached and saw that the car was charging at 11 volts.

I decided as a preventative measure to have the alternator rebuilt. When I removed the alternator, I found that it was an internally regulated Valeo brand. On the rear of the alternator, the red (B+) wire was attached, the blue (D+) wire was attached, and the brown wires (4 of them) were NOT attached via a spade bit. I was not sure if these wires were originally disconnected, or the wires pulled off the spade when I was removing the air horn from the back of the alternator. The black (D-) wire was unattached.

I took the alternator to a quality rebuilder. The diodes, brushes, and slip ring were replaced. The alternator was bench tested and its output was 14.3 volts.

I cleaned all the alternator wire contacts with steel wool to remove any corrosion. I re-installed the alternator, disconnected the external voltage regulator connector, and started the car. The “generator” light illuminated, but did not turn off with an increase in RPMs. I cleaned and retightened the battery cables. Unfortunately, the car was still not charging.

I removed the alternator and checked all the wires again. Using a continuity tester, I confirmed that the ground strap to the alternator was grounding properly, the red B+ wire showed continuity to the battery, and the blue (D+) wire showed continuity to the “generator” dash bulb socket. I tried the alternator with the brown wires connected and disconnected. None of this worked and the car was still not charging.
I also took the alternator back to the rebuilder and bench tested it again. It was working perfectly.

Since my car originally came with an external voltage regulator and had been converted to an internal one, I read the Porsche Technical Bulletin 1 regarding the conversion that our host has posted. This Bulletin verified that the alternator black colored D- wire was not used anymore, as I had seen when I first removed the alternator. I also verified that the external voltage regulator is supposed to be disconnected.

On page 2 of the Bulletin, it directed that a 91 ohm 5W resistor (Porsche part# 911 641 981 00) be installed. Basically, a resistor is added in series to the “generator” dash bulb socket because the regular light bulb is insufficient to cause the exciter current to activate the alternator. My car did not have this addition, but it did not stop it before from charging.

I figured if the Bulletin said that it was needed, then I would install it. I checked with our host and found that this part is unavailable. I looked on this forum and found a basic work-around. I purchased the necessary parts to build a homemade version of the required addition. I installed it and no luck. The car still was not charging, though the resistor did get warm to the touch, meaning it was working. The overall resistance of my set-up was 102 ohms.

While the car was running, I removed the entire dash light bulb socket. I verified that there was no current at the blue D+ wire at the socket. I was getting 12 volts at the other wires (see photo) that attached to the socket. I even installed just the resistor into the wires at the “generator” dash socket, just to see if the increased resistance would do the trick, but again it was not charging.

I have checked the wiring diagram for the car to see what else may be an issue, but I am not very good at figuring it out. I did see that the warm-up regulator is connected to the brown wires that connect to the alternator, but since I do not have a fuel problem, I did want to mess with that and cause another issue.

I am totally at a loss for what to do/check next.
I would appreciate any ideas you have or areas for me to investigate.
Thank you
Jeff K.

The added resistor is placed in PARALLEL not series. It's used to INCREASE the field current to start the alternator charging.

[jkshadow2]

I may have been mistaken saying that I put the resistor in series. I believe that it is in parallel. I've attached a photo of it. Please let me know if this is not correct. Thanks a ton.

[mysocal911]

Originally Posted by jkshadow2

I may have been mistaken saying that I put the resistor in series. I believe that it is in parallel. I've attached a photo of it. Please let me know if this is not correct. Thanks a ton.

Correct. Temporarily have two resistor added, i.e. the internal regulator may need additional startup current.